Abstract
This paper reports influence of isothermal aging on bulk solder and the microstructure evolution of commercial Sn-0.7Cu (Sn-0.7Cu-0.05Ni+Ge) Pb-free solder alloy with addition of bismuth (Bi) and indium (In). Sn-0.7Cu, Sn-0.7Cu-1.0Bi, Sn-0.7Cu-1.0Bi-1.0In, Sn-0.7Cu-1.0Bi-4.0In, Sn-0.7Cu-1.0Bi-7.0 In solder alloys were prepared via casting process. The solder alloys were aged isothermally at 180°C for 500 hours and microstructure was compared with that of as-cast solder. Microstructure of bulk solder and melting temperatures were analysed via scanning electron microscopy, SEM and differential scanning calorimetry, DSC. The addition of In up to 7 wt% reduced the melting temperature from 231.1 to 218.5°C, and crystallisation temperature from 205.8 to 194.7°C with decreasing degree of undercooling which indicates that the In-added solder alloy had faster nucleation rate. Microstructure evaluation showed that as the amount of indium added increased to 7.0 wt%, the ß-Sn grains became smaller suggesting a refinement effect with In addition. Indium addition also encouraged the formation of Sn-In and Bi-In IMC which increased as the amount of In increased. These IMCs could potentially act as blocking mechanism for deformation leading to higher strength.
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