Relationship Between Layer Structure of Three–Layer Reflow Tin Plating and Heat Resistance

Abstract

Reflow tin plating with high heat resistance was developed for in–vehicle connectors. The plating consists of three layers, with tin as the top layer, Cu–Sn intermetallic compounds（IMCs）Cu₆Sn₅ and Cu₃Sn as the middle, and nickel as the bottom layer and lies on surface of the copper alloy substrate. Contribution of each layer to heat resistance was investigated. Cross–sectional microstructure of the plating and surface of the nickel layer were examined by scanning electron microscopy. It was found that degradation of heat resistance in high temperature environment is caused by defects in the nickel layer located immediately below areas where thin IMCs layer was present. By increasing the coverage ratio of Cu₃Sn before heat treatment, the amount of thin areas of IMCs was decreased, and defects in the nickel layer was suppressed.

In addition to increasing the coverage ratio of Cu₃Sn, by thickening the tin and nickel layer a heat resistance of 175°C for 1000h was achieved.