This art icle was downloaded by: [ Georgia Tech Library] On: 08 April 2014, At : 08: 37 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Journal of Adhesion Science and Technology Publicat ion det ails, including inst ruct ions f or aut hors and subscript ion inf ormat ion: ht t p: / / www. t andf online. com/ loi/ t ast 20 Review of Recent Advances in Electrically Conductive Adhesive Materials and Technologies in Electronic Packaging Myung Jin Yim Wong a , Yi Li b , Kyoung-sik Moon c , Kyung Wook Paik d & C. P. e a School of Mat erials Science and Engineering, Georgia Inst it ut e of Technology, 771 Ferst Drive, At lant a, GA 30332-0245 b School of Mat erials Science and Engineering, Georgia Inst it ut e of Technology, 771 Ferst Drive, At lant a, GA 30332-0245 c School of Mat erials Science and Engineering, Georgia Inst it ut e of Technology, 771 Ferst Drive, At lant a, GA 30332-0245 d Mat erials Science and Engineering, Korea Advanced Inst it ut e of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taej on, Korea 305-701 e School of Mat erials Science and Engineering, Georgia Inst it ut e of Technology, 771 Ferst Drive, At lant a, GA 30332-0245; , Email: cp. wong@mse. gat ech. edu Published online: 02 Apr 2012. To cite this article: Myung Jin Yim , Yi Li , Kyoung-sik Moon , Kyung Wook Paik & C. P. Wong (2008) Review of Recent Advances in Elect rically Conduct ive Adhesive Mat erials and Technologies in Elect ronic Packaging, Journal of Adhesion Science and Technology, 22: 14, 1593-1630, DOI: 10. 1163/ 156856108X320519 To link to this article: ht t p: / / dx. doi. org/ 10. 1163/ 156856108X320519 PLEASE SCROLL DOWN FOR ARTI CLE Taylor & Francis m akes every effort t o ensure t he accuracy of all t he inform at ion ( t he “ Cont ent ” ) cont ained in t he publicat ions on our plat form . However, Taylor & Francis, our agent s, and our licensors m ake no represent at ions or warrant ies what soever as t o t he accuracy, com plet eness, or suit abilit y for any purpose of t he Cont ent . Any opinions and views expressed in t his publicat ion are t he opinions and views of t he aut hors, and are not t he views of or endorsed by Taylor & Francis. The accuracy of t he Cont ent should not be relied upon and should be independent ly verified wit h prim ary sources of inform at ion. 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Term s & Condit ions of access and use can be found at ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions Journal of Adhesion Science and Technology 22 (2008) 1593–1630 www.brill.nl/jast Review of Recent Advances in Electrically Conductive Adhesive Materials and Technologies in Electronic Packaging Myung Jin Yim a , Yi Li a , Kyoung-sik Moon a , Kyung Wook Paik b and C. P. Wong a,∗ Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 a b School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0245 Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon, Korea 305-701 Abstract Electrically Conductive Adhesives (ICAs: Isotropic Conductive Adhesives; ACAs: An-isotropic Conductive Adhesives; and NCAs: Non-conductive Adhesives) offer promising material solutions for fine pitch interconnects, low cost, low-temperature process and environmentally clean approaches in the electronic packaging technology. ICAs have been developed and used widely for traditional solder replacement, especially in surface mount devices and flip chip application. These also need to be lower cost with higher electrical/mechanical and reliability performances. ACAs have been widely used in flat panel display modules for high resolution, lightweight, thin profile and low power consumption in film forms (Anisotropic Conductive Films: ACFs) for last decades. Multi-layered ACF structures such as double and triple-layered ACFs were developed to meet fine pitch interconnection, low-temperature curing and strong adhesion requirements. Also, ACAs have been attracting much attention for their simple and lead-free processing as well as cost-effective packaging method for semiconductor packaging applications. High mechanical reliability, good electrical performance at high frequency level and effective thermal conductivity for high current density are some of required properties for ACF materials to be pursued for a wide usage in flip chip technology. Recently, NCAs are becoming promising for ultra-fine pitch interconnection and low cost joining materials in electronic packaging applications. In this paper, an overview of the recent developments and applications of electrically conductive adhesives for electronic packaging with focus on fine pitch capability, electrical/mechanical/thermal performance and wafer level packaging application is presented.  Koninklijke Brill NV, Leiden, 2008 Keywords Electrically conductive adhesives, ICA, ACA, NCA, electronic packaging, fine-pitch joint, flat panel display, flip chip, reliability, wafer-level packaging * To whom correspondence should be addressed. Tel.: 404-894-2846; Fax: 404-894-9140; e-mail: cp.wong@mse.gatech.edu  Koninklijke Brill NV, Leiden, 2008 DOI:10.1163/156856108X320519 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1594 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 1. A typical percolation curve showing the abrupt increase in conductivity at the percolation threshold. 1. Introduction Today, resin based interconnection materials for electronic packaging and interconnection technologies are widely used in manufacturing of electronic devices such as flat panel displays and semiconductor/system package modules [1]. They are attractive as traditional solder alternative due to advantages of low-temperature and low cost process, finer pitch capability and environmentally clean solutions. Electrically conductive adhesives are generally composite materials composed of on insulating adhesive binder resin and a conductive filler. Depending on the conductive filler loading level, they are divided into ICAs, ACAs or NCAs. The differences based on the percolation theory between an ICA and an ACA/NCA is shown in Fig. 1. For an ICA, the electrical conductivity is provided in all x-, y- and z-directions due to high filler content, exceeding the percolation threshold. For an ACA or NCA, the electrical conductivity is provided only in the z-direction between the electrodes of the assembly. Figure 2 shows the schematics of the interconnect structures and typical cross-sectional images of flip chip joints by ICA, ACA and NCA materials illustrating the bonding mechanism for all three adhesives. Especially, ICA materials, typically silver-filled conductive adhesives, have been recommended as solder replacement materials in a surface mount technology (SMT), flip chip, chip scale package (CSP) and ball grid array (BGA) applications. There are still challenging technical issues for full commercialization of ICAs such as low conductivity and reliability, high material cost, and poor impact strength, etc. and extensive research is being performed to enhance the electrical performance and reliability of adhesive joints [2–6]. Interconnection technologies using ACFs are major packaging methods for flat panel display modules with high resolution, lightweight, thin profile and low consumption power [7], and have already been successfully implemented in the forms of Outer Lead Bonding (OLB), flex to PCB bonding (PCB), reliable direct chip Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1595 Figure 2. Schematic drawings and cross-sectional views of (a, b) ICA, (c, d) ACA and (e, f) NCA flip chip bonding. attach such as Chip-On-Glass (COG), Chip-On-Film (COF) for flat panel display modules [8–11], including liquid crystal display (LCD), plasma display panel (PDP) and organic light emitting diode display (OLED). As for the small and fine pitched bump of driver ICs to be packaged, fine pitch capability of ACF interconnection is much more desired for COG, COF and even OLB assemblies. There have been advances in development works for improved material systems and design rules for ACF materials to meet fine pitch capability and better adhesion characteristics of ACF interconnection for flat panel displays. Alternative resin based interconnection materials such as anisotropic conductive pastes (ACPs) and nonconductive films/pastes (NCFs/Ps) have been developed and introduced due to their advantages in terms of process, cost and ultra-fine pitch capability where a conventional ACF has limitations. It is obvious that electrically conductive adhesive materials are required for advanced packaging materials, but formulation, material design and process should be optimized and developed for high electrical, mechanical and thermal performance as well as enhanced reliability performance. In this paper, an overview on recent issues, developments and applications of conductive adhesives for electronic packaging applications with fine pitch capability, high electrical, mechanical, and reliability performance, and wafer level flip chip package applications is presented. 