A Study on Strengthening Mechanisms in Sn-0.7Cu via Microstructural Observation, Elemental Distribution, and Grain-Size Analysis

Sn-0.7Cu is a typical Pb-free solder for electronic packaging, but its strengthening mechanism has not been clear. In this study, we will figure out the main mechanisms for the hardness enhancement of the addition of the 0.7 wt.% Cu on the Sn matrix alloy. The precipitation of Cu-Sn IMC was observed, and the hardness of the Sn-0.7Cu was higher than that of pure Sn. EBSD orientation map shows the large grain size of Sn in the Sn-0.7Cu solder. That proves that the precipitation of the Cu
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Sn
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IMC is in the Sn matrix that resists dislocation slips and enhances the hardness of the solder. After thermal aging, the cluster of Cu
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Sn
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IMC induces the ~20% decrease in the hardness of the Sn-0.7Cu solder. That can support the net-like structure of the Cu
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Sn
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dispersion is the critical mechanism to enhance the strength of the solder.