Effects of Surface Finish on the Shear Fatigue of SAC-Based Solder Alloys

Solder joints in electronic assemblies are subjected to mechanical cycling in normal operating conditions. Testing individual solder joint in mechanical cycling allows monitoring the evolution of hysteresis loop with time and provides the ability to study the factors that affect the reliability. In this article, the effect of surface finish on the fatigue behaviors of four doped lead-free solder alloys (CycloMax, Innolot, Ecolloy, and SAC-X-Plus) and SAC305 was investigated. Individual solder joints with three surface finishes [organic solderability preservative (OSP), immersion silver (ImAg), and electroless nickel immersion gold (ENIG)] were prepared and subjected to various cyclic shear stress loadings until complete failure. Hysteresis loop was systematically recorded using an Instron machine. Characteristic fatigue life and energy dissipation per cycle were analyzed, and the Morrow energy model was applied to empirically predict the fatigue life as a function of work dissipation. Failure mechanisms were investigated as well using SEM/EDS, and the results were correlated with the fatigue behavior of the solder joints. The results indicated that the fatigue resistance of the solder alloys decreased with the increase in stress amplitude. The fatigue resistance of the solder joints with OSP and ImAg surface finishes outperformed ENIG surface finish. CycloMax with OSP surface finish was associated with the best fatigue resistance, while SAC-X-Plus with ENIG surface finish was associated with the worst fatigue resistance. Three failure modes were detected in this study: ductile, brittle, and near-intermetallic compound (IMC) failure. It was also found that Innolot with ENIG surface finish is more susceptible to brittle failure because of the excessive brittleness associated with this alloy.