Fretting Corrosion of Solder-Coated Electrical Contacts

The contact resistance of fretting solder-plated contacts may rise quickly due to the accelerated formation of insulating oxides on their surfaces. This condition becomes worse with increase in length of wipe and with reduction in normal load. The solder-plated versus gold-plated system shows a more rapid rise in contact resistance which can be explained by differences in the mechanics of oxide film fracture compared to that of all-solder-coated contacts. Many fluid lubricants are able to maintain low, stable contact resistance when applied to surfaces in thick (about 20 000 Å) layers, particularly on both contacts of a mated pair. The contact resistance of solder versus gold is more difficult to stabilize with lubricants than that of solder against itself, although lubricants may be useful provided that fretting distances are small. The effectiveness of coatings is due to their ability to greatly reduce oxygen arrival at the surface. When it is impractical to employ finishes other than solder on the contacts of separable connectors and when fretting can occur, it is recommended that coatings of viscous oils or greases be used to prolong contact life. An example of a useful coating is one which is obtained by immersion and withdrawal of the contacts from a mixture by weight of 20 percent polyphenylether fluid and 1 percent of microcrystalline wax in 1,1,1- trichloroethane.