Effects of cooling rate and solder volume on the formation of large Ag3Sn plates in Sn-Ag based solder joints

The relationships between the morphology of Ag₃Sn phases and the cooling rate, solder volume and 0.7 wt.% Cu addition into Sn-3.5Ag solder were investigated in this study. The morphology of Ag₃Sn phases in the solder joints was greatly affected by the cooling rate. In Sn-3.5Ag/ENEPIG (electroless nickel electroless podium immersion gold) solder joints, large plate-like Ag₃Sn phases formed on the interfacial intermetallic compounds (IMCs) under furnace cooling, normal cooling and air cooling conditions, while no plate-like Ag₃Sn phases formed under water cooling condition. The volume of solder ball also had a significant effect on the morphology of Ag₃Sn phases in the solder joints. Large plate-like Ag₃Sn phases formed at the Sn-3.5Ag (50 μm)/ENEPIG (40 μm) and Sn-3.5Ag (100 μm)/ENEPIG (150 μm) interfaces under furnace cooling condition. However, no plate-like Ag₃Sn phases were detected at the Sn-3.5Ag (200 μm)/ENEPIG (150 μm) interface under the same cooling condition. The degree of undercooling increased with downsizing solder joints, which would extend the growth time of Ag₃Sn phases in the solidification process. The addition of 0.7 wt.% Cu into Sn-3.5Ag solder had little effect on the morphology of Ag₃Sn phases in the solder joints.