Conformal coatings are applied to protect electronic assemblies from adventitious environmental factors, which include, for example, corrosive gases, corrosive fluids and high humidity. Whenever there is a significant level of humidity, there is always the opportunity for parts of the assembly to drop beneath the dew point, thus resulting in the formation of condensed water on the surface of the assembly, which can significantly reduce the insulation resistance of the boards surface, resulting in malfunctioning electronics. While the characterisation of coating performance under high humidity conditions is detailed, in well accepted IPC and IEC standards, the performance and testing under condensing conditions is not so well developed. This situation largely reflects the hardware challenge. Most humidity chambers are designed to achieve stable, well controlled humidity and temperature conditions, but none of these offer condensing options. Therefore the user has to improvise. A common approach to attempt to achieve condensing conditions is to ramp at a fast enough rate to cause condensation, a feature the humidity chamber designers have by and large, successfully managed to remove. An immediate drawback of this approach is that chambers of different designs will perform differently, and will be sensitive to small drops in cooling performance. An alternative approach is the BMW test K15 from GS 95024-3-1, where a tray of water is introduced to the chamber and heated to cause excess moisture, and hence condensation. The sensors in the chamber detect the additional moisture and will work to reduce the humidity level to the required set-point. Thus, once again, the level of experimental control is poor, and the actual conditions on the test-sample are not known. The NPL, have developed a new approach, where the test board is mounted on a substrate whose temperature can be independently controlled without changing the ambient condition. Thus, the temperature of the test board can be depressed below ambient to any desired point and hence, produce condensation at different levels. It is then, therefore, straightforward to cycle between condensing and non-condensing conditions on the test board in a constant ambient environment. The technique has been demonstrated to be repeatable and controllable, with the user able to select a temperature differential that matches their worst in-use conditions, or to understand the performance of their system under a range of condensing conditions. The data for a range of conformal coatings are presented, and correlated back to the conformal coating material type, and coverage and thickness by cross-sectioning.
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