Bright Sn, Sn-Cu and Sn-Pb layers, 3, 7 and 16 mm thick were electrodeposited on phosphor bronze cantilever beams in a rotating disk apparatus. Over a period of several days, Sn-Cu deposits develop 50 mm contorted hillocks and 200 mm whiskers, pure Sn deposits develop only 20 mm compact conical hillocks, and Sn-Pb deposit surfaces remain unchanged. Cantilever beam deflection measurements within 15 min of plating proved that all electrodeposits had in-plane compressive stress. The magnitude of compressive stress, as well as the degree of surface disturbance, increased in the composition progression, Sn-Pb, Sn, Sn-Cu. The differences between the initial compressive stresses for the alloys compared to that of pure Sn can be understood in terms of the precipitation of Cu(sub 6)Sn(sub 5) or Pb particles within supersaturated Sn grains shortly after plating. Longer time changes in beam deflection (15 min to 1 month) are caused by two processes: electrodeposit creep and the formation of a Cu(sub 6)Sn(sub 5) on the bronze/Sn interface that respectively decrease and slightly increase the compressive stress in the deposit. The creep is manifest in two forms, uniform and localized. Uniform creep occurs for the Sn-Pb which has an equiaxed grain structure. Localized creep in the form of hillocks and whiskers occurs for Sn and Sn-Cu which both have columnar structures. Compact hillocks form when the columnar grain boundaries are mobile, as for the Sn deposits. Contorted hillocks and whiskers form when the columnar grain boundary motion is impeded, as for the Sn-Cu deposits.
Citation: Acta Materialia
Pub Type: Journals
alloys, electrodeposition, Pb-free solders, stress, whiskers