Process Development of 4-Die Stack Module Using Moldable Underfill

Market is always looking for way to reduce the cost of package. Traditional way of protecting the fragile micro-bumps is by applying capillary underfill (CUF) to mitigate the issue of CTE mismatch between the die and the substrate. However, the use of CUF introduce additional assembly process on top of high material cost as compared to Moldable Underfill (MUF). In this paper, we explore the use of MUF for the 4-diue stack. MUF is very attractive as it combined the step of molding and underfilling into one single step in addition to the low material cost as compared to CUF. The reliability of MUF is much superior to CUF as shown in the simulation study. The simulation study indicates a drastic 1.65 times more fatigue life for MUF as compared to CUF. The 4 die stack is formed using conventional mass reflow process. The dies is stacked on top of each other on a bottom substrate wafer using conventional noclean flux. The whole substrate wafer with the 4 die stack is then send through a reflow oven to form the solder interconnect for all 4 die stacks. This approach is much prefer than individual thermo-compression process in terms of throughput and less thermal loading to the solder interconnects as no heat is applied to each die stacking process. We had demonstrated no void in the region between the solder bump after the MUF molding process.