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

添加Ag对Sn-Bi系钎料与Cu连接接头组织和性能的影响。在Sn-Bi合金中加入0.5mass%的Ag，通过Sn-Bi-Ag 3Sn三元共晶凝固，使共晶组织细化。随着共晶组织的减少，特别是在快速应力速度下，其塑性比Sn-Pb焊料的塑性增加更多。然而，添加超过1质量%的银，形成粗和单轴Ag 3Sn，这降低了焊料的延展性。采用熔点在410 K左右的Sn-Ag-Bi钎料，可以在433 K实现与Cu焊盘的低温回流焊。采用Sn-Bi-Ag钎料的Cu微连接具有与Sn-Ag-Cu钎料相当的抗拉强度和热循环疲劳性能。由于433 K低温焊接的优点，再流焊产生的残余应力变小，Cu焊盘与基板之间的断裂被大大抑制。对于采用Sn-Bi-Ag的接头，拉伸断裂发生在Cu 6Sn 5反应层与钎料的界面处。时效过程中接头强度的下降是由于反应层的生长造成的。动力学分析表明，反应层的生长行为与Sn-Ag-Cu焊料接头的生长行为基本相同。

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.