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 eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations 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 dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within 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.