Isothermal low cycle fatigue behavior of Sn-8Zn-3Bi on single shear solder joint

Tin-zinc solder with the low melting temperature below 200 /spl deg/C is an attractive alternative to eutectic tin-lead. The addition of bismuth improves wettability of Sn-Zn alloy. Low cycle isothermal mechanical fatigue testing of solder joints of Sn-8Zn-3Bi (wt%) and Sn-37Pb were tested in a solder joint of single lap shear sample with a multi-functional Instron 4458-Microtester. Solder joints around 0.5mm high and 1.6mm in diameter was used. The testing was executed in a displacement -controlled mode, at three different amplitudes namely, 40, 50 and 60/spl mu/m. The test frequency was kept constant at 0.2Hz for all tests. The failure of the solder joint was defined as a 50% load reduction. Finite element (FE) modeling were simulated to analyze the strain and stress distribution in solder joint and compared with the experimental results. The cross-section showed that there were mixed phase of small cellular-shape and coarse needle-shape in the microstructure of Sn-8Zn-3Bi. SEM-EDS analysis indicated that the Au-Zn intermetallic compound was observed near the interface. The total average lifetime of Sn-8Zn-3Bi solder joint was 17% increase comparing with Sn-37Pb solder joint from low cycle fatigue testing. The ductile-fracture manner and brittle-fracture manner were both involved in fracture surface, and more features of brittle-fracture manner appeared in small displacement loading and more features of ductile-fracture manner in large displacement loading. From FE simulation, the maximum stress was found at two corners, coinciding with the experimental observation of crack initiation and propagation.