SIR technique improves conformal coating analysis -

SIR technique improves conformal coating analysis

LONDON — Protection performance of conformal coating using a new test method and test board design provides more realistic and useful information on the protection performance of conformal coating against specific harsh environment in which electronic circuits operate according to a report from the National Physical Laboratory (NPL).

Coating protection performance is not only dependent on coating and contaminant type, but also the component type where shape factor and coating coverage is an issue.

NPL's (Teddington, England) report describes developments for a test method for assessing conformal coating protection performance on electronic assemblies using the surface insulation resistance (SIR) technique.

The new method is different to the existing standard in a number of respects and reflects more realistic application of conformal coatings. A test assembly is introduced with a range of components, and has a number of advantages in identifying coating protection problems.

The procedure on how to choose and apply contaminants to test coupons to simulate the contaminants effect from harsh environments on underlying circuit reliability is detailed in the method. The test parameter settings for SIR test and SO2 exposure testing are given. The test method provides a measurement tool to discriminate between coatings.

The use of this method to investigate the protection performance of seven different types of coatings (two water-based acrylics, solvent-based acrylic, fluoroacrylate, silicone, polyurethane, and epoxy) against solvent-based flux, surfactant and SO2 gas, is included.

The results demonstrate that SIR technique is a suitable discriminatory tool for coating performance assessed, and is sensitive in detecting the circuit reliability underneath coatings. Coating type affects the level of protection against specific contaminants i.e. coatings are contaminant-sensitive.

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