Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-te... more Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-term fixation performance. The fixation techniques may inherently aggravate the "stress-shielding" effect of the implant, leading to weakened bone foundation. In this study, two cemented tibial fixation cases (fully cemented and hybrid cementing with cement applied under the tibial tray leaving the stem uncemented) and three cementless cases relying on bony ingrowth (no, partial and fully ingrown) were modelled using the finite element method with a strain-adaptive remodelling theory incorporated to predict the change in the bone apparent density after prosthesis implantation. When the models were loaded with physiological knee joint loads, the predicted patterns of bone resorption correlated well with reported densitometry results. The modelling results showed that the firm anchorage fixation formed between the prosthesis and the bone for the fully cemented and fully ingrown cases greatly increased the amount of proximal bone resorption. Bone resorption in tibial fixations with a less secure anchorage (hybrid cementing, partial and no ingrowth) occurred at almost half the rate of the changes around the fixations with a firm anchorage. The results suggested that the hybrid cementing fixation or the cementless fixation with partial bony ingrowth (into the porous-coated prosthesis surface) is preferred for preserving proximal tibial bone stock, which should help to maintain post-operative fixation stability. Specifically, the hybrid cementing fixation induced the least amount of bone resorption.
A lack of initial stability of the fixation is associated with aseptic loosening of the tibial co... more A lack of initial stability of the fixation is associated with aseptic loosening of the tibial components of cementless knee prostheses. With sufficient stability after surgery, minimal relative motion between the prosthesis and bone interfaces allows osseointegation to occur thereby providing a strong prosthesis-to-bone biological attachment. Finite element modelling was used to investigate the bone–prosthesis interface micromotion and the relative risk of aseptic loosening. It was anticipated that by prescribing different joint loads representing gait and other activities, and the consideration of varying tibial–femoral contact points during knee flexion, it would influence the computational prediction of the interface micromotion. In this study, three-dimensional finite element models were set up with applied loads representing walking and stair climbing, and the relative micromotions were predicted. These results were correlated to in-vitro measurements and to the results of prior retrieval studies. Two load conditions, (i) a generic vertical joint load of 3×body weight with 70%/30% M/L load share and antero-posterior/medial-lateral shear forces, acted at the centres of the medial and lateral compartments of the tibial tray, and (ii) a peak vertical joint load at 25% of the stair climbing cycle with corresponding antero-posterior shear force applied at the tibial–femoral contact points of the specific knee flexion angle, were found to generate interface micromotion responses which corresponded to in-vivo observations. The study also found that different loads altered the interface micromotion predicted, so caution is needed when comparing the fixation performance of various reported cementless tibial prosthetic designs if each design was evaluated with a different loading condition.
Wafer Level Packaging" has been there in microelectronics industry since late 1990's, b... more Wafer Level Packaging" has been there in microelectronics industry since late 1990's, but recently it is accepted as package of choice particularly for applications where the pin count is from low to medium. Wafer Level Chip Scale Package (WLCSP) is growing rapidly in consumer application such as cell phone, digital camera and wireless application. It combines the chip scale advantage
An integrated stress based reliability criterion has been proposed, with application aiming at pr... more An integrated stress based reliability criterion has been proposed, with application aiming at predetermining and improving the moisture/reflow reliability of QFN packages. A brief account on the fundamental concepts of moisture modeling has been discussed to provide greater comprehensiveness in relation to analyzing moisture induced failure, such as during the course of solder reflow. Key strategies on implementing these concepts in the integrated stress analysis have also been discussed. In the current setup, actual test statistics from moisture/reflow reliability is coupled to mechanical simulation based on the finite element method. Correspondingly, an engineering test parameter termed as the stress endurance weightage (SEW) has been developed. SEW provides a comparative measure of stress overload along the epoxy molding compound (EMC)-die paddle interface of the QFN devices under study. The stress state addresses both structural and material (molding compound in particular) implications on device integrity and it serves as a risk indicator of possible interface delamination for the current QFN test vehicle. A clear advantage of this indicator is the establishment of a common reference across all analyzes, regardless of variations in package geometry as well as material selection. The test methodology was implemented on in-house QFN packages ranging from body sizes of 4×4mm to 8×8mm. Good correlations were achieved in separate validations of moisture/reflow reliability conducted under IPC/JEDEC moisture sensitivity (MSL) level 2 and 3 across QFN packages of different body sizes. With the development of a test chart based on SEW parameter, design of experiments (DOE) can be better refined prior to a full-blown execution, relieving burdens on extensive and inefficient testing.
