Fracture Behavior and Reliability of Low-Silver Lead-Free Solder Joints

Abstract

Low-silver solders are increasingly being used because silver improves the tensile strength. In this study, the variation in fracture behavior with silver content in lead-free solder joints was studied using double cantilever beam specimens. Fracture tests were done with solder joints made with Sn-0.7Cu, SACX0307, and SAC305 solder materials. The critical energy release rate for crack-initiation (Gci) of the joint was correlated with the plastic zone just ahead of the precrack tip, intermetallic compound layer thickness, energy dispersive spectroscopy analysis, and scanning electron microscopy based fractography study. The Gci for Sn-0.7Cu solder joint was observed to be significantly higher than the other two solder joints. The fractography study revealed that the failure was ductile for Sn-0.7Cu and a mix of ductile and brittle for the other two solder joints. The extent of the plastic zone ahead of the crack tip, obtained from finite element modeling, was found to be significantly larger and the intermetallic compound layer was relatively thinner for Sn-0.7Cu solder joint compared to the other two solder joints. The ductile failure, significantly larger plastic zone size and thinner intermetallic compound layer resulted in significantly higher Gci for Sn-0.7Cu solder joint.