Humidity acceleration factor for plastic packaged electronic devices

Failure mechanisms associated with the effects of humidity on plastic packaged devices are briefly reviewed. Relevant experimental and theoretical work on these failure mechanisms is then considered, pointing to an incomplete understanding as to why relative humidity is the appropriate variable, along with temperature to use in acceleration factor models. The BET equation (Brunauer, Emmett and Teller), is invoked to provide a theoretical underpinning to the use of relative humidity in such acceleration factor models. The BET equation provides such a connection by relating surface concentration of water to vapour pressure of water. Using the BET equation as a starting point, we derive a relation between surface concentration of water and relative humidity that has no explicit temperature dependence. We then consider a simple kinetic model for a corrosion reaction leading to failure. The result is an acceleration factor that is a typical Arrhenius form, multiplied by a simple function of relative humidity. This form is similar to other power‐law acceleration factor models that have been developed empirically. Now a physical derivation is offered as a theoretical basis for power‐law models of the effects of humidity on plastic packaged device reliability.