The recent advancement in high performance semiconductor packages has been driven by the need for... more The recent advancement in high performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip BGA package with high pin count and targeted reliability has recently been developed by UTAC. The flip chip technology can accommodate I/O count of more than five hundreds, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. Nonetheless, greater expectations on these high performance packages arose such as better substrate land estate utilization for multiple chips, ease in handling for thinner core substrates and improved board level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planer top surface can be formed. And a flat lid can then be mounted on the planer mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multi-chip package and thin core substrate options. Finite element simulations have been employed for the study of its structural integrity, thermal and electrical performances. Detailed package and board level reliability test results will also be reported.
High pin count and superior thermal dissipation are the main driving factors for high performance... more High pin count and superior thermal dissipation are the main driving factors for high performance IC packages. Flip chip interconnects technology can generally achieve I/O count of more than 500, and large amount of heat in the silicon chip can be dissipated efficiently through metal heat spreader attachment. A one-piece cavity lid flip chip BGA package with high pin count and targeted reliability has recently been developed by UTAC. However it was found that solder joint reliability could be compromised due to the rigidity resulted by the one-piece cavity lid. A new design of flip chip BGA package (patent pending) has been looked into for improved board level performance. In this new design, the flip chip will be over-molded (with the die top surface exposed) before the lid is being attached. With the new single flat lid mounted onto the mold compound, the package substrate is thus less rigid under thermal loading. Hence solder joint integrity enhancement is expected. The structural differences between two flip chip BGA designs are being discussed in the paper. Test vehicles (BT substrate based) of size 40x40mm with pin count of 1521 are fabricated for the evaluation of package and board level reliability. The analysis results showed that board level reliability could be improved (of more than 50%) through design change, with no compromise in thermal performance of the package. The easy manufacturability of the flat lid has given an advantage over the complicated powder injection lid molding process, where the package cost can be reduced. Lastly, detailed package reliability of the two flip chip BGA packages is being reported.
Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-te... more Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-term fixation performance. The fixation techniques may inherently aggravate the "stress-shielding" effect of the implant, leading to weakened bone foundation. In this study, two cemented tibial fixation cases (fully cemented and hybrid cementing with cement applied under the tibial tray leaving the stem uncemented) and three cementless cases relying on bony ingrowth (no, partial and fully ingrown) were modelled using the finite element method with a strain-adaptive remodelling theory incorporated to predict the change in the bone apparent density after prosthesis implantation. When the models were loaded with physiological knee joint loads, the predicted patterns of bone resorption correlated well with reported densitometry results. The modelling results showed that the firm anchorage fixation formed between the prosthesis and the bone for the fully cemented and fully ingrown cases greatly increased the amount of proximal bone resorption. Bone resorption in tibial fixations with a less secure anchorage (hybrid cementing, partial and no ingrowth) occurred at almost half the rate of the changes around the fixations with a firm anchorage. The results suggested that the hybrid cementing fixation or the cementless fixation with partial bony ingrowth (into the porous-coated prosthesis surface) is preferred for preserving proximal tibial bone stock, which should help to maintain post-operative fixation stability. Specifically, the hybrid cementing fixation induced the least amount of bone resorption.
A lack of initial stability of the fixation is associated with aseptic loosening of the tibial co... more A lack of initial stability of the fixation is associated with aseptic loosening of the tibial components of cementless knee prostheses. With sufficient stability after surgery, minimal relative motion between the prosthesis and bone interfaces allows osseointegation to occur thereby providing a strong prosthesis-to-bone biological attachment. Finite element modelling was used to investigate the bone–prosthesis interface micromotion and the relative risk of aseptic loosening. It was anticipated that by prescribing different joint loads representing gait and other activities, and the consideration of varying tibial–femoral contact points during knee flexion, it would influence the computational prediction of the interface micromotion. In this study, three-dimensional finite element models were set up with applied loads representing walking and stair climbing, and the relative micromotions were predicted. These results were correlated to in-vitro measurements and to the results of prior retrieval studies. Two load conditions, (i) a generic vertical joint load of 3×body weight with 70%/30% M/L load share and antero-posterior/medial-lateral shear forces, acted at the centres of the medial and lateral compartments of the tibial tray, and (ii) a peak vertical joint load at 25% of the stair climbing cycle with corresponding antero-posterior shear force applied at the tibial–femoral contact points of the specific knee flexion angle, were found to generate interface micromotion responses which corresponded to in-vivo observations. The study also found that different loads altered the interface micromotion predicted, so caution is needed when comparing the fixation performance of various reported cementless tibial prosthetic designs if each design was evaluated with a different loading condition.
