Abstract
Effect of different cooling rates(0°C, 8°C, and 25°C water cooling experiments) on microstructural evolution of Sn-3.0Ag-0.5Cu(SAC305) and Sn-3.8Ag-0.7Cu(SAC387) lead-free solder were investigated. The influence of morphology it produced would be evaluated via tensile strength. The study showed the cooling rates are only a few changes by equipment preheat to 250°C. Crystallization time increased significantly with cooling rates decreasing. SAC solder stood on non-equilibrium solidification process, so the process could be expressed as: L→L+(β-Sn+Ag3Sn)→β-Sn+Ag3Sn +Cu6Sn5. Cu6Sn5 compounds would grow slower than Ag3Sn by different proportion of Ag and Cu content. The decline of the cooling rates and the rise of crystallization time made the morphology of Ag3Sn and Cu6Sn5 eutectic compounds develop as : particle-like → particle and needle mixed → needle-like → needle and plate or long flake mixed → plate-like. In terms of mechanical properties, the size of Ag3Sn and Cu6Sn5 with rapid cooling was tiny that it had the higher hardness. SAC387 added more Ag content caused that eutectic compounds and β-Sn area grew larger. Hardness of SAC305 is lower than SAC387.
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