Microstructure Evolution During the Aging at Elevated Temperature of Sn-Ag-Cu Solder Alloys

In this work, binary Sn-3.5%Ag and Sn-0.7%Cu and ternary Sn-3.5%Ag-0.9%Cu eutectic alloys has been used to investigate the dynamic of microstructure transformation during isothermal artificial aging process at 180¼C. The microstructure was investigated using optical microscopy (OM), scanning electron micrograph (SEM) and EDX (energy dispersive X-ray microanalysis) and followed by Differential Scanning Calorimetry technique (DSC). In the as-cast condition the binary eutectic alloys have a structure composed of Sn primary phase crystals surrounded by an irregular eutectic structure composed of intermetallic sheets of Ag3Sn and Cu6Sn5 in an Sn matrix while the microstructure of the ternary alloy shows the following phases: β-Sn surrounded by Sn-Ag binary eutectic and Sn-Ag-Cu ternary eutectic. After 48 hours of isothermal aging at 180°C both alloys show a noticeable change in their microstructures. In binary alloys a larger grain structure is formed, Sn dendrites coarsen, and in the ternary alloy the intermetallic phases Ag3Sn Cu6Sn5 are randomly distributed in a Sn matrix. There is a coarsening of Sn dendrites and Ag3Sn phase produced by the growth mechanism of Ostwald (Ostwald Ripening).