Corrosive Effects of Solder Flux on Printed-Circuit Boards

A number of tactical radio sets have been reported inoperable as a result of lead breakage of transistor and quartz crystals. The lead material was a Kovar
^TM
-type iron-nickel-cobalt glass sealing alloy (54 percent Fe-29 percent Ni-17 percent Co). The location and type of fractures indicated that these leads failed by stress-corrosion cracking. Such failures result from the combined effects of mechanical stress and a mildly corrosive atmosphere. The only source of specific corrodent for this type of failure was chloride ions found in residual flux on printed circuitry. The corrosive effects of the residual flux was demonstrated by the identification of corrosion products found on the printed wiring. Careful selection of the solder flux, scrupulous cleaning of the printed-circuit boards, and conformal coating is recommended for all similar applications.