THE EFFECT OF LOW GOLD CONCENTRATIONS ON THE CREEP OF EUTECTIC TIN-LEAD JOINTS

The effects of low Au concentrations on the creep properties of a eute ctic Sn/Pb alloy were investigated. Creep testing was performed on dou ble-shear specimens of fine-grained, eutectic Sn/Pb joints with Au con centrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90-degrees-C, 0, 0.2, and 1.0 wt pct Au at 65-degrees-C, and 0.2 wt pct Au at 25-degrees-C. In the absence of Au, the creep of fine-grained eutectic Sn/Pb is dom inated by grain-boundary sliding at high homologous temperature and in termediate stress. The addition of 0.2 wt pct Au or more suppressed th is mechanism; the high-stress, bulk-creep mechanism was dominant at al l stresses tested. Higher concentrations of Au increased porosity with in the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.