Corrosion Factor and Effects of Tin - Zinc Lead-Free Solder on Copper Substrate in Environmental Tests

摘要

We coated copper substrate with tin-zinc lead-free solder (Sn-9Zn and Sn-8Zn-3Bi), and then we performed the following corrosion tests: the salt mist test, the gas corrosion test, and the weathering test. Following the tests, we visually inspected specimen surfaces and cross-sections to determine the causes of corrosion and the extent of the corrosion to the base substrate. By the surface condition of the Sn-Zn solder following the various tests, it was found that corrosive substances in the environment (such as sulfur and chlorine) reacted with the Zn in the solder to form corrosive products on the surface of the solder. After 18 months of the weathering test, cross-sectional analysis revealed that the Sn-Zn solder had been oxidized to a depth of 10um. However, the copper substrate underneath showed no evidence of corrosion. On the other hand, conventional Sn-Pb eutectic solder was oxidized to a depth of 20um, and the copper substrate also showed corrosion. We hypothesize that the corrosion of Zn in the Sn-Zn solder yielded a sacrificial corrosion effect by forming an intermetallic compound layer (Sn-Zn layer) between the base substrate and the solder. As a result, Sn-Zn solders exhibited corrosion resistance far superior to that of conventional Sn-Pb eutectic solder.