Conformal Coating on Circuit Boards| 27 January, 2000
We currently use a silicon conformal coating on many of our manufactured circuit boards and are experiencing situations where the coating doesn't conform. For example, on the sharp edges of components and corners of leads it pulls away before curing and leaves potentially exposed areas for corrosion, electrolytic action, etc. To overcome this, we have started to flood critical areas with a higher viscosity version of the coating on a second pass. This requires a lot of coating (almost to the point of calling it potted) - as the coating has to be deep enough to cover critical chip components and IC lead knees. Has anyone had any similar experiences? The manufacturer of the coating has recommended a primer to get better adhesion (which we have yet to test out), but I was wondering if anyone has any other ideas of ways to overcome this issue? Also, where is the best place to look for objective studies on various types of coatings and pottings and their advantages / disadvantages? Recommendations on consultants in this area would also be appreciated, as I would like to become better educated as to our options for the future.
Re: Conformal Coating on Circuit Boards| 29 January, 2000
All liquid coatings are Newtonian - they follow gravity so they do indeed run away from sharp leads and edges. Silicones are generally worse because they have very low surface energy - although this is an asset when trying to get under components.
Very thin edge coverage does not necessarily equate to poor protection, unless you have proved that this is an area of circuit failure.
Is your silicone heat curable? Is it solvent based? Can you switch to an alternative?
One trick I know, is to pre-heat the assembly to around 120F immediately before coating. This way a solvent based coating or heat curable will be able to "FLASH" dry onto edges more effectively. Spray application will also help this greatly - not robotic dispensing, like Nordson Select-coat, but standard atomised spraying and then applying many "light" coats.
Hopes this helps, but let me know directly off line if you want more.