Fast dissolution and soldering reactions on Au foils

Thin films of Au have been widely used as surface passivation layers and soldering layers to enhance the a wetting of molten solders in the electronic packaging industry. A unique feature of Au is that it has a high dissolution rate in most Pb- and Sn-based solder alloys. We have studied the wetting behavior and interfacial reactions of two Pb-containing (63Sn-37Pb, 95Pb-5Sn) and four Ph-free solders (pure Sn,96Sn-4Ag. 57Bi-43Sn, 77.2Sn-20In-2.8Ag) on Au foils, using scanning electron microscopy and energy dispersive X-ray analysis. The 95Pb-5Sn, having the highest dissolution rate of Au among them, formed sunken interfaces which show a double curvature (positive and negative) near the wetting tip. The formation of Au2Pb is everywhere within the solder cap. Pure Sn/Au showed less compound formation compared to 96Sn-4Ag/Au, but both formed a smaller sunken interface compared to 95Pb-5Sn/Au. The SnInAg showed an inhomogeneous interfacial reaction initially with two different types of interface (flat and curved or sunken interfaces). However, the flat interfaces disappeared after a slightly longer reflow. The only solder which formed a relatively flat interface with Au is the eutectic SnBi, which also showed a very slow rate of intermetallic compound formation. Owing to the sunken interface, we measured the wetting angles from the cross-sections. (C) 1998 Elsevier Science S.A. All rights reserved.