Ionic polarisation layers in polymer electrolytes

When high electrical fields are applied, and especially above the glass transition temperature, ion transport through the epoxy molding compounds that encapsulate the integrated circuit (die) strongly increases, leading to the accumulation of charge at the passivation-epoxy interface, and possibly to leakage currents because of the formation of parasitic transistors. Recovery from this failure mechanism is possible after annealing the package in the absence of the applied voltage, at sufficiently high temperature. The high electrical fields originate either from within the package due to high-voltage bondpads or are due to an external high-voltage source. We have developed a numerical transport model that is able to describe ion transport through IC packages including the formation of 'ionic diffuse layers', or 'polarisation layers', at the passivation-epoxy interface. In these layers the ion concentration strongly increases leading to a modification of the electrostatic boundary conditions (Gauss' law), as well as to a voltage step over the passivation-epoxy interface. The exact distribution of the ions in this diffuse layer is a very important quantity as it strongly influences the voltage profiles in the bulk of the material via the boundary conditions, and therefore the ion transport rates. We will briefly outline how the relation between charge and voltage (integral capacitance) can be obtained experimentally from the dielectric response when a DC voltage is applied across a thin polymer sample. The data that are obtained can be used to derive realistic microscopic models for the ion distribution in the diffuse layer formed within an epoxy resin next to electrodes and passivation layers.