Electrochemical corrosion and electrochemical migration of 64Sn-35Bi-1Ag solder doping with xGe on printed circuit boards

Abstract

Ge-doped effects on electrochemical corrosion (ECC) and electrochemical migration (ECM) behaviors of 64Sn-35Bi-1Ag (SBA) solder on printed circuit boards (PCBs) in 3wt% NaCl solutions were discussed. ECC was conducted by potentiodynamic polarization and electrochemical impedance (EIS) tests. Dendrites growth is a result of electrochemical migration (ECM) process, which is another factor affecting soldering reliability for electronic packagings, it was carried out as a designed schematic diagram coupled with a direct current power supply. Microstructures of products were investigated by SEM coupled with EDAX, XRD techniques. ECC results showed that anti-corrosive capacity increased as an increase of Ge addition in SBA solder which was expressed as the percent corrosion prevention (IE%), there was a maximal IE% value (83.74) when Ge addition was 0.5wt%, this obtainment was consistent to EIS results. The probable reason was that Ge had a “skin collection” effect, that was, it highly enriched and segregated at edge of SBA solder, which protected Sn in SBA solder from being corroded. Microstructure of corrosive products was distinctly different for SBA solder with and without Ge addition. ECM results showed that the dendritic grown rate of pure SBA solder was slower than those of its candidates with Ge dopings. Dendrites of SBA solder with 0.5wt% Ge addition grew to about 100–150nm in length and 20–50nm in width after ECM test for 8h, they were nanorods arranged trimly into arborization. While dendrites was shorter under the same condition for pure SBA solder without Ge doping. The dendritic growth mechanism was tentatively discussed. It showed the loaded electronic field intensity during ECM process played an important role for dendritic formation owing to its controlling the dendritical crystal anisotropic growth rate. Medium concentrations had an another key effect, which affected by controlling the ion diffusion from anode to cathode. From the point of microelectronic packagings, Ge addition was advantage to improve anti-corrosion capacity of SBA solder, but it also reduced soldering reliability if ECM process took place in surrounding of wet and thermal weather.