Q13: Contamination Prior to Conformal Coating When moving PCB assemblies through the factory we use a tray that is not made of anti-static material. In order to minimize static issues we will place a layer of pink antistatic bubble material on the tray placing the boards on it. There is a “rumor” that the pink material can leave a residue on the PCB creating a contamination, inhibiting good adhesion of conformal coating. We have contacted several suppliers of this pink bubble material and none of them believe this could be a problem. Has anyone seen evidence of this problem or have some data to support or deny this rumor? Thanks for any help.

The most likely source of contamination from this type of material would be either plasticizers used in the production of the film, or something like a release additive used to enable easy handling of the film in production.

In my experience, both of these issues are less likely to cause issues than other materials used in manufacturing the bare board and other chemistries used in assembly such as fluxes and staking compounds etc.

The specific type of conformal coating and application method are also factors that will have an impact on the adhesion.

In general, high surface energy is the key to good coating adhesion. This is an easy thing to test and a Google search of dyne pen test will give you plenty of information such as can be found at www.accudynetest.com/adt_introduction.html.

In my experience, 35 Dynes/cm would be the minimum value I would feel comfortable with prior to coating. I prefer to see numbers in excess of 40, which is achievable, but depending upon your board shop, can become an issue to be worked through.

This is a huge area, and like most engineering questions, the answer is ´it depends”… It may be easier to handle offline in a phone call – please feel free to contact us directly.

Monday, 8th March 2010