Microstructure evolution and mechanical reliability of Cu/Au–Sn/Cu joints during transient liquid phase bonding

The microstructure evolution and mechanical reliability of Cu/Au–Sn/Cu sandwich joints during transient liquid phase (TLP) bonding were investigated in this study. The results show that the Au–Sn solder reacted with the Cu substrate to form Au and Au_6.6Cu_9.6Sn_3.8. During the TLP bonding process, (Au, Cu)₅Sn was formed first, following which new α(Au) and Au_6.6Cu_9.6Sn_3.8 phases appeared, to finally form a combination of α’(Au), α(Au), and Au_6.6Cu_9.6Sn_3.8 phases when the Au–Sn solder is exhausted. The volume contraction associated with the consumption of the Au–Sn solder results in pore formation and subsequent shear strength deterioration in the Cu/Au–Sn/Cu joint. The presence of the Au_6.6Cu_9.6Sn_3.8 phase also slightly reduced the shear strength of the joint. However, the overall shear strength of the TLP-bonded joint consisting of α’(Au)/α(Au)/Au_6.6Cu_9.6Sn_3.8 phases without pores was approximately 50 MPa. The TLP-bonded joints possess excellent mechanical reliability, with shear strength of 28 MPa even at 350 °C, a temperature that is 70 °C higher than the melting point of the Au–Sn solder. Moreover, the shear strength of the TLP-bonded joint remains stable at 50 MPa even after exposure to high temperatures such as 250 or 350 °C, for 400 h, and only slightly decreased after 400 thermal cycles.