Developments of high-Bi alloys as a high temperature Pb-free solder

Predominant high melting point solders for high temperature electronics (operating temperatures from 200 to 250 C) are Pb-based but these are hazardous to environment. In this study, we designed and characterized the `new´ Bi-based solder alloys, 78Bi-14Cu-8Sn and 70Bi-20Sb-10Cu, to understand their performance for high temperature die attach applications. The specific alloy compositions were determined by thermodynamics calculations via the Thermo-Calc software to have a solidus temperature above 271 C. Microstructures of the selected as-cast alloys have shown Cu₃Sn and Cu₂Sb intermetallic compounds (IMCs) spread in Bi or Bi-Sb matrix, respectively, which are essential for maintaining good thermal conduction and high temperature strengths. Die-attach solder joints were replicated using a Cu substrate, a solder preform and a Ti/Ni/Au metallized Si die, to study interfacial reactions and joint microstructure. In this paper, microstructure - property relationships of high-Bi alloys are highlighted to assess their potential as a high temperature Pb-free solder. In particular, we examined the reactivity of these solders with common surface finishes (e.g., Ni/Au, Cu) of the die and the substrate at various reflow conditions. It was shown that strong reaction exists between Bi and Ni (BiNi), between Sb and Ni (NiSb), and between Sb and Cu (Cu₂Sb, Cu₄Sb), but the reaction in the case of 78Bi-14Cu-8Sn was rather weak with Cu (forming Cu₃Sn) due to the limited supply of Sn from the Bi matrix. The die shear testing was also performed on these sandwiched coupons to evaluate their solder performance at various reflow conditions. The die attach with 70Bi-20Sb-10Cu showed as-reflowed die shear strengths over 30 MPa, which were comparable to or better than high-Pb alloys.