Microstructural evolution and mechanical reliability of transient liquid phase sintered joint during thermal aging

Solder paste of Ag and SAC0307 (Sn-0.3 wt%Ag-0.7 wt%Cu) mixed powders was fabricated and utilized as the interlayer to achieve a Cu/SAC0307–Ag/Cu transient liquid phase (TLP) sintered joint. The microstructural evolution and mechanical reliability of the joint during thermal aging at 350 °C were systematically studied. A serial transition of Ag–Sn phase occurred from Ag3Sn into Ag4Sn and eventually into Ag–Sn solid solution (Ag) until 15 days, accompanied with the formation of Cu3Sn network inside the Ag–Sn phase layer. It was because that the Cu atoms diffused from the substrate into the Ag–Sn phase layer and then reacted with Ag–Sn phase, including two processes: 4Ag3Sn + 3Cu → 3Ag4Sn + Cu3Sn and Ag4Sn + 3Cu → 4Ag + Cu3Sn. The shear strength of the aged joint decreased from 75 to 60 MPa after 15 days due to the transition of Ag–Sn phases and the formation of contraction voids, and then it remained stable. The comparisons of hardness and elasticity modulus between the three Ag–Sn phases were both as follows: Ag3Sn > Ag4Sn > Ag–Sn solid solution. Thus, the excellent reliability of the TLP sintered joint with SAC0307–Ag powders was experimentally verified.