Metal problems in plastic encapsulated integrated circuits

The long-term reliability of plastic encapsulated integrated circuits in humid environments is ultimately limited by the effects of entering moisture. Degradation of device characteristics may be acceptable in some types of circuits, but catastrophic failures ("opens" and "shorts") are certainly not acceptable. Catastrophic failures can occur in a) the metalization used on the silicon chip itself, b) the internal wires, beam leads or frames, and c) the interfaces between these parts (and others) used in making a complete package. Entry of moisture permits electrochemical corrosion or deplating and replating of metals. This occurs under operating conditions, accelerated by externally applied potentials, and also in the unoperated condition due to EMFs produced by couples between dissimilar metals or by built-in potentials of p-n junctions. Choice of metal systems, platings, and passivation geometries determine the useful life and modes of failure of plastic encapsulated integrated circuits exposed to moisture.