Influence mechanism of Au layer thickness on wettability of Sn–Ag–Cu solder on heated ENIG pads

Solder wettability on substrate is an issue for the electronic devices. In this present, the heated Cu pads with ENIG finish was selected as the study object. Effect of Au layer thickness on ENIG surface morphology, Ni distribution and wettability of Sn-3.0Ag-0.5Cu solder was investigated. An acceptable morphology with nanoscale fluctuation for ENIG surface was achieved, with the absence of interface cracking, pits, holes. Ni diffused from Ni layer to Au surface during heating process, then reacted with the ambience to form Ni oxides, the fraction was increased with the increasing Au layer thickness. The Ni oxides inhibited the interfacial reaction between Sn-3.0Ag-0.5Cu and Ni during reflow, and remained to the interface. It resulted in the formation of "barren zones", where without any intermetallic compounds. With Au layer thickness increased, "barren zones" was shrunk until to disappear. The "barren zones" was responsible to the wettability of Sn solder. The more "barren zones", the poor wettability.