Comparative study on interfacial reactions between Sn-3.5Ag, Sn-3.0Ag-0.5Cu solder balls and ENEPIG pad after multiple reflows

Nowadays, with electronic products tending to become shorter, smaller, lighter, and thinner, solder balls used to join chips and substrates are also downsizing. This may have an adverse effect on the reliability of electronic products. The focus of this study is on the effect of solder volume on the interfacial reactions between Sn-3.5Ag, Sn-3.0Ag-0.5Cu lead-free solder balls and electroless nickel electroless palladium immersion gold (ENEPIG) pads on printed circuit board (PCB) after various reflows. The diameters of solder balls were 200, 300, 400 and 500 μm, respectively. In the interfacial reaction between Sn-3.5Ag solder balls and ENEPIG pads, as the diameters of the solder balls changed, obvious changes in Ni3Sn4 intermetallic compound (IMC) morphologies and thickness were not found. We attribute this phenomenon to the limited Ni concentration in the solder. When a small amount of Cu was added into Sn-Ag lead-free solder, changes in type, morphology and growth kinetics of interfacial IMCs occurred. In the case of Sn-3.0Ag-0.5Cu/ENEPIG reaction, volume effect in the form of IMC type transition occurred. Only (Ni, Cu)3Sn4 IMCs with morphologies of needle and chunk shape were observed at the interface of 200 μm solder ball. Interestingly, chunk type (Ni, Cu)3Sn4 IMCs co-existed with the octahedron-type (Cu, Ni)6Sn5 IMCs at the interface between Ni-P layer and 300 μm solder ball. And in the case of 400 μm and 500 μm solder balls, only (Cu, Ni)6Sn5 IMCs in the needle-type shape were formed at the interface. This phenomenon was related to the absolute Cu content in the solder which was changed with solder volume.