2. Isotropic Conductive Adhesives (ICAs) for Electronic Packaging ICAs are being used to replace the traditional eutectic SnPb solder alloys in electronic packaging and interconnects. They are composites of polymer resins and Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1596 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 3. Schematic structures of (a) surface mount interconnection using ICA and (b) flip chip interconnection using ICA. conductive fillers. The polymer resins, thermoplastic or thermosetting resins, are generally cured at high temperature and provide the shrinkage force, adhesion strength, and chemical and corrosion resistances. Epoxy, cyanate ester, silicone, polyurethane are thermosetting resins, and phenolic epoxy, polyimide are common thermoplastics for an ICA matrix resin. Conductive fillers include silver (Ag), gold (Au), nickel (Ni), copper (Cu) and Sn, SnBi or SnIn coated Cu in various sizes and shapes. Ag is the most common conductive filler for an ICA due to its high conductivity and easy processing, but its high cost is one of drawbacks for wide use of Ag-filled ICAs. ICAs have been used for die attach adhesives [12, 13], adhesives for SMT [14, 15], and flip chip [16] and other applications. Figure 3 shows the schematics of SMT components and flip chip devices interconnected by ICAs instead of solder alloy. 2.1. ICAs for Surface Mount Technologies Surface-mount technology (SMT) is the main technique for interconnecting chip components to substrate by packing and placing the components on the printed circuit board and using the reflow furnace to melt the solder alloy for the electronic system interconnection. Tin–lead (Sn–Pb) solder has been exclusively used as the interconnection material in surface-mount technology, because current commercial ECAs, in spite of their numerous advantages, cannot be used as drop-in replacements for solder in all applications due to some challenging issues. Due to the extreme toxicity of lead and legislations for lead-free electronics, world-wide efforts have been put in the study of ICAs. Significant progress has been made to address different materials properties and reliability issues for the development of high performance ICAs as a potential replacement for lead-containing solders in SMT application as well. M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1597 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 2.2. ICAs for Flip Chip Interconnects Isotropic conductive adhesive materials use much higher loading than ACAs to give electrical conduction isotropically or in all directions throughout the material. In order for these materials to be used for flip chip applications, it is necessary to apply them selectively onto those areas which are to be electrically interconnected, and to ensure that spreading of the materials does not occur during placement or curing which would cause electrical shorts between the separate pathways. ICAs are generally supplied in paste form. To precisely deposit the ICA paste, screen or stencil printing is most commonly used. However, to do this to the scale and accuracy required for flip chip bonding would require very accurate pattern alignment. To overcome this requirement, the transfer method may be used. For this technique, raised studs or pillars are required on either the die or the substrate. The ICA is then selectively transferred to the raised area by contacting the face of the die or the substrate to a flat thin film of the ICA paste. This thin film may be produced by screen printing and the transfer thickness may be controlled by controlling the printed film thickness. This method confines the paste to the area of the contact surfaces and the quantity may be adequately controlled so as to prevent spreading between pathways when the die is placed. Pressure during bonding is not required in this technique, which gives the option of oven curing the assembly. In a high volume environment, the high precision screen printing techniques to print the ICA paste directly onto the I/O pads of the substrate can be used. This would remove the requirement for stud pillars on the substrate track terminations and also quite possibly the need for bumping of the flip chip pads. Once such a process is in place, the ICA technique could then compete with the ACA method on the basis of speed and ease of processing, however, substantial improvements in bond strength will need to be made before the technique can be realistically considered. Unlike ACA flip chip bonding, however, a separate underfilling step would be required with ICA flip chip bonding to improve long-term reliability of the bond. It is shown that reliability is quite good with ICA flip chip joining on rigid substrates [17]. The difficulties with the ICA flip chip joining technology are the poor processibility and small process window in handling of the flip chip module directly after assembly. Although there are many technical advantages of ICAs compared with traditional solder materials, current ICAs still have some limitations on the electrical, thermal, and reliability properties compared with SnPb solders for full replacement for solder. Table 1 shows a general comparison of various properties between SnPb solders and conventional ICAs [18]. Therefore, much research effort has been focused on the improvement of electrical conductivity of ICAs and reliability enhancement of ICA joints, electrically and mechanically. Also the replacement of expensive Ag flakes by new metal flakes is required for wide use of ICAs instead of solder materials. Copper can be a conductive filler metal due to its low resistivity, low cost and improved electromigration performance, but oxidation causes this metal to lose its conductivity [19]. 1598 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 2.3. Electrical Conductivity Improvement of ICAs Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 To enhance the electrical conductivity of metal-filled ICAs, polymer-metal composite properties are controlled and maximized. Typically, increasing cure shrinkage of matrix polymer binder [20], the intimate metallic contacts by removal of lubricant layer on Ag flakes [21], and oxidation layer removal [22], metallurgical bonding between the conductive particles by low melting point alloy coating on Cu powder [23, 24] are representative methods for improvement of ICA conductivity. Recently, nano-sized Ag particles are added as conductive fillers instead of highly loaded micro-sized Ag flakes and the electrical conductivity is enhanced by sintering nano-sized Ag fillers [25]. 2.3.1. Increase of Polymer Matrix Shrinkage In general, ICA pastes exhibit insulative property before cure, but the conductivity increases dramatically after curing. ICAs achieve electrical conductivity during the polymer curing process caused by the shrinkage of polymer binder. Accordingly, ICAs with high cure shrinkage generally exhibit higher conductivity. Table 2 shows the relationship between shrinkage and conductivity for three different cross-link density ECAs, ECA1, ECA2 and ECA3 [26]. With increasing cross-link density of ECAs, the shrinkage of the polymer matrix increased, and, consequently, an obviously decreased resistivity of ECAs was observed. Therefore, increasing the cure shrinkage of the polymer binder could improve electrical conductivity. For epoxy-based ICAs, a small amount of a multi-functional epoxy resin can be added Table 1. Comparison between a Conductive Adhesive and Eutectic Solders [18] Characteristic SnPb solder ICA Volume resistivity ( cm) Typical junction resistance (m) Thermal conductivity (W/mK) Shear strength (psi) Min. processing temperature (◦ C) Environmental impact 0.000015 10–15 30 15.2 MPa 215 Negative 0.00035 <25 3.5 13.8 MPa 150–170 Very minor Table 2. Relationship of shrinkage and electrical conductivity of ECAs [27] Formulation Crosslink density (10−3 mol/cm3 ) Shrinkage (%) Bulk resistivity (10−3  cm) ECA1 ECA2 ECA3 4.50 5.33 5.85 2.98 3.75 4.33 3.0 1.2 0.58 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1599 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 into the ICA formulation to increase cross-link density, shrinkage, and thus increase electrical conductivity. 2.3.2. In Situ Removal of Lubricant on Ag Flakes An ICA is generally composed of a polymer binder and Ag flakes. There is a thin layer of organic lubricant on the Ag flake surface. This lubricant layer plays an important role for the performance of ICAs, including the dispersion of the Ag flakes in the adhesives and the rheology of the adhesive formulations [21, 28–30]. This organic lubricant layer, typically a fatty acid such as stearic acid, forms a silver salt complex between the Ag surface and the lubricant [21]. However, this lubricant layer affects conductivity of an ICA because it is electrically insulating. To improve conductivity, the organic lubricant layer should be partially or fully removed or replaced during the curing of ICA. A suitable lubricant remover is a short chain dicarboxylic acid because of the strong affinity of carboxylic functional group (–COOH) with silver and stronger acidity of such short chain dicarboxylic acids. With the addition of only a small amount of short chain dicarboxylic acid, the conductivity of an ICA can be improved significantly due to the easier electronic tunneling/transport by the intimate flake–flake contacts in the Ag flake networks [25, 31]. 