Effect of Composition and Cooling Rate on the Microstructure of SnAgCu-Solder Joints

In this study the solder alloys SnAg3.5, SnAg3.0Cu0.5, SnAg3.8Cu0.7 and SnAg2.7Cu0.4Ni0.05 have been analysed in order to determine variations in microstructure caused by cooling rate, solder composition and ball diameter. Solder spheres with a diameter of approx. Ø 1100 μm, Ø 590 μm, Ø 270 μm and Ø 130 μm were solidified with cooling rates of 0.14K/s, 1.1K/s and 10.9K/s. Cross sections of these specimens were analysed by optical light microscopy. Interpretations of the analysed microstructure allow a description of the solidification process, which takes place in a solder ball. It could be concluded that this process is divided in three stages: the formation of primary intermetallics, the formation of fine structured regions and the formation of coarse dendritic areas.