Mechanism of Cu6Sn5 layer act as a diffusion barrier layer

Interfacial intermetallic compound (IMC) is a important factor for the reliability of soldering joints. Too thick intermetallic compounds layer will reduce the reliability of soldered joints seriously. And stable intermetallic compound Cu₆Sn₅ phase initially formed at the interface after reflowing is necessary for good quality of soldering joints. In the present study, scallop-like Cu₆Sn₅ was used as diffusion barrier layer between lead-free solder alloy and cooper substrate. Effect of the diffusion barrier layer for reflowing with several holding time was investigated. In addition, the element diffusion behavior and the change of IMC layer in solid-state isothermal aging process have been analyzed from the perspective of intermetallic compounds layer act as a diffusion barrier layer. Results indicate that, 1) The IMC layer initially preparation on cooper substrate act as a diffusion barrier layer between lead-free solder alloys and cooper substrate is feasible. 2) The effect of thinner IMC layer (Average thickness is about 0.87 μm) is more obvious compared with a thicker IMC layer during reflowing process. 3) The impact of the diffusion barrier layer is remarkable for a shorter time reflowing; 4) Cu₆Sn₅ layer can effectively restrain elements diffusion in Sn-0.5Cu soldering joints during thermal aging progress, but not in pure Sn soldering joints.