Effect of Surface Finish on the Fatigue Behavior of Bi-based Solder Joints

Abstract

The reliability of an electronic assembly is often limited by the fatigue failure of one of the interconnected solder joints due to the mismatch of Coefficient of Thermal Expansion (CTE) between the components and Printed Circuit Board (PCB). Fatigue properties of lead-free solder joints are normally studied on the large bulk samples. However, the microstructure and failure mechanism of actual size solder joints are different than bulk samples. The surface finish also affects the fatigue behavior of solder joints. The objective of this study is to investigate the effect of surface finish on the fatigue properties of the high-Bi lead-free solder joints. Lead-free alloys for harsh applications are commonly micro-alloyed with small percentages of metal elements, such as Bi, to enhance the fatigue life. In this study, individual solder joins were cycled using Instron micromechanical testers in stress-controlled experiments. The tested solder materials were Sn-3Ag-0.5Cu (SAC305), Sn-3.5Ag-0.7Cu-3Bi-l.5Sb-0.125Ni (Innolot), Sn-3.41Ag-0.52Cu-3.3Bi (CycloMax), and Sn-0.92Cu-2.46Bi (Ecolloy). The surface finishes were OSP, ImAg, and ENIG. Characteristic fatigue life and work dissipation per cycle were measured and compared. The results showed that characteristic fatigue life decreased with the increase of stress amplitude. Solder joints with high Ag and Bi contents demonstrated outstanding fatigue properties. OSP and ImAg surface finishes demonstrated more stable fatigue properties, while ENIG surface finish tended to show an embrittlement fracture between the Ni layer and solder joints during high-stress-amplitudes fatigue test.