2.3.3. Incorporation of Reducing Agents Silver flakes are by far the most used fillers for conductive adhesives due to the high conductivity of silver oxide compared to other metal oxides, most of which are insulative. However, the conductivity of silver oxide is still inferior to metal itself. Therefore, incorporation of reducing agents would further improve the electrical conductivity of ICAs. Aldehydes were introduced into a typical ICA formulation and obviously improved conductivity was achieved due to reaction between aldehyde and silver oxide that exists on the surface of metal fillers in ECAs during the curing process: R–CHO + Ag2 O → R–COOH + 2Ag. (1) The oxidation product of aldehydes, carboxylic acids, which are stronger acids and have shorter molecular length than stearic acid, can also partially replace or remove the stearic acid on Ag flakes and contribute to the improved electrical conductivity [22]. 2.3.4. Low-Temperature Transient Liquid Phase Fillers Another approach for improving electrical conductivity is to incorporate transient liquid-phase metallic fillers in ICA formulations. The filler used is a mixture of a high-melting-point metal powder (such as Cu) and a low-melting-point alloy powder (such as Sn–Pb or Sn–In). The low-melting-alloy filler melts when its melting point is reached during the cure of the polymer matrix. The liquid phase dissolves the high melting point particles. The liquid exists only for a short period of time and then forms an alloy and solidifies. The electrical conduction is established through Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1600 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 4. Schematic of an ECA joint with metallurgical connections in conductive filler network by transient liquid phase sintering. a plurality of metallurgical connections in situ formed from these two powders in the polymer binder (Fig. 4). The polymer binder with an acid functional ingredient fluxes both the metal powder and the metals to be joined and facilitates the transient liquid bonding of the powders to form a stable metallurgical network for electrical conduction, and also forms an interpenetrating polymer network providing adhesion. High electrical conductivity can be achieved using this method [32, 33]. 2.3.5. Low-Temperature Sintering of Nano-silver Fillers Recently, nano-sized conductive particles have been proposed as conductive fillers in ICAs for fine pitch interconnects. Although the nano-silver fillers in ICAs can reduce the percolation threshold, there has been concern that incorporation of nano-sized fillers may introduce more contact spots due to high surface area and consequently induce higher resistivity compared to micro-sized fillers. A recent study showed that nano-silver particles could exhibit sintering behavior at curing temperature of ICAs [34]. Typically, application of nano-fillers increases the contact resistance and reduces the electrical performance of the ICAs. The number of contacts between the small particles is larger than that between the large particles. The overall resistance of an isotropic conductive adhesive (ICA) formulation is the sum of the resistance of filler, the resistance between filler particles, and the resistance between filler and pads (equation (2)). In order to decrease the overall contact resistance, the reduction of the number of contact points between the particles may be obviously effective. If nano-particles are sintered together, then the number of contacts between filler particles will be fewer. This will lead to smaller contact resistance. By using effective surfactants on these nano-sized silver fillers for better filler dispersion in ECAs, obvious sintering behavior of the nano-fillers can be achieved. The sintering of nano-silver fillers improved the interfacial properties of conductive fillers and polymer matrices, and reduced the contact resistance between fillers. Therefore, an improved electrical conductivity of nano-silver-filled M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1601 ICAs can be achieved at a lower loading level than that of micro-filler-ICAs with a filler loading of 80 wt% or higher: Rtotal = Rbtw fillers + Rfiller to bond pad + Rfillers . (2) Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 2.4. Reliability Enhancements of ICA Interconnects Critical reliability concerns of ICA joints in electronic packaging applications are mainly due to unstable contact resistance between ICA and metal finished components under environmental attacks, such as humidity and temperature cycling/aging. For high temperature and humidity aging environment, the galvanic corrosion rather than simple thermal oxidation at the interface between metallic fillers in ICA and non-noble metal finish is known as the most detrimental underlying mechanism for unstable contact resistance [35]. Therefore, most research works for improving the stability of electrical conductivity of ICA joints have focused on the methods to avoid or minimize the unstable contact resistance mechanism of ICA joints. Several possible methods are: development of polymer matrix resin with low moisture absorption [36], use of oxygen scavengers [35] and corrosion inhibitors [36] in the ICA formulation, the corrosion control by adding metal fillers with low corrosion potential, sacrificial anode [37], and oxide-penetrating particles in the ICA formulation [38]. Also, for the reliability improvement of Ag-based ICA joints, Ag migration is most serious concern. Several methods are proposed to reduce Ag migration and improve the reliability of ICA joints such as Ag alloying with an anodically stable metal [39], hydrophobic polymer coating over the PWB [40], surface coating of tin, nickel, gold or organic compounds on silver particles. 2.4.1. ICA With Low Moisture Absorption Moisture in polymer composites has been known to have an adverse effect on both mechanical and electrical properties of epoxy laminates [41, 42]. Effects of moisture absorption on conductive adhesive joints include degradation of bulk mechanical strength; decrease of interfacial adhesion strength causing delamination; promoting the growth of voids present in the joints, giving rise to swelling stress in the joints; and inducing the formation of metal oxide layers resulted from corrosion. The water condensed from the adsorbed moisture at the interface between an ECA and metal surface forms the electrolyte solution required for galvanic corrosion. Therefore, one way to prevent galvanic corrosion at the interface between an ICA and the non-noble metal surface and achieve high reliability is to select ICAs with lower moisture absorption. ICAs with a low moisture absorption generally exhibit more stable contact resistance on non-noble metal surfaces compared with those with high moisture absorption [36]. 2.4.2. ICA With Oxygen Scavengers Since oxygen accelerates galvanic corrosion, oxygen scavengers could be added into ECAs to slow down the corrosion rate [35]. When ambient oxygen molecules diffuse through the polymer binder, they react with the oxygen scavenger and are consumed. The main mechanism for oxygen scavengers to inhibit the corrosion Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1602 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 5. Shifts of contact resistance of conductive adhesives on Sn/Pb surface with and without oxygen scavengers. is the cathodic mechanism which is based on the lowering of oxygen concentration. Therefore, the reactivity of an oxygen scavenger with oxygen is an important consideration. Some commonly used oxygen scavengers include sulfates such as sodium sulfate (Na2 SO4 ), hydrazine (H2 N–NH2 ), carbohydrazide (H2 N–NH– CO–NH–NH2 ), diethylhydroxylamine ((C2 H5 )2 N–OH), and hydroquinone (HO– C6 H4 –OH) [43–46]. Figure 5 shows the effect of oxygen scavengers on the contact resistance between an ICA and a Sn/Pb surface. The application of oxygen scavengers reduces the contact resistance increase obviously, especially in the first 200 h test time. However, with continuing aging test when the oxygen scavenger within the ECA is depleted, oxygen can again diffuse into the interface and accelerate the corrosion process. Therefore, oxygen scavengers can only delay the galvanic corrosion process, but do not solve the corrosion problem completely. 2.4.3. ICA With Corrosion Inhibitors Another method of preventing galvanic corrosion and stabilizing contact resistance is the use of corrosion inhibitors in ICA formulations [35, 36, 47, 48]. In general, organic corrosion inhibitors are chemicals that adsorb on metal surfaces and act as a passivation barrier layer between the metal and the environment by forming an inert film over the metal surfaces [49–52]. Thus, the metal finishes can be protected. Some chelating compounds are especially effective in preventing metal corrosion [51]. Appropriate selection of corrosion inhibitors can be very effective in protecting the metal finishes from corrosion. However, the effectiveness of the corrosion inhibitors is highly dependent on the types of contact surfaces. Effective corrosion inhibitors have been discovered for Sn/Pb, Cu, Al and Sn surfaces [35, 47, 53]. 