Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 Thick-Film and LTCC Thermistors Andrzej Dziedzic, Leszek J. Golonka Institute of Microsystem Technology, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland Keywords: thick film, Low Temperature Co-fired Ceramics, thermistor, pulse durability, temperature field Abstract Self-made and commercially available NTC thermistor compositions were investigated. The influence of kind of substrate as well as component configuration and placement on basic electrical properties (resistivity at room temperature, R(T) dependence) and long-term stability was analysed. The durability of LTCC and thick-film thermistors on short high-voltage pulses was described. Application of thermistors matrix for characterization of temperature distribution in LTCC and thick-film microcircuits was presented. 1. Introduction The application of thermistors for temperature measurements and temperature compensation circuits is widely known. Typical commercial NTC and PTC thermistors are realized using ceramic technology. Therefore it is nothing strange that these devices were adapted very quickly for realisation in thick-film technology – the first paper describing thick-film NTC thermistors was published in 1969. Moreover amongst about 700 papers connected with thick-film sensors, which were published between 1968 and 1995, 33 were devoted to NTC thermistors [1]. Thick-film NTC thermistor pastes consist of the powder of spinel-type semiconductor metal oxides (or spinel-type semiconductor is formed during firing process from powders of various metal oxides) blended with the powder of precious metal or conducting oxide, the glass binder and the organic vehicle [2-4]. One should note that modern chip thermistors for surface mounting technology are made using standard thick-film technology [5]. Therefore at present the thermistor inks are commercially available. For example Du Pont developed NTC thermistor series NTxx composed of 3 members: 1 (NT30), 10 (NT40) and 100 (NT50) k / [6] whereas D-NTC-2100 series from ESL is designed for sheet resistance (at room temperature) from 30 up to 1 M [7]. Therefore it is rather unexpected that as yet there was no information about application of thermistors in LTCC circuits. This paper tries to fill the above-mentioned gap. The basic electrical and stability properties of self-prepared and commercially available NTC thermistor compositions were described. The influence of substrate (Al2O3, Du Pont 951 and Ferro A6M tapes), device configuration (planar or 3-dimensional) and placement (both placed on the surface of LTCC structure or alumina substrate or buried inside LTCC) on resistance versus temperature dependence and long-term stability were tested. Moreover behaviour of LTCC and thick-film thermistors under short rectangular high-voltage pulses was examined. The analysis of temperature field in various microcircuits and characterization of thermal conductivity of LTCC substrate with the aid of thick-film thermistors were also presented. 77 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 2. Test samples fabrication Self-made thermistor composition consisted of Mn1.6Co0.8Ni0.35Ru0.25O4 (44 wt %), RuO 2 (16 wt %) and glass (40 wt %) [4,8]. It was used for fabrication of 3D (all geometrical dimensions of the same order) and planar thermistors placed both on the surface of Al2O3, Ferro A6-M or DP 951 tapes as well as embedded inside LTCC structure (Fig.1). Ag- or PdAg-based conductor pastes compatible with mentioned tape systems, were used as electrodes. The second group of thermistors were made using NT40 ink from Du Pont. Planar topology was used for them. They were prepared on alumina as well as surface and buried components on/in DP 951 tape. 60 min firing cycle with 10 min dwell time at 850oC was used for firing of thick-film thermistors on alumina. LTCC thermistors were made according to tape manufacturers` recommendations. This means that the lamination process was carried out in an isostatic press at 210 bar for 10 minutes held at 70oC. Next laminates were fired. For example it was one step process for A6-M tape with the following procedure: ramp rate v1 = 2oC/min T1 = 450oC/120 min v2 = 8oC/min T2 = 850oC/10 min. Surface planar thermistor Buried (embedded) planar thermistor Buried 3D thermistor Surface 3D thermistor Electrodes Electrodes LTCC foil LTCC foil Fig. 1. Planar and 3D configuration of LTCC thermistors 3. Basic electrical and stability properties Results presented in Table 1 and in Fig. 2 indicate that metallurgy of electrodes, i.e. the interface conductor/thermistor, affects the electrical properties of LTCC thermistors much stronger than configuration. The resistivity of 3D