Microstructure evolution and mechanical properties of Cu/Sn/Ag TLP-bonded joint during thermal aging

Low-temperature transient liquid phase (TLP) bonding is a promising joining technology for high-temperature electronic devices packaging. However, the effect of thermal aging on the joint microstructure and performance has been rarely studied so far. In this paper, the relationship between microstructural evolution and mechanical property was developed for the Cu/Sn/Ag UP-bonded joint during 350 degrees C aging. Experimental results show that an interfacial reaction occurs between the intermetallic compounds (IMCs) of Cu3Sn and Ag3Sn, which induces that a great amount of Cu3Sn particles or wedges embed into the Ag-Sn phase layer. The reaction process is controlled by a volume diffusion of Cu elements from the Cu substrate, and capable of promoting the transformation from Ag3Sn into Ag(Sn) solid solution. Cu41Sn11 phase can be formed at the expense of Cu3Sn and Cu, of which transformation process is completely achieved after aging for 480 h, and the Cu(Sn) solid solution intimately contacts with the opposite Ag(Sn) solid solution in many areas after aging for 960 h. The joint mechanical property initially declines due to the formation of Cu41Sn11 phase and Cu-Sn IMCs grain coarsening, and then increases because of the complete depletion of Cu3Sn and substantial precipitation of Cu(Sn) solid solution. After aging for nearly 1000 h, the joint still maintains a high shear strength of > 40 MPa, indicating an excellent thermal reliability.