Electrodeposition of Sn-Ag alloys and its connecting reliability for automotive connectors

Electrodeposition behavior of Sn-Ag alloys was investigated at 1 to 1000 A/m ² in both sulfate and pyrophosphate-iodide baths of 298 K, and the contact resistance of Sn-Ag alloys deposited on Cu connector was evaluated. In both baths, Ag behaved as more noble metal than Sn, showing the typical feature of regular type codeposition. The difference of deposition potential between Ag and Sn was 0.4 V in pyrophosphate-iodide bath, while it was 0.2 V in sulfate bath containing thiourea as complexing agent for Ag ⁺ ions. The deposits obtained from pyrophosphate-iodide bath consisted of blocks of a few micron in size, while those from sulfate bath showed grains smaller than 1 μm. The deposits containing Ag less than 45 mass% were composed of Ag ₃Sn metallic compound and Sn in accordance with the equilibrium phase diagram of binary Ag-Sn system. The contact resistance of deposited Sn-Ag alloys after heating at 433 K for 120 hours was somewhat smaller at Ag contents less than 45 mass% than that of reflow Sn plating. The connecting reliability of connector after abrasion was better in deposited films of Sn-Ag alloys than in reflow Sn plating.