Internal Stress in Electrodeposited Tin-Nickel Alloy

SYNOPSIS

Measurements have been made of the internal stress in deposits of tin-nickel alloy. The method of Brenner and Senderoff was used. The alloy can be deposited over a range of 36,000 lb/in² tensile to 40,000 lb/in² compressive stress. The factors affecting stress are plating bath temperature, current density and chloride content. The change from tensile to compressive stress, brought about by either raising the bath temperature or adding choride, is in both cases accompanied by a small increase in the nickel content of the deposit. When plated on flat panels of steel, the deposit tends to crack when the stress exceeds 7,000 lb/in² tensile. Examination of the effect of chloride suggests that deposition occurs from a complex ion of the type (NiSnF₄Cl₂)--, in chloride-fluoride solutions and from (NiSnF₆)-- in all-fluoride solutions. A simple equation relates stress, bath temperature and current density in an all-fluoride electrolyte. For certain applications, it is recommended that, in making up a bath from the published instructions, 50 g/l ammonium chloride may be added to avoid all possibility of obtaining tensile stressed deposits initially. Bath maintenance increases the chloride concentration in worked baths and these naturally tend to give compressive stress in the deposit; no chloride addition is thus necessary in tin-nickel baths in present use.