Finite difference modelling of moisture diffusion in printed circuit boards with ground planes

A finite difference model has been applied to diffusion modelling of molecular moisture diffusion in Printed Circuit Boards (PCB) that have ground planes. Capacitance measurements between two ground planes within a PCB during moisture uptake and removal allowed diffusion coefficients to be determined. These diffusion coefficients were then used to look at the effect of ground plane width, hole density and meshed ground plane dimensions on bake time. The effect of plating holes on the bake time has also been investigated. Results show that whilst a few hours are sufficient to bake PCBs with no ground planes, days, or even months are required to remove moisture from within ground planes due to the extra perpendicular distance the moisture must diffuse. The results demonstrate the importance of storage conditions of the PCB, as once moisture has diffused into a board with ground planes; it is often not feasible to remove it.

► Moisture diffusion has been modelled in printed circuit boards with ground planes.
► Ground planes without openings require unrealistically long bake times.
► The decrease in bake times for meshed ground planes and holes is presented.
► Under certain conditions, baking can increase local moisture content.
► Diffusion coefficients for FR4 are fitted to the Arrhenius equation.