Interfacial reactions of BGA Sn–3.5%Ag–0.5%Cu and Sn–3.5%Ag solders during high-temperature aging with Ni/Au metallization

The joint strength and the microstructure of Sn–3.5Ag and Sn–3.5Ag–0.5Cu (wt.%) solders on Cu/Ni/Au ball-grid-array (BGA) pad metallization were investigated after high-temperature solid-state aging at 190 °C (around 0.86Tm of solder alloys). Sn–Ag solder gave better results in terms of shear strength on high-temperature aging than Sn–Ag–Cu. Very high consumption of Ni was observed in the case of Sn–Ag–Cu solder alloys. After 16 days of aging at the afore mentioned temperature, 5 μm Ni layer was fully consumed from the substrate pad and a thick layer of Cu–Sn intermetallic compounds (IMCs) was found at the base of the interfacial IMCs. Much less consumption of Ni substrate was observed for Sn–3.5Ag solder during high-temperature aging for longer time. The mean thickness of the intermetallics at the interface was higher for Sn–Ag–Cu solder alloy. For both cases Ni diffused through the interfacial IMCs and formed quaternary compounds for Sn–Ag–Cu system and ternary compounds for Sn–Ag system within the bulk solder. It appeared that Sn–Ag–Cu solder alloy was more vulnerable in high-temperature solid-state aging.