Journal of SMT Article

Thermal Preconditioning, Microstructure Restoration and Property Improvement in Bi-Containing Solder Alloys

Authors: André M. Delhaise, Polina Snugovsky, Ivan Matijevic, Jeff Kennedy, Marianne Romansky, David Hillman, David Adams, Stephan Meschter, Joseph Juarez, Milea Kammer, Ivan Straznicky, Leonid Snugovsky, Doug D. Perovic
Company: Celestica, Department of Materials Science & Engineering, University of Toronto, Rockwell-Collins, BAE Systems, Honeywell Aerospace, Curtiss-Wright
Date Published: 1/30/2018   Volume: 31-1

Abstract: It has been well-established that the properties of lead-free solder alloys such as SAC 305 (Sn-3.0Ag-0.5Cu) degrade over time as the result of the coarsening of the intermetallic phases such as Ag3Sn and Cu6Sn5. In earlier studies, it was shown that the inclusion of bismuth (Bi) in Sn, Sn-Ag, Sn-Cu, and SAC-based alloys leads to a stabilization of the as-solidified mechanical properties after aging at temperatures above the alloy’s solvus (where all precipitates are allowed to dissolve into and diffuse through the ß-Sn matrix, leading to a uniform, homogenous microstructure). Further, the ß-Sn grain structure becomes more refined, transitioning from consisting of only a few large grains to having many smaller, randomly-oriented grains. However, most practical operating conditions lie below the solvus temperature. This results in Ostwald ripening of the Bi precipitates, which may render solder joints sufficiently brittle to be a reliability concern.

This paper contains details from a recently patented process which allows for the improvement of the properties of the solder joint, either post-reflow, or after some amount of product lifetime. The treatment was analyzed by comparing the creep properties of Violet, an alloy containing 2.25% Ag, 0.5% Cu, and 6.0% Bi, with SAC 305. An above-solvus aging treatment was performed on each alloy, either after solidification, or after below-solvus aging. It was found that the creep resistance of both alloys is reduced after below-solvus aging, but the creep resistance of Violet is significantly improved after above-solvus aging (regardless of prior thermal history). These results show that this aforementioned thermal treatment is a viable method to improve the long-term reliability of solder joints in electronic assemblies.

Keywords: 

creep, thermal treatment, bismuth, aging



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