Analysis of Dynamic Moisture Diffusion in Packaging Materials During Steam Sterilization

We present a model, verified by experiment, for predicting the moisture absorption and desorption of a range of plastic encapsulant and organic circuit board materials under both nonsaturated and supersaturated steam conditions. This paper was carried out as part of the development of embedded plastic-encapsulated modules (PEMs) for monitoring the internal atmosphere parameters of medical steam sterilizers (autoclaves). The rapid cycles of temperature, pressure, and humidity that are a feature of autoclave operation present unique challenges for the reliability of embedded PEMs. Analysis of moisture absorption and desorption in candidate encapsulant and circuit board materials was, therefore, required for prediction of both moisture ingress and of hygroscopic swelling. This analysis was carried out at three different temperature/humidity/pressure levels (85 °C/85%RH/atmospheric, 121 °C/100%RH/2 bar, and 134 °C/100%RH/3 bar), while desorption was analyzed at both ambient temperature/pressure and in a vacuum oven (0.6 and 0.3 bar) at 85 °C, 121 °C, and 134 °C. Interval weighing was used to monitor the moisture diffusion in the samples and to identify when saturation/desiccation was reached. The moisture diffusion coefficients and the saturated moisture concentrations of the materials were extracted from the test results and a model, verified by experiment, was developed to predict these values for any temperature and humidity combination between 85 °C/85%RH and 134 °C/100%RH. Measurements of hygroscopic swelling were also carried out as part of the analysis. This is believed to be the first detailed analysis of moisture diffusion in these types of materials over this range of parameters.