Microstructure and strength of Sn-Bi coated Sn-3.5 mass% Ag solder alloy

Sn-Bi coated Sn-3.5 mass%Ag solder alloy was investigated as a possible low melting temperature solder. Electroplating method was used to form Sn-Bi coated solder alloy on the Sn-3.5 mass%Ag alloy. Sn-Bi coated solders were bumped on the FR-4 substrate and Cu plates by a reflow machine at different temperatures. The die shear strength and microstructure were evaluated with scanning electron microscope (SEM). The compositions of Sn-Bi coated solder, and Bi distribution of solder were studied by electron probe X-ray analyzer (EPMA). The Sn-Bi coated Sn-Ag solder was possible to be bonded at low temperature such as 473-523 K and it is comparable to Sn-37%Pb solder. Intermetallic compounds (IMC's) along the bonded interface were well formed in the coated solder composed of high Bi and the compositions consisted of 59.49 at%Sn-37.20 at%Cu-3.31 at%Ag. Cu ₆Sn₅ and Ag₃Sn intermetallics were observed at the interface as well as inside of the solder. Bi segregation in the solder and at the IMC was not observed. The die shear strength was 3.6384N and increased as coated Sn-Bi thickness increases.