A comparison of the theory of moisture diffusion in plastic encapsulated microelectronics with moisture sensor chip and weight-gain measurements

Abstract
In this paper, the issues pertaining to moisture diffusion in PEMs are explored and discussed. The existing models of moisture diffusion in plastic molding compounds and PEMS are reviewed. Results, modeling and analysis of moisture sorption experiments performed in this study are presented. The moisture sorption experiments were conducted on a set of PEM samples with a common type of encapsulant material to 1) characterize sorption behavior; 2) compare weight gain measurement to the measurement of moisture concentration using a moisture sensor device at the die surface; 3) assess the moisture sensor measurement method. In the case of PEM samples tested in this study, simple Fickian diffusion was shown to agree closely with the experimental results. In one case, a relatively small anomaly from Fickian diffusion was observed and was attributed to swelling and relaxation phenomena at later stages of moisture sorption in the molding compound. The calibration constants determined for the sensors in this study were found to be significantly different from those collected by the manufacturer prior to the encapsulation of the devices. This problem is believed to be degradation in sensitivity of the moisture sensor due to exposure to high temperatures and storage conditions.

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