Experimental studies of non-Fickian moisture diffusion in plastic packages

Moisture penetrating into the polymer and subsequent hygroswelling stress play an important role in the integrity and reliability of plastic electronic packages. In this paper, moisture absorption experiments of four types of plastic encapsulated microcircuits (PEMs) were conducted. Moisture diffusion characteristics of PEMs available by experiments were measured and typical moisture diffusion characteristics were calculated. Fickian second law controls distribution of moisture concentration in plastic packages, and it was found that moisture transport deviations from ideal Fickian behavior under the higher temperature and humidity stresses, which is the so-called non-Fickan diffusion. It was attributed to the “two-stage” sorption. Analysis of moisture diffusion coefficients shows that different packages have values varying largely. Besides, comparing the saturated moisture and vapor concentration, it can be found that the moisture exists with combining state of liquid and vapor in plastic electronic packages, which probably a main cause of non-Fickian moisture diffusion. So the experimental results indicated that it's necessary to measure the moisture diffusion coefficient in plastic electronic packages in order to reduce potential risk during their assembly process and long-term applications.