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Characterisation of Plastic Encapsulants for Packaging of Non-Hermetic Electronic Modules for Monitoring of Steam Sterilizers
Keywords: Steam Diffusion Characteristics in Encapsulants, Packaging for Sterilizable Electronics, Steam Sterilization Monitoring
Steam sterilizers present a challenging operating environment of 134C and 3 bar saturated steam. As part of the development a plastic-encapsulated sensor module for monitoring of sterilizers, the moisture absorption and steam diffusion characteristics of a range of encapsulant and PCB materials (Table 1) were measured at 134C/100%RH, 121C/100%RH and 85C/85%RH. The weights of the samples were monitored at regular intervals until saturation and the steam diffusion coefficients and saturated moisture concentration of the material were extracted. [Types of Encapsulants: a) FP4460 from Henkel, b) FP4650 from Henkel, c) OD2002 from EPO-TEK, d) 353ND from EPO-TEK, Types of Substrates: a) RF-35A from Taconic, b) NX9520 from Nelco, c) HTFR4 from Nelco, Table 1: List of Encapsulants and Substrates] Below 100C at room pressure, moisture diffusion and saturated moisture concentration are dependent on RH. However, above 100C and at elevated pressure, the relative humidity is always 100% regardless of temperature and consideration of absolute humidity (AH) is more logical since AH is different at each temperature and vapour pressure. From the test results presented, it was found that the steam diffusion coefficient is increased by 8-15 times at 121C/100%RH and by 10-20 times at 134C/100%RH compared with 85C/85%RH. Also, saturated moisture concentration of the material is directly dependent on the AH. A model was developed from these results for predicting steam absorption rates and saturated moisture concentration of materials at any temperature and vapour pressure and the model was verified at test conditions different to those of the original experiments: 126C/100%RH and 100C/100%RH. The model was able to successfully predict both steam diffusion and saturated moisture concentration with accuracy less than 10% for the different materials. The work reported in this paper puts in place a model which can be used to predict the steam diffusion characteristics of encapsulated modules.
Jinto George, Ph.D. Student
Cork Institute of Technology
Bishopstown, Cork

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