2.4.4. ICA With Sacrificial Anode To improve the contact resistance stability, applying a sacrificial anode is another efficient method. For galvanic corrosion of ECAs during aging, the larger the dif- Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1603 ference in electrochemical potentials, the faster the corrosion develops. Also, the self-corrosion rates of both metals are different: the comparably active metal (the anode) corrodes faster while the other (the cathode) corrodes slower. Generally, metals with a low potential tend to corrode faster and show increased contact resistance than those with a high potential value. Therefore, when applying sacrificial materials with lower electrochemical potential than those of electrode-metal pads into ECAs, the sacrificial materials are preferentially corroded first and, thus, can protect the metal finishes [37]. This corrosion control is very important in reliability issues of the conductive adhesive joints. The addition of individual metals with low corrosion potential, metal mixtures or metal alloys greatly reduces the electrode potential of ECAs, or, in other words, narrows down the potential gap between the ECA and the metal finishes. Thus, these sacrificial anode materials act as an anode in this configuration and they are corroded first instead of the metal finishes, resulting in protecting the surfaces at the cathode [37, 54, 55]. 2.4.5. ICA With Oxide-Penetrating Particles Another approach for improving contact resistance stability during aging is to incorporate some electrically conductive particles, which have sharp edges, into the ICA formulations. Such particle is called oxide-penetrating filler. Force must be provided to drive the oxide-penetrating particles through the oxide layer and hold them against the adherend materials. This can be accomplished by employing polymer binders that show high shrinkage when cured as shown in Fig. 6 [38]. This concept is used in polymer-solder which has good contact resistance stability with standard surface-mounted devices (SMDs) on both solder-coated and bare circuit boards. 2.5. Adhesion Strength and Mechanical Reliability Enhancement of ICA Interconnects Another critical reliability issue regarding conductive adhesives is their low adhesion strength. High adhesion strength is a critical parameter in fine pitch interconnection that is fragile to shocks encountered during assembly, handling and lifetime. Figure 6. A joint connected with an ICA containing oxide-penetrating particles and silver particles. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1604 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 There are two types of adhesion mechanisms, physical bonding and chemical bonding, which contribute to the overall adhesion strength of a polymer on a surface [56]. Chemical bonding involves the formation of covalent or ionic bonds to link the polymer and the substrate [56]. In other words, a chemical reaction must take place for the formation of chemical bonds. Physical bonding involves mechanical interlocking or physical adsorption between the polymer and surface of substrate. In cases where the molecules of the polymer are highly compatible with the molecules of the substrate, they interact to form an inter-diffusion layer. In mechanical interlocking, polymer and substrate interact on a more macroscopic level, where the polymer flows into the crevices and the pores of substrate surface to establish adhesion [56]. Therefore, a polymer is expected to have better adhesion on a rougher surface because there is more surface area and “anchors” to allow for interlocking between the polymer and the substrate. Under thermal cycling (TC) environment, the failure mechanisms of unstable ICA performance are generally the thermal stress in the ICA joint and the interfacial delamination due to the adhesion degradation. The TC performance of ICA joints can be improved by reducing the thermal stress by incorporating flexible molecules in the epoxy resin [57]. 2.5.1. Adhesion Improvement by Coupling Agents in ICAs The most useful approach to improve the adhesion of ICA joints is by using coupling agents [58]. Coupling agents are organofunctional compounds based on silicon, titanium, or zirconium. Their general structure is R–X–(O–R′ )3 , where X = Si, Ti or Zr, R = organic group that interacts with the polymer, and R′ = a hydrolysable methoxy or ethoxy group that interacts with the substrate. A coupling agent consists of two parts and acts as intermediary to “couple” the inorganic substrate and polymer. Silane coupling agents have been commonly used to improve the adhesion performance [59]. For example, chemically etched 304 stainless steel can react with γ -aminopropyltrimethoxysilane. The methoxy groups of the silane coupling agent can first hydrolyze to hydroxyl groups which are quite reactive and can react with the metal surface hydroxyl groups, forming an M–O–Si chemical bond. The other organo-reactive group (e.g., amine end of the coupling agent) can react with poly(amic acid), a precursor of the polyimide polymer, to form a strong polymer-metal interfacial bond. As such, it produces mechanically stronger polyimide/stainless steel interfaces [60]. Other approaches use the formation of a thick metal oxide layer prior to application of the silane coupling agent to improve the adhesion to organic films [61]. In conductive adhesives, application of specific silane coupling agents with appropriate concentration can increase the adhesion strength on different metal surfaces [58]. As an example of effect of coupling agents on adhesion strength of ECAs, Fig. 7 shows the adhesion data of polyarylene ether derivative (PAE-2E) with a coupling agent (CA-4), and the obvious increase in adhesion strength on NiAu and Sn surfaces was observed. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1605 Figure 7. Adhesion data of PAE-2E with coupling agent (CA-4) [58]. Figure 8. The relationship between the adhesion strength and the elastic modulus of ECA [63]. Although silane coupling agents are mostly used, some other coupling agents with various functional groups, such as thiol, carboxylate coupling agents, are also used [60]. 2.5.2. Adhesion Improvement by Optimization of Elastic Modulus of the Adhesive In order to enhance the adhesion, another approach is to lower the elastic modulus of adhesive resins. By using low elastic modulus resins, the thermal stress at the adhesion interface can be reduced which should result in improved adhesion strength [61, 62]. Figure 8 shows the relationship between the adhesion strength against COF (Chip on flex) and the elastic modulus. The adhesion strength increases with lowering the elastic modulus. However, too low modulus value deteriorates the cohesive 1606 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 force and, thus, decreases the adhesion strength. Therefore, the elastic modulus needs to be optimized to improve the adhesion properties. In addition to the methods listed above, some other factors such as curing conditions and structures of IC packaging may also affect the adhesion strength of conductive adhesives. 2.5.3. Improving the Thermo-cycling Reliability of ICA Interconnects The poor thermal cycling (TC) performance of the ECA joints has been another reliability issue for board level interconnects. Generally, the failure of the electrical interconnection during the TC test can be caused by many factors such as coefficient of thermal expansion (CTE) mismatch between the IC component chip/the interconnection materials/the substrates, elastic moduli difference of these components, adhesion strength of the interconnect materials on the IC chip and the substrate, the mechanical properties of the IC chips, the glass transition or the softening point of the ECA materials, moisture uptake both at the interface and in the bulk ECAs, the surface or interface property change and so forth. Especially, the thermal stress in the ECA joints generated by a huge temperature difference during the TC and the interfacial delamination due to the adhesion degradation could be the critical reasons. In this aspect, a feasible solution to the TC failure problem is to introduce flexible molecules into the epoxy resin matrix. By releasing the thermal stress with the flexible molecules, the thermomechanical stresses can be dramatically reduced and the ECA/component joint interfaces can stay intact through the thermal cycling test [64]. 3. Anisotropic Conductive Adhesives (ACAs) for Electronic Packaging Anisotropic conductive adhesives (ACAs) or anisotropic conductive films (ACFs) provide uni-directional electrical conductivity only in the vertical or Z-axis. This directional conductivity is achieved by using a relatively low volume loading of conductive filler (5–20 volume percent) [65–67]. The low volume loading is insufficient for inter-particle contact and prevents conductivity in the X–Y plane of the adhesive. The ACA, in film or paste form, is interposed between the surfaces to be connected. Heat and pressure are simultaneously applied to this stack-up until the particles bridge the two conductor surfaces. Because of the anisotropy, an ACA/ACF can be deposited over the entire contact region, greatly facilitating materials application. Also, an ultra-fine pitch interconnection (<0.04 mm) can be achieved easily. The fine pitch capability of ACA/ACF would be limited by the particle size of the conductive filler, which can be a few micrometers or a few nanometers in diameter. 