Microstructural development and mechanical properties of a near-eutectic directionally solidified Sn-Bi solder alloy

Sn-Bi solders may be applied for electronic applications where low-temperature soldering is required, i.e., sensitive components, step soldering and soldering LEDs. In spite of their potential to cover such applications, the mechanical response of soldered joints of Sn-Bi alloys in some cases does not meet the strength requirements due to inappropriate resulting microstructures. Hence, careful examination and control of as-soldered microstructures become necessary with a view to pre-programming reliable final properties. The present study aims to investigate the effects of solidification thermal parameters (growth rate - VL and cooling rate - ṪL) on the microstructure of the Sn-52 wt.%Bi solder solidified under unsteady-state conditions. Samples were obtained by upward directional solidification (DS), followed by characterization through metallography and scanning electron microscopy (SEM). The microstructures are shown to be formed by Sn-rich dendrites decorated with Bi precipitates surrounded by a complex regular eutectic mixture, with alternated Bi-rich and Sn-rich phases. Experimental correlations of primary (λ1), secondary (λ2), tertiary (λ3) dendritic and eutectic spacings (λcoarse and λfine) with cooling rate and growth rate are established. Two ranges of lamellar eutectic sizes were determined, described by two experimental equations λ = 1.1 VL− 1/2 and λ = 0.67 VL− 1/2. The onset of tertiary branches within the dendritic array along the Sn-52 wt.%Bi alloy DS casting is shown to occur for cooling rates lower than 1.5 °C/s.