Thermal Stability of Bi in Sn-Cu Based High Strength Pb-Free Solder Alloys

Commercial solder alloys with the addition of bismuth (Bi) have gained prominence in the electronics manufacturing industry, because of their low melting point and the strengthening effect associated with Bi. If the Bi concentration exceeds the solubility of Bi in Sn at room temperature (approximately 1.7 wt” in typical Sn-based solder alloys), there will be a driving force for precipitation of Bi within the Sn phase. However, the solubility of Bi in Sn is strongly temperature dependent and dissolution of some or all of the Bi precipitates may occur at higher temperatures, depending on the alloy composition. To study the thermal stability in solders with different Bi content, two samples: Sn-3.SAg-3Bi-2Sb-O.8Cu and Sn-l.SBi-O.7Cu-O.OSNi (in wt.%) were characterised by in-situ scanning electron microscopy (SEM) and in-situ powder x-ray diffraction (PXRD). Bi dissolution was observed in the sample with higher Bi addition upon heating and pores remained when the sample was cooled down. On the contrary, in the sample with lower Bi concentration the microstructure remained identical before and after heating, indicating better thermal stability. Furthermore, the microstructure change caused by Bi dissolution is reflected in a volume expansion in the Sn phase lattice obtained by PXRD data.