The Effect of Thermal Aging on the Mechanical Properties of Gold-Containing Solder Joints

The reliability of SnPbAg solder joints containing gold (Au) under different aging conditions have been investigated. The solder joints were prepared by printing Sn62Pn36Ag2 solder paste on gold-plated copper (Cu) pads and then reflow soldering them. Subsequently, the joints underwent aging at temperatures of 75°C, 100°C, 125°C, and 150°C for durations of 100 h, 250 h, 500 h, and 1000 h, respectively. During the aging process, there was a gradual transition of Au from the AuSn₄ phase to the (Cu,Au)₆Sn₅ phase, resulting in a phenomenon known as the “return of Au”. The intermetallic compound (IMC) layer within the solder joint primarily consisted of the Cu₃Sn phase and the (Cu,Au)₆Sn₅ phase, with the IMC layer progressively thickening as the aging time extended. Through the calculation of the growth rate constant (k) and activation energy (Q) for both the Cu₃Sn phase and the (Cu,Au)₆Sn₅ phase, it was found that the growth rate of the (Cu,Au)₆Sn₅ phase surpassed that of the Cu₃Sn phase. Upon reaching an aging parameter of 150 °C for 1000 h, an essentially continuous Pb-rich layer formed above the IMC layer within the solder joint. Following shear force tests, the solder joint experiences ductile fracture, with the mechanical properties of the joint significantly compromised due to the presence of the Pb-rich layer.