Thermal Preconditioning and Restoration of Bismuth-Containing, Lead-Free Solder Alloys

Abstract

Bismuth (Bi)-containing solder alloys have emerged as prime candidates to replace traditional lead (Pb)-free alloys such as SAC 305 (Sn-3.0Ag-0.5Cu, in wt.%); unlike SAC alloys, these alloys show stability in mechanical properties after aging. A process was developed in which a Bi-bearing alloy is intentionally subjected to a short above-solvus thermal treatment, to potentially extend the life of the solder joint and improve device reliability. This treatment may be implemented after reflow, or periodically in service as a form of preventive maintenance. In this work, we evaluated this treatment as both a preconditioning and restoration process to compare Violet (Sn-2.25Ag-0.5Cu-6.0Bi, in wt.%) and SAC 305 alloys. Firstly, the solvus temperature of Violet was evaluated experimentally; this result was used to determine an appropriate heat treatment temperature for preconditioning and restoration. It was found that the heat treatment produces a highly equiaxed β-Sn grain structure, likely caused by particle stimulated nucleation during Bi precipitation. The creep resistance of the alloy was found to improve after the treatment. Finally, it was observed that both Bi and preconditioning can have varying effects on the thermal cycling reliability of the alloy, depending on the harshness of the thermal cycle (ΔT).