Better Warpage Control by Using Low Temperature Solder for Large FCBGA Application

Large die size with high I/O density is the trend for IC product design, especially for 5G networking and AI products which need more function integration. How to decrease package warpage risk is the key to ensure solder joint performance in the future. SnAgCu alloy (SAC305) whose melting point at 217°C is the most common solder material. The peak temperature of reflow profile is around 245 to 260°C. For large package device, the package warpage is the unavoidable problem at these process temperatures, because the Coefficient of Thermal Expansion (CTE) mismatch between substrate and PCB becomes bigger and bigger. Using Sn-Bi alloy solder material can reduce the peak temperature below 200°C due to the melting point is only 139~174°C. It can get good warpage behavior and better ball placement yield at the same time. In this study, we will arrange large FCBGA test vehicle to collect standard SAC305 and LTS (low temperature solder) performance including the package warpage performance, ball placement yield and ball shear/pull ability. Especially, we also arrange the test vehicle without lid attach. Exposed die structure is the latest trend for better heat dissipation which directly connects with external heatsink (system level). However, the warpage control is a big issue for OSAT, LTS is a good solution for it. Finally, board level reliability is another qualification item with temperature cycle test (TCT) condition.