Abstract

Corrosion in hermetically sealed electronic devices is a well-known failure mechanism that can occur when moisture in a sealed cavity condenses and destroys metallization by chemical or galvanic action. Early military standards imposed a 5000 part per million by volume (ppmv) limit on internal water vapor (IWV) in hermetic parts measured by mass spectrometry. In a recent product lot IWV non-conformance, 15 of 37 product lots and 56 of 369 units were nonconforming, with IWV ranging from 5000 to 32,000 ppmv. Root cause was outgassing from a non-robust die adhesive. Functionality of non-conforming units was investigated. Of 149 units selected from non-conforming product lots and lifetested under both static and 17.7V bias conditions at 2°C (assuring condensate in non-conforming units) for 2000 hours, none failed functionally. Subsequent analysis of 27 of these units found 17 exceeding 5000 ppmv IWV with one unit as high as 90,000 ppmv IWV. Despite the certainty of condensate internally during life tests, all units maintained functionality. Units were further investigated by internal visual inspection, glass integrity testing of die passivation, and SIMS analysis of internal surfaces for ionic impurities. The units passed all inspections and were free of any ionic impurities detectable to the few ppm level by SIMS. IWV is clearly a necessary precursor to, but not a sufficient condition for, corrosion failure. This paper discusses these results and proposes an alternative IWV criterion for product assurance.

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