Measurements and FE-Simulations of Moisture Distribution in FR4 based Printed Circuit Boards

Abstract

Studies made by PCB material suppliers concerning the behaviour of PCB towards moisture are mainly done on "raw" material. They don't take into account the presence and repartition of other materials for instance conductive layers. To get knowledge of the behaviour of the actual PCB's with humidity, different measurements were carried out. The desorption during baking at temperatures from 80deg to 120degC, the absorption curves in ambient air (23degC/45%RH), as well as in dry pack storage conditions, and forced absorption under controlled atmosphere (85degC/85%RH) were measured. The absorption and the adsorption were determined by weighting. The saturation concentration at 85%RH/85degC for pure FR4 samples was 11,600 ppm. The moisture ingress depends on the internal track repartition, and thus cannot be directly assessed by measurements. The use of FE-simulations can give information about the moisture distribution in the structures. Under the assumption that the uptake of humidity and the desorption, in composites follow more or less a Fick's law, the measured diffusivities were used as input parameters for FE-simulations. A very good agreement between measurement and simulation for absorption as well as desorption was found for the different temperatures. From the FE-simulations it was found that 8 hours of baking to achieve a dryness of 800 ppm at T=120degC are only sufficient for pure FR4. For samples with copper layers, the baking must be longer. This result proves that copper layers act as blocking planes regarding moisture absorption or desorption. Delamination risks are also investigated. With this simplified model, an easy and fast determination of the diffusion process in actual PCB's, including the conductive layers, is possible