3.1. ACAs for Flat Panel Displays ACF materials are mostly wide used in connecting the tape-carrier packages (TCPs) with driver IC to the LCD glass panel and PCB boards, as well as other interconnection areas for flat panel display manufacturing. Figure 9 shows various packaging Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1607 Figure 9. Various packaing technologies using an ACF in LCD modules (a) TCP: Outer Lead Bonding (OLB) and PCB bonding, (b) COG bonding and (c) COF bonding. technologies using ACFs for LCD modules: TCP, COG and COF bonding. Since the interconnection input/output (I/O) pitch of driver IC electrode has been decreased and the number of output electrodes per IC increased for the high resolution LCD modules, ACF materials and packaging technologies have also been developed to meet high density interconnection capability. ACF bonding process is a thermo-compression bonding as shown in Fig. 10. In case of TCP bonding, the ACF material is attached on glass substrate after release film is removed and TCP with driver IC is pre-attached. Then final bonding is established by thermal cure of ACF resin, typically at 180◦ C, 20 s and 30 kg f/cm2 and conductive particle deformation between the electrodes of TCP and glass substrate by applied bonding pressure. The kind, size, and density of conductive filler, and adhesive resin system are different according to packaging technology for LCD module. When TCP is mounted using an ACF on LCD glass substrate, the CTE mismatch between TCP and the panel should be considered for thermal bonding, and this is more serious for finer pitch TCP bonding, i.e., below 50 µm. For flex to glass bonding below 50 µm pitch, 1608 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 Figure 10. Thermo-compression bonding using an ACF. COF using an ACF becomes more attractive due to several advantages like fine pitch capability, design flexibility and low CTE base material. ACFs are also used in attaching fine-pitched driver IC on COF substrates. The geometry of COF is very similar to that of TCP. However, the substrate is different, i.e., it is a two-layer structure, normally Cu and polyimide (PI) which is thinner, higher density, better flexible and more durable in high temperature than TCP with a three-layer structure (Cu, adhesive and PI). COF’s two-layer structure without adhesive layer normally has weak adhesion property with ACF materials. Therefore, there has been development in ACF adhesion improvement to two-layer COF substrate. In COG technology, the bare driver ICs are flip chip bonded on glass substrate using an ACF, and it is most advantageous technology for low cost and compact size LCD module production [68]. The CTE difference between driver IC and glass substrate is relatively small compared with that in TCP applications and it provides more reliable COG connections. 3.2. ACFs for Fine Pitch Interconnections As the function of driver IC for high-resolution LCD modules increases, the bump density on IC is also increased and this means that bump size and pitch are reduced. For fine pitch COG connection using an ACF, the number of conductive particles trapped between the bump and the substrate pad should be sufficient. Therefore, conductive particle density in the ACF for COG is much higher than that of ACF for TCP OLB. But due to high density of conductive particles, there is high possibility of electrical short between adjacent bumps, mainly due to electrical path formed by a chain of conductive particles accumulated after being flowed into the bump gap during COG bonding process. Therefore, a double-layer ACF, which is composed of an ACF layer and an NCF layer without conductive filler, was developed for high electrical conductivity between the bump and ITO electrode and electrical insulation between adjacent bumps [69]. As bump size and pitch of driver IC decreased more and more, insulating layer coated conductive particles were introduced instead of conventional conductive particles in the ACF layer, and non-conductive Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1609 Figure 11. The relationship between the short occurence frequency and the type of conductive particle [70]. fillers were incorporated together with conductive particles to ensure electrical insulation [70]. Figure 11 shows the relationship between the short occurence frequency and the type of conductive particle. The double-layer ACF with conventional conductive particle and insulating layer coated conductive particle, both in 4 µm diameter size and 35 000/mm2 density. Insulating layer coated conductive filler ACF is more advantageous than normal conductive particle ACF by reducing electrical shorts more effectively, and it achieves insulation capability at 10 µm gap level. In double-layer ACF structure, ACF and NCF layer thicknesses are 7 µm and 18 µm, respectively. The viscosity, formulation, thickness of adhesive layers, conductive filler density, type and hardness should be optimized for high performance COG package. COF, another fine pitch ACF bonding area, is a relatively new technology compared with COG and COB in the production of flat panel modules. LCD module production using COF technology is accelerated due to its advantages of fine-pitch interconnection, low contact resistance and pre-test capability compared with COG in the high-density, multi-functional LCD modules. In COF technologies, there are several alternatives for interconnect materials and processes, such as Au–Sn joining [71], stud bump bonding (SBB) joining [72], ACF joining [73], and NCF and NCP joining [74]. Among them, ACF joining method has been applied as the main bonding method similar to COG technology. As mentioned before, COF’s substrate is a two-layer structure without the adhesive layer, and, therefore, normally has weak adhesion property with ACF materials. It is necessary to improve the adhesion property between IC chip, ACF and twolayer flex substrate for ever-increasing reliability requirement of COF modules. In addition, fine-pitch interconnection is the basic requirement in COF using ACF for driver IC packaging. A triple-layered ACF has been developed, which has func- Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1610 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 12. (a) Cross-sectional view of triple-layered ACF obtained by SEM and (b) COF bonding process using a triple-layered ACF [75]. tional layers on both sides of conventional ACF layer to improve interface adhesion and control bonding property for fine pitch application during thermo-compression bonding as shown in Fig. 12, and the resulting reliability enhancement of COF module assembly [75]. 3.3. ACAs for Reliability Enhancement in Flip Chip Assembly Flip chip assembly on organic board using anisotropic conductive adhesives (ACAs) has received much attention due to many advantages such as simple and lead-free processing, low cost, fine pitch interconnection and low-temperature processing [76–78]. Especially highly improved electrical and thermal performance as well as high frequency characteristics are anticipated due to reduced interconnection distances. Most of all, flip chip using an anisotropic conductive adhesive should provide acceptable reliability level in harsh environment together with good processability. This requires the use of polymer materials which have CTE value close to the chip and the board, and strong adhesion for better reliability. For better mechanical properties without degradation of strong adhesion, non-conductive fillers are incorporated and optimized. As the content of filler increased, CTE values decreased M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1611 Table 3. The Tg, CTE and modulus of ACA composites [79] ACA composite Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 ACA 1 with 10 wt% filler ACA 2 with 30 wt% filler ACA 3 with 50 wt% filler TgTMA (◦ C) 87.6 93.5 98.8 CTE (ppm/◦ C) α1 α2 87.9 76.1 60.7 3960 3630 3920 Modulus (GPa@25◦ C) 5.3 3.2 2.5 and storage moduli increased, but the DSC behavior did not change. Table 3 summarizes the material properties of ACA composites with different filler contents showing lower CTE and higher modulus as filler content increases [79]. For the test IC chip and substrate, gold stud bumps were formed on each I/O pad of test chips and 1 mm-thick FR-4 substrates were prepared. Flip chip assembly was performed by bonding the chip on the substrate with an appropriate bonding pressure of 50 kg f/cm2 at 180◦ C for 30 s. The chip was electrically connected to the substrate via the contacts between compressed gold stud bumps and conductive fillers in the ACA. Non-conductive fillers with smaller size than conductive fillers do not contribute the electrical contacts and affect other properties such as Young’s modulus and CTE because the electrical path is formed by the conductive fillers larger than non-conductive fillers which are dispersed in the polymer resin. Reliability tests in terms of temperature cycling, high humidity and temperature, and high temperature and dry condition tests were performed by measurement of contact resistance variation. Figure 13 shows that flip chip assembly using modified ACA composites with lower CTEs and higher modulus by loading non-conducting fillers exhibits more stable contact resistance behavior than conventional ACAs without non-conducting fillers during the temperature cycling test. An ACF with a lower CTE and higher modulus can reduce the thermally induced shear strain in the ACF layer as measured by Moiré interferometry during thermal cycling environment as shown in Fig. 14, and thus can increase the overall thermal cycling lifetime of ACF joints [79]. There are still more demanding requirements for reliability enhancements such as low moisture absorption, high temperature electrical stability during reflow process, low degree of process-induced voids, high cure density, and stability of interface adhesion strength in an ACA [80–85]. 