Effect of Cu content on the interfacial reliability of SnAgCu solder joints

NEMI recommended Sn3.9Ag0.6Cu to replace the eutectic SnPb solder, but solder joints of this alloy have poor performance in reliability tests of electronic packages, especially the board level drop test. In this paper, the effect of Cu content in solder is studied on the interfacial reliability of SnAgCu solder joints on the Ni/Au plated pads. After preconditioning (three reflows post package assembly), some solder balls fell off during routine handling of the packages. Cracking occurred between the IMC layers of (Cu,Ni)₆Sn₅ and Ni₃Sn₄. Formation of the dual IMC structure is explained from the perspective of interfacial equilibrium. It is suggested that to improve the drop performance of the packages that have Ni/Au plated pads the key is to avoid the formation of the layer-type of (Cu,Ni)₆Sn₅ at the interface. Approaches to do that are discussed. A rule for selecting solder alloy and surface materials on both sides of the joints is proposed for BGA packages.