The solder reaction of Cu and molten pure Sn is studied by scanning and transmission electron microscopy. Similar as reported for SnPb solder, the intermetallic Cu 6Sn 5 product is formed in a scallop-like morphology and a growth kinetic proportional to the cube root of time is found. Size distributions and shape of the scallops are determined experimentally. Comparing the binary reaction couple Cu/Sn with the technical combination Cu/SnPb, a significant difference in the length to width aspect ratio of individual scallops is noticed. We demonstrate by transmission electron microscopy that neighbored scallops are not separated by channels of solder, but by grain boundaries. Thus, grain boundary transport is the rate controlling step and the observed variation in scallop shape is due to differences in the interface tensions of the two reaction couples.