Microstructural evolution of Sn-Ag-Cu-Al solder with respect to Al content and heat treatment

The Pb-free Sn-Ag-Cu-Al solders were investigated for microstructural evolution with respect to Al% and heat treatment. The Al% varies from 0.1% to 0.45% while the contents of Ag are 3.1%-2.53% and of Cu are 0.41%-0.33%. Differential scanning colorimetry (DSC) was applied to identify the melting behavior. A monotectic temperature of 224°C and a eutectic temperature of 220°C are deduced from the DSC results. The microstructure was characterized with x-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy. Ag₃Sn and the γ₂ phase of Al-Cu system are the intermetallic compounds formed in the as-cast solders. Cu₆Sn₅ formed upon heat treatment for 1000 h at 150°C in the 0.45 Al-containing solder, while not found in the other solders or other heat treatment conditions. Sn whiskers were detected in a 0.45 Al-containing specimen after aging for 50 h.