Equilibrium phase diagram design and structural optimization of SAC/Sn-Pb composite structure solder joint for preferable stress distribution

Abstract

The Package-on-Package (PoP) technology is an efficient knowhow to increase the efficiency of electronic components, hence to secure the continuation of the validity of Moore's law. This novel form of packaging technology with bottom stackable narrow pitch Ball Grid Array (BGA) contributes to the integrity of the package. However, the high solder joint density introduces a great challenge on the packaging reliability. Therefore, more stringent requirements must be put forward for achieving the increased performance. That can be achieved by using composite solder joints. In this study, a novel composite SAC (Sn-3Ag-0.5Cu)/Sn-Pb (Sn37Pb) solder joint was designed for improving the stress distribution and its overall mechanical integrity. The formation process and a phase distribution of composite solder joint were investigated, indicating that the phase distribution mainly depends on the reflow temperature. The dissolution ratio of SAC into the composite solder at different temperatures was estimated by the phase diagram. The predicted values were consistent with the experimental results. A review of mechanical property test results, the best performance solder joint could be obtained at 200 °C for the dwell time of 60 s if the composite includes 10 wt% SnPb. The stress distribution of composite solder joint was analyzed by finite element simulation. By actively concentrating the stress in the middle area of the solder joint, the maximum stress in the composite solder reduced for up to 29% when compared to full SAC solder joint.