Gold removal from component terminations via solder dipping

Conversion of gold plating into tin/lead coating on interconnecting pads via solder dipping, i.e., the pretinning process, is investigated with X-ray diffraction and SEM/EDX methods. Under certain conditions, the resultant pads form a gold-tin intermetallic compound layer, AuSn⁴, at the interface between the substrate and the solder coating. The formation of this intermetallic causes two problems. First, when this compound is exposed at the surface by abrading the solder, it renders the pad surface rough and deviant from the eutective composition. This degrades solderability and ultimately leads to soldering failure. Concern is warranted, since certain production requirements call for a leveling procedure for the solder pads, making intermetallic protrusion possible. The second problem is that long-term solder joint reliability for interconnections made from these pads is compromised, as the AuSn⁴ layer is always trapped along the joint/substrate interface