Dvelopment of Sn-Bi Based Low Temperature Solders which Enables Step Soldering

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Effect of Ag Addition to Sn-Bi Eutectic Solders on the Microstructure and Properties of Joints with Cu. About 0.5mass% of Ag addition to the Sn-Bi changes the eutectic microstructure fine through Sn-Bi-Ag3Sn ternary eutectic solidification. With decreasing the eutectic microstructure, the ductility increased more than that of Sn-Pb solders especially at the rapid stress speed. Addition of Ag more than 1 mass%, however, formed coarse and uniaxial Ag3Sn which decreased the ductility of solders. rand improved its ductility.By using Sn-Ag-Bi solders with low melting temperature about 410K, low temperature reflow soldering with Cu pad can be achieved at 433K. Microjoint with Cu using Sn-Bi-Ag solder exhibited the comparable tensile strength and thermal cycle fatigue property with that of Sn-Ag-Cu solders. Be the advantages of the low temperature soldering at 433K, residual stress caused by reflow soldering became smaller and fracture between Cu pad and substrate was drastically supressed. For the joints using Sn-Bi-Ag, fracture by tensile test occurs at the interface between Cu6Sn5 reaction layer and solder. The degradation of joint strength during the aging was caused by the growth of reaction layer. The growth behavior of reaction layer, however, was eastimated to be almost same as that for the joints using Sn-Ag-Cu solders by the kinetic analysis.