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 120h, 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.