Condensation Testing—A New Approach

Estimated reading time: 20 minutes

The results in Figure 12 show that the two polyurethane coatings behave in a very similar mode as shown in Figure 10. The nano coating results are not shown here as they also performed similarly to before. However, now the two acrylic results are different, with the single coated acrylic showing a large drop in SIR, indicating that the corners of the tracks are not covered. Supporting this conclusion, the image in Figure 13 shows corrosion at the track edge, and with corrosion products forming across the top of the coating between the anode and cathode. The acrylic-1 results show the SIR dropping to 106Ω from the first condensation event, with the behavior remaining broadly consistent through the following condensation cycles. With the double coated acrylic the SIR results are not as bad, but SIR values are dropping below 108Ω, and with a trend of decreasing SIR with each condensation cycle. The 1oz copper SIR pattern boards the results in Figure 9 do not show any difference between the single and double coated boards, but with the 3-oz copper SIR pattern boards the results do show a clear difference, and furthermore the acrylic coating can be seen to failing in either single or double coated condition. Hence the geometry challenge of the 3-oz copper tracks for the acrylic-1 coating has been shown by it failing the condensation test. From above this coating passed the humidity test at 40°C/93%RH, and the same coating with the 1oz copper tracks also passed. Thus the incremental geometry challenge and the fine control of the condensing condition in this experimental arrangement allows detailed and robust characterization of coating performance.

Figure 13: The 3-oz SIR pattern coated with acrylic-1 with evidence of corrosion from the track edges.

Discussion

It is well known that water films will lead to anodic corrosion on powered circuitry and that conformal coatings are a suitable mitigator. However, this requires complete coverage of all the parts so that the water film has no access to the energized parts of the boards. Hence there is an issue with achieving complete perfect conformal coating coverage.

The technique described here that has just been developed and offers a wide range of flexibility in the test conditions. In the current setup controlled levels of condensation can be achieved above 30°C and a range of humidities. A crucial aspect of any condensation technique is the ability to achieve a number of aims: repeatability, controllable and stable conditions, and maintain those conditions over any desired time period. Within the many existing testing methods that were mentioned above there is a great challenge to achieve a fully controlled condensing condition across the wide range of equipment manufacturers, and hence there is a struggle to achieve all four aims in the wider context of standards testing. However, the approach described here has this ability, and furthermore these four aims are easily met.

The challenge of achieving stable and controlled condensing conditions over large areas is understood by a consideration of the dew point and the onset of condensation. This work shows that the SIR of the uncoated coupons, begins to drop noticeably when the temperature of the platen was dropped below the dewpoint. At 40°C and 85% RH the dewpoint is 37°C, and the onset of degradation was noticeable at 36.5°C. At 40°C/93% the dewpoint is 39°C and the SIR drop was noticeable at 38.5°C. These results are important in showing that there is good agreement between the platen and dew point temperature, and confirms that the platen arrangement is performing as expected.

The experiments described here show clearly that while test boards may pass simple humidity testing, as the geometry of the surface becomes more challenging test boards will fail under condensing conditions if complete coverage is not achieved, as was observed with the acrylic-1 results. In Figure 8 the SIR results for acrylic-1 are very good, for the 1-oz copper tracks in Figure 9, there is some susceptibility to condensation cycles but the SIR values are still high and above 109Ω. However, in Figure 12 with the 3-oz copper track the SIR values are now at the 106Ω level, which is classed as an SIR failure. Hence, the difficulty in achieving good coverage with these acrylic materials became apparent as the copper track thickness increased, and even with the application of two separate coating layers the performance was problematic.

These results clearly demonstrate the need for condensation type testing for finding the weaknesses in coating coverage, and furthermore the control that can be achieved with this experimental setup allows this to be readily achieved than has hitherto been possible. However, while specific condensing conditions can be achieved the desired test parameters still need to be developed, and this will be the scope of further work.

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 utilizes 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.

References

1. BMW GS 95024-3-1 (Test K15).

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