Effect of Surface Finish on the Shear Properties of SnAgCu-Based Solder Alloys

The reliability of an electronic assembly is typically limited by the failure of one of the solder interconnections. One of the key factors that define the quality of solder interconnections is the strength of the solder joint attachment to the printed circuit board (PCB). The ball shear testing is commonly used as a quantitative approach to evaluate the integrity of the solder. Several factors control the shear strength and failure mechanism of solder joints, including the surface finish, solder alloy composition, and speed of shearing (shear strain rate). In this paper, the effect of surface finish on the shear properties of various "microalloyed" SnAgCu-based solder materials was investigated. Individual solder joints were fabricated on a PCB with a solder mask defined configuration. A series of shear testing was conducted at four different strain rates of 0.001, 0.01, 0.1, and 1 s(-1). Shear stress-strain curves were recorded for each test, and both shear strength and shear energy were measured. Following the shear testing, fractured top surfaces and cross sections were inspected using SEM/EDS microscopy to characterize the failure mechanism. The results showed that both shear strength and shear energy increase with increasing shear strain rate due to the viscoplasticity of the solder materials. Failure mode analysis indicated the existence of three failure mechanisms, including ductile failure, brittle failure, and mixed failure. Higher occurrence of brittle failure was observed when the shear strain rate is high regardless of surface finish. It was also found that the combination of solder alloy with high silver (Ag) and bismuth (Bi) content and electroless Ni/immersion Au (ENIG) surface finish is more susceptible to brittle failure compared to the rest of solder alloy-surface finish combinations.