Ion transport in encapsulants used in microcircuit packaging

Most semiconductor devices are encapsulated in epoxy molding compounds. These molding compounds contain ionic contaminants including chloride ions from epichlorohydrin used in the epoxidation of the resin and bromine ions incorporated into the resin as a flame retardant. Chloride ions are known to break down the protective oxide on the surface of aluminum metallization and accelerate corrosion. The encapsulant material is hydrophilic and will absorb moisture from the environment. When the absorbed moisture is combined with ions, there is an opportunity for electrolytic corrosion to occur on the metal surfaces of the device and package elements. However, the rate of corrosion in an encapsulated microcircuit may be expected to depend upon the rate of ion transport through the encapsulant. This paper evaluates two techniques for the measurement of ion diffusion in epoxy molding compounds used for microelectronic encapsulation. The data suggests that ion diffusion rates vary with molding compound formulation, the solution pH and the ion concentration. SEM-EDX analysis and TOF-SIMS analysis indicate that the mode of diffusion of ions in the encapsulants is primarily through the polymer resin matrix as opposed to diffusion at the interface of the resin and the filler particles. Calculated diffusion coefficients were slower than the literature values for moisture diffusion or the diffusion of gases. In fact, under basic conditions, the ions tend to diffuse through the molding compound almost as a front suggesting that the ions bind to the encapsulant and that the diffusion of ions in molding compounds can be modeled using a Type II non-Fickian model.