Journal of SMT Article

Microstructure and Hardness of Bi-containing Solder Alloys After Solidification and Ageing

Authors: André Delhaise, Leonid Snugovsky, Doug Perovic, Polina Snugovsky, and Eva Kosiba
Company: University of Toronto and Celestica, Inc.
Date Published: 7/1/2014   Volume: 27-3

Abstract: This paper is focused on the microstructure and hardness analysis of new bismuth (Bi) containing alloys that have lower melting and process temperature than the conventional SAC305 solder alloy. One of the main SAC solder drawbacks is fast coarsening of microstructure resulting in degradation of the mechanical and thermomechanical properties during ageing. Seven Bi-containing alloys were used as listed in Table 1.

Samples were aged for 25 and 100 hours at 100°C. Alloy hardness was measured after solidification ageing and quenching using a Rockwell hardness tester (HR15T scale with a 1/16” carbide ball indenter). The microstructures after solidification and after ageing were compared using scanning electron microscopy (SEM). The microstructures undergo significant changes upon ageing. Bi particles become uniformly distributed in the Sn matrix. While the hardness of SAC alloys is reduced after annealing, the hardness of these experimental alloys increases. The hardness gradually increases with increasing Bi content, as well as with increasing Ag content. The effect of solid solution strengthening and precipitation hardening mechanisms on microstructure and properties is discussed.

Keywords: 

Bi-containing solder, Ageing, Microstructure, Hardness



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