Moisture Measurements in PCBs and Impact of Design on Desorption Behaviour

Published:

September 21, 2018

Author:

Chris Hunt, Owen Thomas, Martin Wickham

Abstract:

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....

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Company Information:

National Physical Laboratory

NPL is the UK's National Measurement Institute, and a world-leading centre of excellence in developing and applying the most accurate measurement standards, science and technology available.

Middlesex, United Kingdom

School

  • Phone +44 20 8977 3222

National Physical Laboratory website

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