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

Tin-zinc solder with the low melting temperature below 200 °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 60μm. 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.