The impact of interfacial void formation on Pb-free Sn-4.0Ag-0.5Cu BGA solder joint integrity

As the microelectronics industry prepares for implementation of Pb-free solder materials, a great deal of attention is being placed on utilization of the Sn-rich Ag and Cu-bearing alloys. The Sn-Ag-Cu (commonly referred to as SAC) alloys have achieved widespread adoption as the alloys of choice for second-level interconnection, including BGA solder spheres, SMT pastes, and preforms. Non Cu-bearing alloys such as eutectic Sn-3.5Ag have also found acceptance for these applications, although not to the same degree as their Cu-bearing counterparts. Both these alloy groups have been found to demonstrate acceptable joint strength and reliability in first and second level testing. Recent investigations into their elevated temperature reliability have revealed a tendency for void formation in the Cu-bearing alloys at various joint interfaces, under various thermal conditions. This paper presents results of package-level high temperature storage and temperature cycling studies performed on SAC405 and eutectic Sn-3.5Ag BGA solder ball alloys. These results characterize the formation and growth of voids at the interface of the SAC405 sphere alloy and underlying substrate metallurgy of electrolytically-plated Ni/autocatalytic Au plated pads, and compares the joint micro structural evolution between the Cu-bearing and non-Cu-bearing alloys. In addition, the formation of complex intermetallic compounds in the area of the joint interfaces is discussed, and the package susceptibility to solder sphere separation due to brittle fracture is defined as a function of process conditions and materials