Electrodeposition of Sn–Ag Alloys and Evaluation of Connection Reliability for Automotive Connectors

Electrodeposition behavior of Sn–Ag alloys was investigated at current density 1–1000 A·m−2 in both sulfate and pyrophosphate-iodide solutions at 298 K, and the contact resistance of Sn–Ag alloys deposited on a Cu connector was evaluated. In both solutions, Ag behaved as a more noble metal than Sn, showing regular codeposition. The difference in deposition potential between Ag and Sn was 0.4 V in the pyrophosphate-iodide solution and 0.2 V in a sulfate solution containing thiourea as a complexing agent for Ag+ ions. The deposits obtained from a pyrophosphate-iodide solution consisted of blocks of a few microns in size, while those from a sulfate solution exhibited grains smaller than 1 μm. The deposits containing Ag less than 45 mass% were composed of an Ag3Sn intermetallic compound and Sn. This is in accordance with the equilibrium phase diagram of the binary Ag–Sn system. The contact resistance of deposited Sn–Ag alloys, after heating at 433 K for 120 h, was slightly smaller at Ag content below 45 mass% than that of reflow Sn plating. The connection reliability of connectors after abrasion was better in deposited films of Sn–Ag alloys than in those with reflow Sn plating.