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 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 Pb-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. © 1998 Elsevier Science S.A. All rights reserved.