3.4. ACAs for High Frequency Interconnections Recently, high frequency modeling and characterization for ACA flip chip interconnects have been performed to understand high frequency characteristic of flip chip interconnect using ACFs, and thereby design better ACF and bump materials for enhanced high frequency performance of the ACF joint [86–88]. The effect of low dielectric filler incorporation on high frequency behavior of ACFs was investigated. The extracted impedance model parameters for a 100 × Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1612 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 13. Contact resistance of flip chip interconnects using ACAs with different filler contents during thermal cycling test from −60◦ C to 150◦ C for 700 cycles [79]. Figure 14. Thermal shear strain of ACF layers between the chip and FR-4 substrate of flip chip assembly [79]. 100 µm bonding pad are presented in Fig. 15 for a conventional ACF with conductive particle only, and ACF with conductive particle and SiO2 filler. In an ACF flip chip interconnect at high frequency, interconnection capacitance formed between CPW of PCB and the test chip pad is relatively high due to the high dielectric constant of the ACF resin and the large area, small gap of the parallel metal line structure in the test vehicle, compared with the solder ball flip chip structure. Therefore, the resonance frequency of the ACF flip chip interconnect is lower than that of the solder ball flip chip interconnect. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1613 Figure 15. Impedance parameter, resistance (R) of flip chip interconnect using electroless Ni/Au bumped chip and two different ACFs in high frequency range [89]. Both ACFs show resonance frequencies, around 13 GHz for the conventional ACF and 15 GHz for the SiO2 containing ACF. This resonance phenomenon is dominantly affected by the inductance of conductive particles and capacitance of polymer matrix. In particular, capacitance of polymer matrix is induced by the proximity effect between chip and substrate. Interestingly, the ACF with SiO2 has resonance frequency slightly higher than the conventional ACF. ACF containing SiO2 filler exhibited the resonance phenomenon around 15 GHz. This difference originated from dielectric constant change of polymer matrix. By adding SiO2 filler into the ACF formulation, dielectric constant of polymer matrix in the ACF was lowered and the ACF resonance frequency was shifted to higher frequency [89]. The effect of bump metallurgy on high frequency behavior of ACF interconnects was also investigated. Figure 16 shows the impedance parameter of Au stud bumped chip packaged by ACF method, and compared with Ni/Au bumped chip. As shown in Fig. 16, Au stud bumped chip does not exhibit resonance phenomenon up to 20 GHz. This means that Au stud bumps maintain a constant impedance in the high frequency range up to 20 GHz. Capacitive coupling of the Au-stud bump interconnect between the chip and substrate is relatively low due to the different bonding structure such as large gap of epoxy resin and the small area in the parallel pad structure, compared to the ACF flip chip using Ni/Au bumped chip. Consequently, the resonance frequency of the Au stud bump interconnects using an ACF is higher than that of ACF flip chip interconnect using electroless Ni/Au bump, and it was not observed up to 20 GHz. High frequency performances of several flip chip interconnects using ACFs at RF and high frequency range were demonstrated and ACF flip chip assembly was shown as a simple and cost-effective method for high frequency devices [89]. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1614 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 16. Impedance parameter, resistance (R) of flip chip interconnect using ACF without silica and two different chips, electroless Ni/Au bumped and Au stud bumped chips in high frequency range [89]. 3.5. ACAs With Improved Thermal Properties for High Current Density Interconnections As the current density for the ACA flip chip assembly is increasing for high current and high power dissipation device applications, the current carrying capability of the ACA is one of the important properties which have been characterized [90]. An ACA, normally a thermally poor conductor, is required to be a thermal transfer medium which allows the board to act as new heat sink for the flip chip package and improve the lifetime of ACA flip chip joint under high current density application. The effect of thermal conductivity of ACA on the current carrying capability of flip chip joints was investigated [91]. Figure 17 shows comparison results of I–V characteristics for two different ACA materials: one is a conventional ACA without any thermal filler and the other is the thermally conductive ACA with 100 phr SiC filler. The ACA flip chip joint was bias-stressed at a pair of ACA joints with Au stud bumps and the I–V curves were plotted. The conventional ACA flip chip joint shows the typical I–V curve with maximum allowable current level of 4.53 A. In contrast, the flip chip joint using a thermally conductive ACA shows almost linear increase of current with increase of voltage and the maximum allowable current level is 6.71 A. Therefore, the current carrying capability of the ACA flip chip joint was improved by using thermally conductive ACA material. Figure 18 shows the resistance changes of flip chip joints using a conventional ACA and a thermally conductive ACA as a function of time under constant current of 4.1 A. The contact resistance of the conventional ACA flip chip joint increased abruptly as time passed 50 h and had open circuit before 100 h. But the thermally conductive ACA flip chip joint showed stable contact resistance behavior without any open circuit. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1615 Figure 17. I–V test (bias stressing) results at flip chip joints with Au stud bumps for a conventional ACA and a thermally conductive ACA [90]. Figure 18. Contact resistance changes of flip chip joints with Au stud bump using a conventional ACA and a thermally conductive ACA after 20, 40, 60 and 100 h under current stressing [90]. The failure or degradation mechanism of ACA flip chip joints under current biasing test is suggested as follows; (1) Au–Al intermetallic compounds (IMCs) formation, (2) Crack formation and propagation along the Au/IMC interface, and (3) Al or Au depletion due to electromigration [92]. All these causes of electrical degradation of ACA flip chip joints are due to the heat accumulation at the Au stud bumps/PCB pads and thermal degradation of the adhesive due to Joule heating under high current bias. Similar discussion on the heat induced failure mechanism of flip chip joints using isotropic conductive adhesives (ICAs) under high current density was presented [93]. If the local temperature of the flip chip joint by ACA/Au stud bump is relatively low due to effective heat dissipation throughout the thermally conductive ACA, the Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1616 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 19. Chip surface temperature of flip chip assemblies using conventional and thermally conductive ACAs as a function of time under high current application condition [90]. thermal degradation process due to local Joule heating and the electrical degradation of the ACA flip chip joint are slowed down, and electrical stability is obtained. This is verified by the behavior of junction temperature on the surface of flip chip IC assemblies under current stressing condition as a function of time in Fig. 19. The chip surface temperature increases sharply and becomes stable at around 50 s of high current application time. The chip surface using a thermally conductive ACA became hot faster than the conventional ACA joint, which means that the thermally conductive adhesive dissipates the heat from the source more easily than the conventional ACA. The maximum temperature of the chip surface of the flip chip joint using a thermally conductive ACA is lower than that of conventional ACA under constant current stressing. Therefore, the electrical reliability of a flip chip joint under high current bias condition can be improved by dissipating the heat from the hot spot and keeping the chip temperature as cool as possible. 3.6. ACAs With Improved Electrical Properties As the conductivity of ACA joints is directly determined by the mechanical contact between the terminals of chips and the electrodes on chip carriers, the bonding force plays a critical role in the electrical performance. High bonding pressure is certainly favorable to an intimate contact, and thus to a low contact resistance. In addition, all ACA flip chip joints should have uniform electrical conductivity, requiring the bonding situation of every joint to be completely the same because the bonding of all the bumps of a chip is performed simultaneously. 