Immersion plating of bismuth on tin-based alloys to stabilize lead-free solders

Bismuth was deposited on tin and tin-copper alloys by immersion plating, with the purpose of protecting lead-free solder balls from a destructive sub-ambient phase transformation ("tin pest"). Deposition in aqueous solutions, or in non-aqueous solutions at ≤130°C, yielded dark, poorly adhering deposits which could not be reflowed. By contrast, use of a glycerol solution above the Sn-Bi eutectic temperature (139°C) yielded reflective deposits with good adhesion to the substrate and amenable to fluxless reflow. The concentration of Bi in the reflowed solder ball could be controlled by changing the immersion time. Unlike usual exchange processes, the rate of deposition did not decrease markedly with time. Extensions of this "immersion eutectic plating" process are suggested.