Wetting of Cu Pads by Bi-2.6Ag-xCu Alloys and Phase Equilibria in the Ag-Bi-Cu System

The phase equilibrium in the Ag-Bi-Cu system was experimentally determined at 573 K, 773 K, and 973 K by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) on annealed alloys and liquid/solid couples. The experimental results indicate that the mutual solubility of the components is limited. Based on the present results and literature data, phase equilibria in the Ag-Bi-Cu system were thermodynamically assessed. Wetting of Bi-2.6Ag-xCu alloys on Cu substrates was studied with the sessile drop method in the presence of flux at 573 K and 623 K. It was found that the wetting to non-wetting transition corresponds to the solubility limit of Cu in liquid. Selected solidified solder–substrate couples were cross-sectioned and their interfacial microstructure examined with SEM–EDS. There are no reaction products at the interface, but the copper surface becomes rough because of dissolution by liquid solder.