Highly reliable and low-cost multi-chip module composed of wafer process packages

Abstract

A high-performance. low-cost, and highly reliable multichip module (MCM) was developed. It is composed of a MPU and a synchronous DRAM [i.e., wafer process packages (WPPs)] on a substrate, To reduce the cost of the MCM, a low-cost FR-4 base is used a substrate on which the WPPs are mounted. Since the mismatch between the thermal expansion coefficients (TEC) of the MPU and DRAM chips and the substrate increases, thermal deformation due to the mismatch may cause cracking of the chip or decrease the life of the solder joints between the MCM and a FR-4 base motherboard. To minimize the stress caused by thermal deformation in the MCM and, thus, to improve its reliability, the structure of the MCM was therefore optimized by finite element analysis (FEA). The FEA results show that thermal stresses in both the chips and the solder bumps are a function of the ratio of chip thickness (tc) and substrate thickness (ts), i.e., tclts, and these stresses decrease when tclts is decreased. Furthermore, thermal-cycling test results show that MCM reliability is assured when tclts is 0.5 or less. A MCM with tclts of 0.4, including a margin of safety, was developed. It was found that the life of this MCM is over 1000 cycles (no chip cracking and no solder joint failure) under thermal-cycling between -55°C and 125°C.