The Influence of Plastic Encapsulants and Passivation Layers on the Corrosion of Thin Aluminium Films Subjected to Humidity Stress

The study of moisture induced aluminium corrosion in the presence of passivation layers and plastic encapsulants has shown that the corrosion rate is determined by the surface conductivity of the water layer that forms on the die surface. The surface conductivity is determined by the external environment and the adhesion of the layer in contact with the die surface (which together control the water film), the impurities that enter the water film from the plastic or passivation layer and the applied bias. The impurities can accelerate the corrosion rate by breaking down protective oxides on the aluminium but they also can delay the onset of severe corrosion by forming space charges that limit the electrolytic currents that can flow. Silicone and silicone-epoxy resins give the best protection against corrosion when used alone followed by epoxy anhydride and epoxy novolac resins in that order. With the best passivation layer used, 1.8% phosphorous content CVD silicon dioxide, the lifetime was dependent on the choice of plastic encapsulant. The lifetime was only independent of the type of plastic encapsulant for die passivation layers containing high levels of impurity. The temperature dependence of the anodic and cathodic aluminium corrosion mechanisms is independent of the nature of the impurities found in a plastic encapsulated component and can be described by an activation energy of about 0.75 ev.