Microstructural coarsening of lead free solder joints during thermal cycling

The present work mainly focuses on the microstructure evolution of Sn-3.5 Ag, Sn-3.4 Ag-3.0 Bi and Sn-3.2 Ag-0.5 Cu solder joints in Pb-coated lead frame and Ni/Au plated Cu surface in thermal cycling test. Eutectic Sn-Pb alloy is also studied as a reference material. The solder joints were made using typical surface mount assembly process and thermal cycling was carried out between -40°C and 100°C for 1000 cycles. The microstructure observation was performed by scanning electron microscope (SEM). It was found that microstructure of eutectic Pb-Sn solder joints coarsened dramatically after 1000 cycles, which caused deterioration of solder joint shear strength. The coarsening is related to the relatively high solid solubilities of Pb in Sn and vice versa, especially at elevated temperatures. Whereas, the Sn-Ag based lead-free solders have limited solubility of Ag in Sn, making them more resistant to coarsen, and the formation of Ag₃Sn and other precipitates in Sn-Ag, Sn-Ag-Bi and Sn-Ag-Cu alloys also hinder the grain growth during thermal cycling. As a result, the shear strength change of these solder joints is relatively minor. However, due to the Pb-containing lead frame, Pb-rich phase is detected for every lead-free solder joint. The microstructural coarsening effect of Pb-rich phase caused by the thermal cycling is obvious