Evaluation of the Moisture Sensitivity of Molding Compounds of IC’s Packages

Plastic encapsulated devices are of great interest against their ceramic or metallic counterparts, as they permit significant cost reductions. However, they are more sensitive to moisture ingress, which can lead to reliability problems. Moisture can penetrate either through the polymer or along the interface between the leads and the encapsulant. Samples of 3 different polymers usually used for IC’s encapsulation, either pure or containing a lead frame with the lead terminations, were aged under different RH conditions. Results show that the penetration through the polymer is preponderant face to the progression along the leads. In a first approach, this penetration can be considered as a diffusion process, following the Fick’s law; the diffusion coefficient $D$ and the saturation coefficient $S$ can be deduced from the measurements as a function of the relative humidity. These values can be used in finite element simulations, in order to evaluate the moisture ingress as a function of time within an IC of a given geometry. Moisture diffusion within these new resins is a very long process. In fact, a first “saturation” occurs after a few hours of aging, but if samples are left longer in the oven, a new diffusion process occurs, not leading to a real saturation. Hence processes different than pure fickian diffusion occur in the polymers.