thermistors is lower than for planar ones in the case of Ag contacts. Moreover, silver electrodes strongly influence the R(T) characteristics both for planar and 3D components, decreasing B (thermistor constant) 2 2.5 times in comparison with thermistors possessing PdAg contacts. Contrary to many papers, where R(T) dependence is presented only in the range above room temperature, we measured the dependence of resistance as a function of temperature in the range from –170oC to 130oC. The typical NTC thermistor characteristic (described e.g. by Formula (1)) usually starts at temperatures higher than 273 K (Fig. 3). The resistance changes are much weaker below this temperature. The 44/16/40 termistor film is based on the spinel-type semiconducting oxide, RuO 2 and glass. The spinel and precious metal oxide grains create conductive network in the film. Moreover, hopping conduction could take place in thin glass layer between grains. Electrical conduction model with parallel connection of regions with negative (spinel) and positive (RuO 2) temperature coefficient of resistance explains the phenomena observed below the room temperature. The conductivity of spinel grains dominates at higher temperature. 78 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 Very simple electrical equivalent circuit (Fig. 3) was used in order to check the above suggestion. Thick-film thermistor was replaced by resistors R and RT connected in parallel. If we assume that R is temperature independent and temperature dependence of RT is given by Eq. (1) RT R298 exp B 1 T 1 298 (1) then the R(T) dependence of thermistor in the whole measured temperature range is described by (2) 1 T 1 298 1 R298 exp B T 1 298 R R298 exp B Rz R (2) Figure 3 shows significant agreement between measurements and so simple thermistor equivalent circuit. Table 1. Resistivity (at 298 K) and thermistor constant for various thermistor configurations and electrodes (self-made thermistor paste 44/16/40, substrate - Ferro A6-M tape, B – buried and S – surface placed device) Configuration, Planar, B, Planar, S, 3D, B, 3D, S, electrodes Ag Ag Ag Ag [ m] 4.28 2.79 0.92 0.83 B[K] 1140 1090 various 1040 3D, B, PdAg 2.94 2640 3D, S, PdAg 4.31 2560 Planar, PdAg, on Al2O3 3.00 2660 7 10 3D, B, PdAg 3D, S, PdAg planar, PdAg 5 10 THERMISTOR 6 10 4 10 R[ ] R[ ] 5 10 4 10 3 10 3D, B, Ag 3D, S, Ag planar, B, Ag planar, S, Ag 3 10 2 10 2 10 2 3 4 2 3 4 5 1000/T [1000/K] 5 1000/T [1000/K] Fig. 2. The temperature characteristics of resistance for 44/16/40 thermistor with Ag and PdAg electrodes The long-term stability of various types of thick film planar and 3D thermistors are compared in Figure 4. Almost all tested elements exhibit small resistance changes, less than 1% after 500 hours exposure at 150 oC. The level of stability is similar for 3D and planar thermistors. 79 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 4 10 lg R [ ] measurements model R RZ RT 3 10 2 10 2 4 6 8 10 1000/T [1/K] Fig. 3. Temperature dependence of resistance of real thermistor and model as well as the electrical equivalent circuit of thick-film thermistor 1,0 THERMISTOR R/R [%] 0,5 0,0 -0,5 planar, S, Ag planar, B, Ag 3D, B, Ag 3D, S, PdAg 3D, B, PdAg -1,0 -1,5 -2,0 1 10 Time [h] 100 1000 Fig. 4. Long-term stability of various thermistors 4. Pulse durability of thick-film and LTCC thermistors So-called load hyperbola, this means the dependence between electrical load and component life time, is very useful in fixing of admissible power of electronic devices. This paper presents the behaviour of thick-film and LTCC thermistors during single “short” rectangular pulse (pulse width changed between 20 and 5000 s). Programmed Pulse Generator, described in details in [9], was used for measurement of voltage and current in the course of so short pulse and than to calculate resistance changes and energy stored by thermistors listed in Table 2. The resistance of NTC thermistors is decreased during the pulse very quickly (Fig. 5). Because of preliminary character of these investigations the reason of such changes is not clear yet. Such characteristics could be connected both with temperature rise inside thermistor volume as well as with changes of potential barrier shape (its height and/or width) in a very high electric field. When the pulse duration increases the surface density of energy is increased, too 80 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 (Fig. 6) – prolongation of pulse width from 20 to 5000 s leads to increase of the total energy dissipated during the pulse by more than one order prior to thermistor catastrophic damage. It is interesting to note, that buried LTCC thermistors withstands more severe voltage conditions than