Effect of aging on the interfacial reactions of BGA Sn-Ag-Cu and Sn-Ag solders with Ni(P) / Au surface finish on Cu pad

Abstract

In this study, the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu (wt.%) solders with Cu/Ni(P)/Au ball grid array (BGA) pad metallization were systematically investigated after solid state aging at 150℃ (around 0.7Tm of solder alloys). The Sn-3.5%Ag solder gave better results in terms of shear strength on high temperature aging than Sn-3.5%Ag solder with 0.5%Cu. It was found that a relatively high consumption of the Ni(P) was observed in the case of Sn-3.5%Ag solder alloys during long time aging. It was also noticed that the mean thickness of the intermetallic compounds (IMCs) at the interface and the dark P rich Ni layer was higher for the Sn-3.5%Ag solder alloy due to high interfacial reaction. With the addition of 0.5% Cu in the Sn-3.5%Ag solders, the overall reaction rate at the interface was diminished during high temperature aging. For both cases Ni diffused through the interfacial IMCs and formed quaternary compounds in the Sn-Ag-Cu system and ternary compounds in the Sn-Ag system within the bulk solder. A simplistic theoretical approach is carried out to find out the transferred amount of Ni in the bulk of the solder by the measurement of the Ni consumption from the substrate and the thickness of the IMCs and P rich Ni layer at the interface.