Growth of η phase scallops and whiskers in liquid tin-solid copper reaction couples

In the microelectronics industry, many solder junctions rely upon reaction between a copper substrate and a molten tin-based alloy. For the tin/copper system, interfacial continuity is afforded by the formation of the eta (Cu6Sn5) and epsilon (Cu3Sn) phase intermetallic compounds. The eta grows in a scalloped morphology along the tin interface with whiskers emanating from their tops. This article quantitatively describes the unusual growth behavior of the eta phase scallops and whiskers formed during reaction of liquid tin with a solid copper substrate.