3.6.1. Enhancement by Organic Self-Assembly Monolayers (SAMs) In order to enhance the electrical performance of ACA materials, organic monolayers have been introduced at the interface between metal filler and metal-finished bond pad of ACAs [94, 95]. These organic molecules adhere to the metal surface M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1617 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 Table 4. Potential organic monolayer interfacial modifiers for different metal finishes Formula Compounds Metal finish H–S–R–S–H* N≡C–R–C≡N O=C=N–R–N=C=O Dithiols Dicyanides Diisocyanates Au, Ag, Sn, Zn Cu, Ni, Au Pt, Pd, Rh, Ru Dicarboxylates Fe, Co, Ni, Al, Ag Imidazole and derivatives Organosilicone derivatives Cu SiO2 , Al2 O3 , quartz, glass, mica, ZnSe, GeO2 , Au R–SiOH* * R denotes alky or aromatic groups. and form physi-chemical bonds, which allow electrons to flow, and thus, it reduces electrical resistance and enables a high current flow. The unique electrical properties are due to their tuning of metal work functions by these organic monolayers. The metal surfaces can be chemically modified by the organic monoalyers and reduced work functions can be achieved by using suitable organic monolayer coatings. An important consideration when examining the advantages of organic monolayers pertains to the affinity of organic compounds to specific metal surfaces. Table 4 gives the examples of molecules preferred for maximum interactions with specific metal finishes; although only molecules with symmetrical functionalities for both head and tail groups are shown, molecules and derivatives with different head and tail functional groups can also be used for interface modification of different metal surfaces. Dicarboxylic acids and dithiols have been introduced into ACA joints for silver-filled and gold-filled ACAs, respectively. For dithiol containing ACAs with micro-sized gold fillers, significantly lower joint resistance and higher maximum allowable current (highest current applied without inducing joint failure) were achieved for low-temperature curable ACAs (<100◦ C). For high curing temperature ACAs (150◦ C), the improvement is not as significant as for low curing temperature ACAs, due to the partial degradation of organic monolayer coating at the relatively high temperature. However, when dicarboxylic acid or dithiol was introduced into the interface of nano-silver filled ACAs, significantly improved electrical properties could be achieved for high temperature curable ACAs, suggesting the coated organic monolayers did not suffer degradation on silver nanoparticles at the curing temperature (150◦ C), (Fig. 20). The enhanced bonding could be attributed to the larger surface area and higher surface energy of nano-particles, which enabled Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1618 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 20. Joint resistance of nano-Ag filled ACA with dithol or dicarboxylic acid (acid M) [95]. the monolayers to be more readily coated and be relatively thermally stable on the metal surfaces [95]. 3.6.2. Low-Temperature Sintering of Nano-Silver (Ag) filled ACA One of the concerns for an ACA/ACF is the higher joint resistance since interconnection using an ACA/ACF relies on mechanical contact, unlike the metal bonding in soldering. An approach to minimize the joint resistance of an ACA/ACF is to make the conductive fillers fuse with each other and form metallic joints such as metal solder joints. However, to fuse metal fillers in polymers does not appear feasible, since a typical organic printed circuit board (Tg ∼ 125◦ C), on which the metal filled polymer is applied, cannot withstand such a high temperature; the melting temperature (Tm) of Ag, for example, is around 960◦ C. Research showed that the Tm and sintering temperature of materials could be dramatically reduced by decreasing the particle diameter size of the materials [96, 97]. It has been reported that the surface pre-melting and sintering processes are a primary mechanism of the Tm depression of the fine nano-particles (<100 nm). For nano-sized particles, sintering could occur at much lower temperatures, and, thus, the use of fine metal particles in ACAs would be promising for fabricating high electrical performance ACA joints through eliminating the interface between metal fillers. The application of nanosized particles can also increase the number of conductive fillers on each bond pad and result in more contact area between the fillers and bond pads. Figure 21 shows the SEM micrographs of nano-Ag particles annealed at various temperatures. Although very fine particles (20 nm) were observed for as synthesized (in Fig. 21a) and 100◦ C treated particles (in Fig. 21b), dramatically larger particles were observed after heat treatment at 150◦ C and above. With increasing temperature, the particles became larger and appeared as a solid matter rather than as porous parti- Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1619 Figure 21. SEM micrographs of 20 nm-sized Ag particles annealed at different temperatures for 30 min: (a) room temperature (no annealing); (b) annealed at 100◦ C; (c) annealed at 150◦ C and (d) annealed at 200◦ C, [94]. cles or agglomerates. The particles shown in Fig. 21c–d were fused through their surface and many dumbbell type particles could be found. The morphology was similar to a typical morphology of the initial stage in the typical sintering process of ceramic, metal and polymer powders. This low-temperature sintering behavior of the nano-particles is attributed to the extremely high inter-diffusivity of the nanoparticle surface atoms, due to the significantly energetically unstable surface status of the nano-sized particles with large proportion of the surface area to the particle volume. For the sintering reaction in any material system, temperature and duration are the most important parameters, in particular, the sintering temperature. Current– resistance (I –R) relationship of the nano-Ag filled ACAs is shown in Fig. 22. As can be seen from the figure, with increasing curing temperature, the resistance of the ACA joints decreased significantly, from 10−3  to 5 × 10−5 . Also, higher curing temperature ACA samples exhibited higher current carrying capability than the low-temperature samples. This phenomenon suggested a higher degree of sintering of nano-Ag particles and consequently superior interfacial properties between nano-Ag filler particles and metal surfaces of the bond pads were achieved at higher temperatures [98], yet the x–y direction of the ACF maintained an excellent electrical insulation. Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 1620 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 22. Current–resistance (I –R) relationship of nano-Ag-filled ACAs with different curing temperatures [98]. 3.7. ACAs for Wafer Level Packaging Applications Flip chip technology is extending its applications to the fields of smart cards, displays, computers, mobile phones, communication systems, etc. However, the flip chip technology has a drawback that the production efficiency is poor in terms of process complexity and product cost because it requires conventional solder-using complex connection processes, i.e., solder flux coating, chip/board arranging, solder bump reflowing, flux removing, underfilling and cure process. In order to reduce these complex processes, particular attention has recently been paid to wafer-level packaging technology in which wafers are coated with polymeric materials having flux and underfill functions [99, 100]. More recently, in developing new, improved flip chip connection technology, advantage has been taken of conductive adhesives, which are of lower price than solders and enable the formation of ultra-fine pitches with the potential to realize environmentally friendly, fluxless and low-temperature processes. In spite of extensive research activity, flip chip technology using these environmentally friendly ACFs or ACAs as connecting materials suffers from the disadvantage of being inefficient in production costs requiring many processes, including chip design and bump formation for ACA flip chip packaging, mass production of connecting materials, and automation of connecting processes. Therefore, wafer level flip chip package using ACAs was developed to provide advantages in terms of production cost by simplifying the processes steps compared to the fabrication of conventional solder bump flip chip packages [101, 102]. Fabrication of wafer-level flip chip package using ACAs is comprised of forming a low priced non-solder bump on an I/O pad of each chip of a wafer, coating the ACA over the wafer, dicing the ACA coated wafer into individual chips using Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1621 Figure 23. (a) Schematic views showing a wafer with non-solder bump formation and cross-sectional view along the line A–A′ , (b) schematic views showing a wafer after ACA deposition on the wafer with non-solder bump and its cross-sectional view and (c) schematic views showing a dicing process and cross section of diced chip with ACA deposition [101]. a wafer dicing machine, and subjecting the individual chips to flip chip bonding as shown in Fig. 23. The application of ACA to the wafer with non-solder bumps can be achieved by a spraying, a doctor blade, or a meniscus coating process using ACP solution, and lamination of ACF. The film was laminated on the wafer to a thickness of 20–50 µm. In wafer dicing process, the wafer with pre-applied ACA is mounted on a wafer dicing machine to check the scribe line of the wafer through pre-applied ACA layer, after which the wafer is diced into individual chips. In this regard, the ACA layer is required to be transparent and to have such high adhesion so as not to exhibit delamination during the process. After removing the protective layer from the diced chips, it is heat pressed against a circuit board so that the individual chips are electrically connected via the conductive particles of the ACA onto the substrate pads. Wafer level flip chip package using ACAs is economically favorable owing to its simplicity and environmentally friendly process. 