surface ones and these seconds are more durable than components made on alumina surface. Probably lamination process, necessary for formation of LTCC structure, creates much more compact structure in comparison with thermistors made on alumina. Table 2. List of investigated LTCC and cermet thick-film thermistors (PdAg electrodes, screen density – 325 mesh) Kind of ink Substrate Configuration Dimensions of and position device [mm2] Al2O3 Planar, surface 2 2 DP 951 tape Planar, buried 1.75 1.75 DP 951 tape Planar, surface 1.75 1.75 NT40 NT40 NT40 ] 11 700 11 900 4 200 Thermistor constant, B [K] 2370 2080 1650 4000 600 0.1 Voltage 3000 2 wc [J/mm ] 500 2000 300 R[ ] 400 U [V] R [ 0.01 NT 40/Al2O3 NT 40/DP 951 S NT 40/DP 951 B 200 Resistance 1000 1E-3 100 0.01 0 0 100 200 300 400 500 600 0 700 0.1 tp [ms] 1 t [ s] Fig. 5. Changes of resistance of buried NTC thermistors during “short” pulses with 100, 300 or 1000 s duration Fig. 6. Critical density of energy cumulated by LTCC and standard thick-film thermistors in dependence of pulse duration 5. Thermistors in temperature field analysis of LTCC and thick-film microsystems Thick film thermistors can be easily used for analysis of the temperature distribution inside or on the top of the LTCC structure. Special test structures consisted of four Du Pont DP 951 foils with five DP NT40 thermistors on each were described in [10,11]. The thermistors played role of heaters or temperature sensors. The temperature inside the structure was measured by thermistors and was calculated using thermal simulation tool TRESCOM II [11] or neural network method [10]. Very good agreement between experimental and calculated data was found in both cases. The temperature distribution inside or on the top of the structure was calculated. Moreover, the surface emissivity of various test structures materials was determined at Vienna University of Technology using a thermovision system [11]. It was found that emissivities of DP 951 tape, PdAg conductor and NT40 thermistor films were equal to 0.92, 0.90 and 0.50, respectively. The emissivities were temperature independent in the used temperature range (up to 250oC). The thermal conductivity in horizontal and perpendicular direc- 81 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 tion of the multilayer tape structure was calculated to be equal to 3 W/mK and 1.6 W/mK, respectively [11]. Probable it was connected with various LTCC tape shrinkage in x, y and z direction during cofiring process. Another network of NT40 thick film thermistors is presented in Figure 7. The thermistors were made on 2”x2”x0.01” 96% alumina substrate. They were used for determination of the temperature at various points on the alumina surface. Experimentally found dependence of temperature versus heated resistor power is presented in Figure 8. 400 7 6 1 4 2 380 8 R1 360 9 R2 T [K] 5 T2 T4 T8 T10 10 340 11 320 3 300 0.0 0.5 1.0 1.5 2.0 2.5 P [W] Fig. 7. Test circuit with heating sources R1, R2 and 11 thick-film thermistors Fig. 8. Temperature measured by sensors 2, 4, 8 and 10 versus power emitted by resistor R1 6. Conclusions Thermistors are still attractive and useful electronic components. This paper presents electrical, thermal and stability properties of thick-film and LTCC thermistors. During the investigations the following were found: 1. Standard thermistor inks can be applied in LTCC circuits. 2. Basic electrical parameters of thermistors (resistivity, thermistor constant B) are dependent on the kind of substrate as well as device configuration and placement in/on the substrate. 3. Metallurgy of electrodes affects thermal properties of thermistors much stronger than parameters listed in point2. 4. Devices based on 44/16/40 composition exhibit typical NTC characteristic at temperatures higher than 273 K. Below this temperature their resistance is almost temperature independent. Electrical equivalent circuit with parallel connection of regions with negative temperature coefficient of resistance and temperature independent explains the R(T) dependence in a very wide temperature range. 5. Buried LTCC thermistors withstand more severe pulse voltage than surface ones and these seconds are more durable than components fabricated on alumina. Probably lamination process creates much more compact structure of LTCC thermistors. 6. The LTCC thermistors are useful in measurement of temperature distribution not only on the surface of LTCC circuits but also inside them. 82 Proc. XXIV Int. Conference IMAPS-POLAND 2000, Rytro, 25-29 Sept. 2000, p.77-83 References 1. A. Dziedzic; Examinations of thick-film sensors and actuators – chosen statistical data, Proc. 21st Int. 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