Developments of Bi-Sb-Cu 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 our previous study, we have shown the alloy design and characterization of the `new´ bismuth (Bi) - based solder alloys; 78Bi-14Cu-8Sn, 70Bi-20Sb-10Cu. It was shown that these solders have strong reactivities with Ni (forming intermetallic phases containing Bi or Sb), and with Cu (forming Cu₂Sb, Cu₄Sb). Of the two alloys, 70Bi-20Sb-10Cu exhibited the potential for forming good bonding with both Ni and Cu surfaces. The aim of this study was to establish the microstructure - property relationship by exploring the microstructure features involving solid solution behavior between Bi and Sb and the interfacial intermetallic layers. For this, the sandwiched coupons were fabricated using high Bi solders to bond the silicon die having Ti/Ni/Au backside metallization to the Cu substrate. These sandwich samples were then tested for thermal conductivity, die shear testing and liquid thermal shock between -50°C and 150°C. The current alloy matched the performance of high Pb alloys in thermal and die shear testing. In particular, the role of interfacial intermetallic layer on the reliability of the solder joint was investigated. From the findings of this study, Bi solder alloys showed some potential as an alternative to high-temperature Pb-free solders.