Authored By:
Carolyn Taylor, Phil Kinner
Electrolube Ltd
Leicestershire, UK
Summary
Conformal Coatings are often used to increase the reliability of electronic assemblies operating in harsh or corrosive environments where the product would otherwise fail prematurely. Conformal coatings are often qualified to international standards, intended to enable users to better differentiate between suitable conformal coating chemistries, but always on a flat test coupon, which is not representative of real world use conditions.
In order to better correlate international standards with real world-use conditions, three-dimensional Surface Insulation Resistance (SIR) test boards have been manufactured with dummy components representative of those commonly used on printed circuit assemblies.
A variety of commercially available, internationally qualified, conformal coatings have been applied to these coupons by a variety of common methodologies including dip, spray and selective-spray, at a variety of thicknesses. The applied conformal coatings were cured in accordance with the manufacturer's recommendations. The conformal coating thickness and coverage over critical areas was assessed by non-invasive optical methods.
The coated samples were then subjected to 1000 thermal shock cycles (-65 degrees C to + 125 degrees C) and salt-mist cycles to represent typical end use qualification testing. Voltage was applied to the SIR boards during the salt-spray test regime to better correlate to real use conditions. The corrosion evident on assemblies was visually assessed by optical microscopy under 4- 40X magnification and compared with the measured SIR to assess corrosion resistance of the various process combinations.
Lead-free solder was used exclusively for this test, and water-washable, cleanable 'no-clean' and no-clean flux samples were included, to investigate the effect of cleaning on the overall reliability of the coated system.
Conformal coating thickness and coverage were assessed for the various coating techniques. The results of the thermal shock and powered salt-spray test results are correlated back to the application method, coating thickness, flux, cleaning and coating chemistry to determine the best overall process and material combinations for high reliability applications.
Conclusions
IPC-CC-830 is intended to discriminate high performance materials from materials that provide lower performance. However, all qualification data is produced on flat 2D, FR4 test coupons finished with bare Copper.
This work shows clearly that the flat-test board used for IPC-CC-830 qualification does not adequately stress conformal coatings during thermal shock testing, and that materials meeting the requirements of IPC-CC-830 cannot necessarily be assumed to meet customer expectations in real-life testing on populated assemblies, even when water-washable chemistry is used to eliminate the variable associated with no-clean residues.
Once the conformal coating is cracked, it will provide greatly reduced protection to the assembly against its operating environment. This has been highlighted through the use of a powered salt-spray test, on an admittedly small and incomplete sample set at this time. The ENIG finish is likely to be more corrosion resistant than other finishes, so it is expected that an alternative finish would yield even more corrosion products.
This work has been a first attempt to combine 'process-qualification' like test methodology with material performance evaluations of conformal coatings and has shown that the methodologies can be used together, but that there are possible refinements to be made on a continuous basis, both to the test vehicle and the evaluation criteria.
When the test matrix has been completed, it should be possible to make correlations between application method, coating thickness and performance under more real world conditions.
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