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 Au6.6Cu9.6Sn3.8. During the TLP bonding process, (Au, Cu)(5)Sn was formed first, following which new alpha(Au) and Au6.6Cu9.6Sn3.8 phases appeared, to finally form a combination of alpha'(Au), alpha(Au), and Au6.6Cu9.6Sn3.8 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 Au6.6Cu9.6Sn3.8 phase also slightly reduced the shear strength of the joint. However, the overall shear strength of the TLP-bonded joint consisting of alpha'(Au)/alpha(Au)/Au6.6Cu9.6Sn3.8 phases without pores was approximately 50 MPa. The TLP-bonded joints possess excellent mechanical reliability, with shear strength of 28 MPa even at 350 A degrees C, a temperature that is 70 A degrees 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 A degrees C, for 400 h, and only slightly decreased after 400 thermal cycles.