Effects of Post-annealing Condition on Interfacial Reaction and Bonding Strength of Cu/Sn-3.0Ag-0.5Cu/Al Joint for Heat Dissipation Module Application

Abstract

The effects of post-annealing times at 150°C on the bonding strength and interfacial characteristics of Cu/Sn-3.0Ag-0.5Cu (SAC305)/Al were systematically evaluated. In the as-bonded sample, Cu₆Sn₅ intermetallic compound (IMC) was formed at the upper Cu/SAC305 interface, while Ag₃Al IMC was formed at the lower SAC305/Al interface, and a black layer of Al₂Cu IMC was formed at a distance of about 20 µm from the Al interface. The Al atoms from the Al pad diffused towards the SAC305 solder during the bonding process and reacted with Cu to form the Al₂Cu IMC. After post-annealing for 500 h at 150°C, the growth of Cu-Sn and Al₂Cu IMCs followed a linear relationship with the square root of the annealing time, which could be understood by a diffusion-dominant mechanism. To measure the bonding strength, the lap shear test was evaluated on the Cu/SAC305/Al structure with time during post-annealing at 150°C. Initial bonding strength of around 118.80 MPa decreased to 92.4MPa after post-annealing for 200h, and then remained constant until 500 h. And the fracture locus was the mixed mode of SAC305/Al₂Cu interface and cohesive inside SAC305 solder. Also, when the post-annealing time was increased from 150 h to 200 h, the fracture toughness decreased sharply from 438 J/m³ to 100 J/m³. This decrease seems to be closely related to an increase in the SAC305/Al₂Cu interface fracture mode. This is believed to be due to the increase in the SAC305/Al₂Cu interface fracture mode due to the growth of the Al₂Cu layer thickness.