Interface reaction and evolution of micron-sized Ag particles paste joining on electroless Ni-/Pd-/Au-finished DBA and DBC substrates during extreme thermal shock test

Abstract The fracture behaviors and Ag–Au joint interface evolution of sintered micron-sized Ag particles paste joined on an electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) and direct-bonded copper (DBC) substrates are evaluated during an extreme thermal shock test (TST) from −50–250 °C. The die shear strength of the as-sintered Ag joint on the DBA substrate is evaluated to be 33.5 MPa at a sintering temperature of 200 °C without any assisted pressure, which decreased gradually with an increase in the thermal cycling number. The shear strength declined slightly but remained at approximately 20 MPa; subsequently, it decreased considerably to 11.2 MPa after 1000 cycles. Coarsening of the sintered Ag layer is observed as the microstructure inhomogeneity and vertical cracks increased after 1000 cycles. In addition, the Al layer induced a greater undulate deformation, resulting in a sintered Ag layer exhibiting partial compression and tension after a TST. The sintered Ag layer became dense with a significant decrease in porosity at the compression parts and large horizontal cracks appeared at the tension parts. Both of horizontal cracks and vertical cracks led to fracture mode change and shear strength decrease. The Ag–Au joint interdiffusion layer became thicker during thermal shock with the gradual diffusion of the Au atoms into the sintered Ag layer. The die shear strength of the as-sintered Ag joint on the DBC substrates is evaluated as 34.4 MPa at 200 °C sintering but decreased to 0 MPa after 250 thermal shock cycles owing to the stress-induced delamination between the Ag and Au interdiffusion layer and sintered Ag layer. This study provides insights into the interface reaction and evolution of sintered Ag on ENEPIG-finished DBA and DBC substrates for applications at high temperatures.