1622 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 4. Non-conductive Adhesives (NCAs) for Electronic Packaging Electrically conductive adhesive joints can be formed using non-filled organic adhesives, i.e. without any conductive filler particles. The electrical connection with an NCA is achieved by sealing the two contact partners under pressure and heat. Thus, the small gap contact is created by approaching the two surfaces to the distance of the surface asperities. The formation of contact spots depends on the surface roughness of the contact partners. Under small bonding pressure, the two surfaces enable only a small number of contact spots to form which allow the electric current to flow. When the parts are pressed together under large bonding pressure during the sealing process, the number and area of the single contact spots are increased according to the macroscopic elasticity or flexibility of the parts and the microhardness and plasticity of the surfaces, respectively. 4.1. NCAs for Improved Electrical Properties Since the electrical conductivity of an NCF is achieved through physical/mechanical contact and no metallurgical joints are formed, it has limited electrical conductance and current carrying capability. Low contact resistance and high current carrying capability of NCF joints are demanding properties for lead-free solder alternatives and high current density application. To ensure low contact resistance and high current density, the interface between electrodes plays an important role. The interfaces between electrodes for NCF joints must be defect free and occupy a stable contact area even under high electrical current and harsh environments. This interface control in NCF joints contributes to their performance and reliability in electronic packaging. In most NCF joints, electrical conductance between contacts depends on the constriction resistance and tunneling resistance due to the presence of ultra-thin insulating film between contacts. The control of the tunneling resistance is important in reducing the contact resistance for NCF joints [103–105]. Self-assembled monolayers (SAMs) have been extensively studied in the last decade, and recent discoveries on the capability of SAMs to functionalize materials and tune their physical and chemical properties have attracted great interest in this research area [106–110]. In particular, conjugated molecules which have a small gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and possess delocalized π -electrons can contribute to conduction. In semiconductors, self-assembled molecular wires have been shown to be effective in tuning the metal work function () and electrical conduction of metal-molecule contact [111–115]. To enhance the electrical performance of non-conductive adhesives, conjugated molecular wires are incorporated into the NCF formulation, and the current– resistance (I –R) relationships of NCF joints are shown in Fig. 24 [116]. The untreated NCF joints showed a contact resistance of 0.15 × 10−3  and current carrying capability (maximum current below which the I–V relationship remains linear) of 2.7 A. After incorporating conjugated molecular wires, the joint resistance of NCF could be reduced to 0.1 × 10−3  and 0.05 × 10−3  with benzo Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 1623 Figure 24. Electrical properties (I –R relationship) of NCF joints with molecular wires [116]. dithiol (BDT) and biphenyl dithiol (BPD), respectively. BDT containing NCF decreased the joint resistance by 1/3 and BPD containing NCF decreased the joint resistance by 2/3. In addition, the current carrying capabilities of BDT and BPD containing NCFs were increased to 2.9 A and 3.1 A, respectively. The significantly improved electrical properties of NCFs could be attributed to the enhanced interface properties, such as reduced tunneling resistance with molecular wires and the conjugated molecular wires assisted electron tunneling and current flow between the joints [117]. 4.2. NCAs With Improved Reliability Properties for Flip Chip Assembly NCAs, materials basically composed of an adhesive polymer resin and a curing agent, have attracted much attention as an alternative for ACAs for flip chip on organic boards due to the advantages of low cost and ultra-fine pitch capability [118–121]. For more wide use of flip chip technology using NCAs, it is necessary to provide good reliability data to prove the advantages of NCAs flip chip technology. The most commonly observed flip chip failure occurs during the thermal cycling test, which is due to the thermal expansion mismatch between chip and substrate. Therefore, the problem of CTE mismatch between chip and substrate becomes serious with the NCAs flip chip assembly because of high CTE of NCA materials without any filler. For this reason, novel NCAs that have low CTE for underfilllike function have been developed. Figure 25 shows the schematic drawing of flip chip chip size package (FCCSP) using an NCF for first level interconnection and its cross-sectional view of NCF joint with Au stud bump. The addition of non-conducting silica filler in the NCA composite materials has control on the curing behavior, thermo-mechanical properties, and reliability for the NCA flip chip assembly on an organic substrate. The content of non-conducting filler was optimized for the desirable thermo-mechanical properties of NCA com- 1624 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 Figure 25. Schematics of flip chip CSP using an NCF and cross section of NCF interconnection [122]. posite materials, such as proper curing profile, high Tg, low CTE and modulus, and strong adhesion. These effects of non-conducting filler addition on the NCA material properties were verified by reliability tests. The reliability of an NCA flip chip assembly using modified NCA with non-conducting filler is significantly better than that of flip chip assembly using commercial ACFs as shown in Fig. 26. Therefore, the incorporation of non-conductive fillers in the NCA composite material significantly improves the reliability of flip chip CSP using NCA materials [122, 123]. NCA materials continue to increase their applications because of their low cost, finer pitch interconnection, high reliability and processability. But the technical concerns for processing and performance of NCAs such as high bonding pressure required and electrical instability at high temperatures should be resolved [124]. Recently, nano-sized metallic fillers have been incorporated into NCA formulations for improved electrical and thermal conductivity, and reduced bonding pressure [95]. 5. Conclusion This review paper has described the recent developments, research activity and applications of electrically conductive adhesives as one of promising lead-free alternatives for electronic packaging and interconnection applications in terms of materials, processing, and reliability concerns. Electrically conductive adhesive materials have evolved to meet the higher electrical/mechanical/thermal performance, fine pitch capability, low-temperature processing and strong adhesion/reliability requirements for electronic packaging modules and assemblies. ICAs are becoming attractive in replacing SnPb or Pb-free solder alloys in die attach, SMT and flip chip assemblies with electrical, mechanical and reliability enhancements. More research investigations in improving these performances of ICAs together with comparable material cost to solder are being pursued for electronics manufacturing without solder. ACAs have been successfully used for fine pitch and Pb-free interconnection areas for flat panel displays and semiconductor packaging. New material systems for conductive fillers and adhesive matrices are being developed for ever-increasing 1625 Downloaded by [Georgia Tech Library] at 08:37 08 April 2014 M. J. Yim et al. / Journal of Adhesion Science and Technology 22 (2008) 1593–1630 Figure 26. Open occurrence frequency of NCA flip chip interconnects (a) during 85◦ C/85%RH test and (b) −55◦ C–160◦ C thermal shock test [122]. demands of electrical, thermal, and reliability performances with fine-pitch, lowtemperature and fast cure ability, etc. Especially multi-layered ACF structures such as double and triple-layered ACFs for fine pitch COG and COF package technologies have been developed, and underfil-like ACA/F and thermally conductive ACAs have been developed for the reliable flip chip assembly under thermal cycling and high current density environments. High frequency characteristic of ACF flip chip interconnects was also investigated and was found to be useful for RF and high frequency interconnects. Wafer level flip chip package using pre-applied ACFs was 1626 M. J. 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