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Condensation Testing - A New Approach



Condensation Testing - A New Approach
The data for a range of conformal coatings are correlated back to the material type, and coverage and thickness by cross-sectioning.
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Authored By:


Chris Hunt, Ling Zou
National Physical Laboratory

Phil Kinner
Electrolube Ltd

Summary


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.

Alternatively chambers run very close to 100% relative humidity and hence at this condition condensation will occur in various parts of the chamber. An immediate drawback of these approaches is that chambers of different designs will perform differently, and will be sensitive to small drops in cooling performance.

There are many alternative approaches to achieving condensation, and these are described in ISO, IEC, ASTM and others, and commonly attempt to drive a chamber into producing condensation, against the anticipated use condition, and hence sensors in the chamber detect the additional moisture and will work to reduce the humidity level to the required set-point. Thus, the level of experimental control will be very dependent on the chamber performance, and variability across chamber manufacturers can be expected.

A new approach has been developed 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.

Conclusions


A review of condensation testing reveals that there are many approaches to achieving the required condition. These approaches struggle to achieve a known and uniform steady state across the test vehicle, and typically attempt to achieve condensation while fighting the inherent control system of the humidity chamber

This paper presents a new technique for achieving condensing conditions on circuit boards, and utilises an approach of using a platen as an independent means of controlling the substrate temperature to induce condensation, which has been shown to be uniform. Hence complete control of the condensing conditions can be achieved in a wide range of temperature and relative humidity climatic conditions, and the level of condensation can be easily adjusted and maintained over long periods of time and readily cycled through those conditions. Since the whole test board is cooled on a platen the condensation film that forms will be uniform across the surface.

This paper has shown that as the test becomes more severe the condensing conditions will find weaknesses in the coating quickly. While humidity SIR and a low profile condensing test, the test vehicle passed, as the geometry became more challenging weaknesses and failures were observed. The results confirmed the susceptibility of exposed edges under condensing conditions, and that complete coating coverage is crucial

The thicker polyurethane materials demonstrated much greater resistance to condensing environments, with polyurethane-1 in particular showing little change in SIR during the condensation events.

It was surprising how quickly the insulation resistance of the acrylic materials and polyurethane-2 dropped with the onset of condensation - the drop in SIR being almost instantaneous. Condensation conditions will find weaknesses in the coating quickly.

Of particular surprise was the poor performance of the nano-coating. It showed no resistance to the condensing environments whatsoever, and corrosion was evident on the traces at both 1oz and 3oz copper track thickness. Although the coating contained a fluorescent trace it was impossible to determine coverage by cross-section.

Therefore it is clear from both the SIR results, the cross-sections and the visual inspection that conformal coating coverage is crucial in providing protection under condensing environments. There was a clear correlation between coating thickness and coverage and SIR under condensing environments.

Initially Published in the IPC Proceedings

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