Moisture Measurements in PCBs

Chris Hunt, Owen Thomas & Martin Wickham
National Physical Laboratory
Teddington, UK

High levels of residual moisture in PCBs are problematic and can result in delamination during soldering and rework. Moisture accumulates during storage and industry practice recommends specific levels of baking to avoid delamination. This paper will discuss the use of capacitance measurements to follow the absorption and desorption behaviour of moisture. The PCB design used in this work, focused on the issue of baking out moisture trapped between copper planes. The PCB was designed with different densities of plated through holes and drilled holes in external copper planes, with capacitance sensors located on the inner layers. For trapped volumes between copper planes, the distance between holes proved to be critical in affecting the desorption rate.

For fully saturated PCBs, the desorption time at elevated temperatures was observed to be in the order of hundreds of hours. Finite difference diffusion modelling was carried out for moisture desorption behaviour for plated through holes and drilled holes in copper planes. A meshed copper plane was also modelled evaluating its effectiveness for assisting moisture removal and decreasing bake times. Results also showed, that in certain circumstances, regions of the PCB under copper planes initially increase in moisture during baking.

Copper planes have a significant effect on moisture desorption and hence consideration to their design should be given to bake out requirements. Increasing the bake time, whilst removing more moisture and reducing the delamination risk, can have detrimental effects to the solderability and mechanical properties of the PCB, such that a compromise in bake time should be sought. In many cases, when moisture has diffused under copper planes, it is not feasible to remove the moisture. Hence preventative measures are required to stop moisture entering the PCB in the first place.

The removal of moisture may not be straight forward, as was shown for two equal 12 mm square copper planes where in the central region the moisture initially increased during bake out, as the moisture initially equilibriates before diffusing out of the board. If such moisture increases are into a delamination prone region, it is conceivable that baking may in some cases have an unwanted effect of increasing delamination risk.

Modelling has shown that meshed copper planes can significantly reduce bake out times, with a clear functional dependence on the open area of the mesh.