Reliability Assessment of a High Performance Flip-Chip BGA Package (organic substrate based) using Finite Element Analysis

The continuous rise in performance requirements of highend computers and networks has resulted in a rise in demand for high pin count and more sophisticated thermal solutions. Flip chip technology naturally presents a good option in offering I/Os of more than 500. With the addition of a heat spreader (of either one-piece or two-piece lid), the thermal performance of the package can be improved tremendously. However due to the differences in material type for one-piece and two-piece designs, the mismatch in thermal expansion between substrate and lid has constituted to different stress distributions in the package. In the current work, finite element simulation was used to perform structural analysis and comparison between the two different designs. Results showed that the two-piece design generated a lower package warpage, thus allowing better surface mounting. On the other hand, the one-piece lid constituted to lower package induced stresses. The increase in lid width improves package performance in terms of warpage and heat spreader to substrate interfacial shear stresses. Solder joint reliability for the one-piece design was evaluated using Darveaux’s volume weighted average viscoplastic strain energy approach. To account for the effect of ramp rate and frequency effects of a temperature cycle, frequency-modified low cycle fatigue relationship developed by Shi et al was employed for comparison with Darveaux’s computation. Lastly, parametric effects of lid material, bond pad structure, die size, substrate and PCB thickness on solder joints reliability have been documented. The trends presented would be useful for both the package and board level reliability assessments and containing possible solder joint failures.