The role of oxide layers in solder joints

In soldering processes, oxide contamination often prevents suitable wetting. In order to determine basic mechanisms, the reaction of SnPbAg solder with Ni under bulb metallization is studied. Prior to soldering, the metallization is artificially coated with NiO surface layers of controlled thickness. The soldering reaction on oxide-contaminated Ni surfaces proceeds in three stages: (i) melting of solder and instantaneous formation of a spherical solder bulb with a large contact angle in the de-wetting regime, (ii) incubation period of constant contact angle, and (iii) wetting with exponentially decreasing wetting angle. It is demonstrated that the incubation time depends parabolically on the thickness of the oxide layer. Reducing atmosphere used as flux is suitable to prevent the solder from oxidation but its effect on the pre-existing NiO is negligible. In consequence, the atomic transport through the existing oxide layer becomes the controlling factor for the soldering kinetics.