Studies of the solidification characteristics in Sn-Ag-Cu-Bi solder alloys.

Abstract

Because of the environmental health implications of Pb, and legislation backing Restriction on Hazardous Substances (RoHS) in electronic devices, Sn-based lead-free solders are developed. Near eutectic Sn-Ag-Cu (SAC) compositions is the most widely used Pb-free solder in the electronic industries. However, its high melting temperatures (>217 ℃) remains a concern. The addition of Bi to a Sn-based alloy can be used to lower melting temperature because of the presence of a deep valley eutectic in the Bi-Sn system. The effects of such additions in SAC on solidification path are rarely considered. The effects of Bi substitution for Sn on solidification of near eutectic SAC alloy are followed with differential thermal analysis (DSC). It is found that the solidus temperature of SAC-Bi alloy mixes is considerably lowered. For low concentrations of Bi, melting range (ΔTr) were generally less than 4 ℃. However, (ΔTr) considerably widens when the Bi content exceeds 2 wt. %. The DSC and X-ray diffraction (XRD) studies suggested that a higher concentration of Bi in SAC would lead to far eutectic solidification that is attended by a formation of primary Bi phase. The existence of primary Bi phase in a solidified solder would lead to embrittlement and thus would affect the integrity of soldered joints. This work suggests that the use of Bi in SAC solders should be restricted to less than 2 wt. % (by varying %Wt of Bi).