Role of Wetting Front in Dewetting of Liquid Solder Drop on Cu Thin Films

The thermodynamic conditions for dewetting of a liquid solder drop on copper thin films were examined under a hot-stage optical microscope in a flowing protective atmosphere. Dewetting of liquid solder was found to depend strongly on the copper film thickness and preceded by spalling of Cu6Sn5 intermetallic compounds. However, the loss of interfacial bonding by spalling was not sufficient to cause immediate dewetting of solder drops if the wetting tip was still strongly bonded to the copper film. By introducing a pinning force on the wetting front, a sufficient condition was found from a force balance analysis for dewetting of the liquid solder drop, in general agreement with the experimental results.