Prevention of Stress-Corrosion Failure in Iron-Nickel-Cobalt Alloy Semiconductor Device Leads

It is now known that the iron-nickel-cobalt alloy, commonly used for semiconductor device leads, is susceptible to stress corrosion cracking. Though this failure mechanism is a common afflication of many metal alloys, stress corrosion cracking of the iron-nickel-cobalt glass sealing alloys (Kovar, Rodar, Therlo, etc.) with 28-29% nickel was not reported until recently. The work reported here confirms that stress corrosion cracking of this alloy can occur very rapidly in the presence of a combination of condensed atmospheric moisture and stress, such as that resulting from normal device manufacture and/or conventional circuit installation procedures. Detection of this phenomenon by accelerated tests is relatively simple and has been the basis for evaluating and introducing corrective measures to surmount this failure mechanisms. Factors intensifying the alloy's susceptibility to stress corrosion when used as semiconductor encapsulation leads have been identified. The porosity of gold finishes conventionally applied to facilitate device assembly, and impairment of the finish's integrity by cracks induced in the surface of the basis lead material through flexure, provide potential corrosion sites. The latter phenomenon results from the lack of ductility of a work-damaged surface layer created during the encapsulation manufacture.