The Effect of Gold-Tin Intermetallic Compound on the Low Cycle Fatigue Behavior of Copper Alloy C72700 and C17200 Wires

Many connectors and springs fabricated from copper base spring materials require a gold-plated surface finish. Often such parts are joined or interconnected to the electronic package by soldering with tinlead solder. Upon application of heat, the gold plating may interact with the tin contained in the solder to produce a brittle intermetallic compound. Such a compound may adversely affect the fatigue resistance of the connector or spring. The effect of gold-tin intermetallic compound on the low cycle fatigue behavior of two high strength copper base alloys in wire form was studied. Alloys C72700 (Cu, 9 Ni, 6 Sn) and C17200 (Cu, 1.9 Be) in the form of 0.009-0.011 inch (229-279µm) diameter wires were gold plated using two techniques. Immersion plating was used to produce a gold plating thickness of 5-10 µinch (0.12-0.25 µm). Electroplating was used to produce a gold plating thickness of 30 µinch (0.75 µm) on a second sample lot. Tin-lead solder (60 Sn 40 Pb) was then electroplated over the gold. The samples were then heated to a temperature just slightly below the melting point of the solder to produce the desired surface layer of gold-tin intermetallic compound. Fatigue tests were made on a commercial wire fatigue tester which is based on the principle of a rotating Euler column. The fatigue data are presented in the form of Weibull probability distribution plots of the fatigue life, N
t
at a given maximum bending stress level S
b
. The effect of either type of gold plating, gold and solder plating with and without intermetallic compound formation, on fatigue was studied. Immersion plating of gold had no effect on fatigue while electroplating caused a significant decrease in fatigue of the alloy wires studied. The formation of the gold-tin intermetallic compound caused a drastic decrease in fatigue life (by a factor of five to one). It is therefore recommended that every effort be made to eliminate or prevent the occurrence of the gold-tin intermetallic compound in critical areas where connector or spring surfaces are subjected to fatigue.