Wafer Level Packaging" has been there in microelectronics industry since late 1990's, b... more Wafer Level Packaging" has been there in microelectronics industry since late 1990's, but recently it is accepted as package of choice particularly for applications where the pin count is from low to medium. Wafer Level Chip Scale Package (WLCSP) is growing rapidly in consumer application such as cell phone, digital camera and wireless application. It combines the chip scale advantage
An integrated stress based reliability criterion has been proposed, with application aiming at pr... more An integrated stress based reliability criterion has been proposed, with application aiming at predetermining and improving the moisture/reflow reliability of QFN packages. A brief account on the fundamental concepts of moisture modeling has been discussed to provide greater comprehensiveness in relation to analyzing moisture induced failure, such as during the course of solder reflow. Key strategies on implementing these concepts in the integrated stress analysis have also been discussed. In the current setup, actual test statistics from moisture/reflow reliability is coupled to mechanical simulation based on the finite element method. Correspondingly, an engineering test parameter termed as the stress endurance weightage (SEW) has been developed. SEW provides a comparative measure of stress overload along the epoxy molding compound (EMC)-die paddle interface of the QFN devices under study. The stress state addresses both structural and material (molding compound in particular) implications on device integrity and it serves as a risk indicator of possible interface delamination for the current QFN test vehicle. A clear advantage of this indicator is the establishment of a common reference across all analyzes, regardless of variations in package geometry as well as material selection. The test methodology was implemented on in-house QFN packages ranging from body sizes of 4×4mm to 8×8mm. Good correlations were achieved in separate validations of moisture/reflow reliability conducted under IPC/JEDEC moisture sensitivity (MSL) level 2 and 3 across QFN packages of different body sizes. With the development of a test chart based on SEW parameter, design of experiments (DOE) can be better refined prior to a full-blown execution, relieving burdens on extensive and inefficient testing.
The recent advancement in high performance semiconductor packages has been driven by the need for... more The recent advancement in high performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip BGA package with high pin count and targeted reliability has recently been developed by UTAC. The flip chip technology can accommodate I/O count of more than five hundreds, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. Nonetheless, greater expectations on these high performance packages arose such as better substrate land estate utilization for multiple chips, ease in handling for thinner core substrates and improved board level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planer top surface can be formed. And a flat lid can then be mounted on the planer mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multi-chip package and thin core substrate options. Finite element simulations have been employed for the study of its structural integrity, thermal and electrical performances. Detailed package and board level reliability test results will also be reported.
High pin count and superior thermal dissipation are the main driving factors for high performance... more High pin count and superior thermal dissipation are the main driving factors for high performance IC packages. Flip chip interconnects technology can generally achieve I/O count of more than 500, and large amount of heat in the silicon chip can be dissipated efficiently through metal heat spreader attachment. A one-piece cavity lid flip chip BGA package with high pin count and targeted reliability has recently been developed by UTAC. However it was found that solder joint reliability could be compromised due to the rigidity resulted by the one-piece cavity lid. A new design of flip chip BGA package (patent pending) has been looked into for improved board level performance. In this new design, the flip chip will be over-molded (with the die top surface exposed) before the lid is being attached. With the new single flat lid mounted onto the mold compound, the package substrate is thus less rigid under thermal loading. Hence solder joint integrity enhancement is expected. The structural differences between two flip chip BGA designs are being discussed in the paper. Test vehicles (BT substrate based) of size 40x40mm with pin count of 1521 are fabricated for the evaluation of package and board level reliability. The analysis results showed that board level reliability could be improved (of more than 50%) through design change, with no compromise in thermal performance of the package. The easy manufacturability of the flat lid has given an advantage over the complicated powder injection lid molding process, where the package cost can be reduced. Lastly, detailed package reliability of the two flip chip BGA packages is being reported.
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