Improving Sn-0.3Ag-0.7Cu low-Ag lead-free solder performance by adding Bi element

Bi element addition effects on Sn-0.3Ag-0.7Cu low-Ag solder properties are studied in this work. Solder alloy with different Bi adding amount, Sn-0.3Ag-0.7Cu-XBi (X = 0, 1.0, 3.0, 4.5), were fabricated. The melting point, wettability, and strength tests were carried out to find the effects of Bi addition on current low-Ag solder. Thermal aging of the solder interconnects with different Bi addition was conducted. The intermetallic compound growth and shear strength were used to evaluate the Bi effects on thermal aging resistance. Test results of the newly fabricated solder were compared with that of original Sn-0.3Ag-0.7Cu. Results show that Bi addition has striking positive effects on improving wettability and decreasing melting point. With proper adding amount as X = 3.0, Bi element significantly improve both the wettability and melting point performance simultaneously. However, too much Bi addition could increase the melting range between liquidus and solidus, which may lead to the initiation of solidification crack inside the solder. The mechanical test also verified the embrittlement of the solder with too much Bi addition. The intermetallic compound growth at the interface of solder and copper during thermal aging was slow down with Bi addition. The shear test results after thermal aging also shows that Bi addition prevent the severe degradation of solder mechanical properties.