Agradecimentos: The authors acknowledge the financial support provided by FAPESP (São Paulo Research Foundation, Brazil: grants 2013/13030-5 and 2013/08259-3), CNPq and CAPES-COFECUB (grant 857/15)

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...

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

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ

857/15

COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP

2013/13030-5; 2013/08259-3

Fechado