Correlation between interfacial microstructure and bonding strength of sintered nanosilver on ENIG and electroplated Ni/Au direct-bond-copper (DBC) substrates

• Nanosilver paste was used to connect dies and substrates with ENIG and Ni/Au finishes.
• A solid solution layer and a delamination region generated in ENIG joint but not in Ni/Au joint.
• The ENIG joint exhibits higher diffusion coefficients than the Ni/Au joint.
• A modified sintering process can effectively suppress the formation of the delamination region in ENIG joint.

We studied the diffusion bonding mechanism and interfacial reactions between sintered nanosilver and direct-bond-copper substrates with different surface finishes, i.e., electroless nickel immersion gold (ENIG) and electroplated nickel gold (Ni/Au). The correlation between the interfacial microstructure and bonding strength of sintered nanosilver joints was discussed. The shear test results indicated that the strength of the Ni/Au joints was approximately three times higher than that of the ENIG joints. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images showed that the interface was bonded well in the Ni/Au joint. However, a delamination region appeared along the interface in the ENIG joint. Excess diffusion of Ag element into the immersion gold layer during sintering at high temperatures resulted in poor densification of the sintered nanosilver adjacent to the Au surface. As a consequence, a continuous AgAu layer and the delamination region formed between the sintered Ag layer and the AgAu layer and a weakened the ENIG joints. A modified sintering process involving a preheating stage at 150 °C for 5 min and a sudden increase to 275 °C held for 30 min effectively reduced the poor densification and increased the shearing strength of the ENIG joints.