When we conformal coated our boards we used the spray method which prevented most material from getting into the holes and we were spraying 3 coats of UR. We also were using compliant pins from AMP which work very good from electrical properties as well as no masking and very good coverage between pads.

The easiest way to avoid conformal coating spreading into connectors is to use a masking agent. Liquid masking agents are easy to apply and can be room temperature or heat cured.

For more information please follow the link to Electrolube.

You did not say how you are doing the coating now, so I have no idea if you are doing this by spraying and masking, just using a brush or dipping.

The trick is not to get conformal coating in the holes are near them in the first place. So you need to look at your coating equipment. There are some very nice jet tools on the market that can put down a bead of material in a well defined area. The bead can be used to DAM fluid from flowing towards to connector pins. There is an example video on the Asymtek web site of the conformal coating jet.

Boards that are conformal coated have the areas that they don't want conformal coating on masked or taped off. There are several way to do this, people will use capton tape to tape off areas that they do not want the coating on. There are also solder mask materials that are room temp cure and also UV that are also used in this application.

This only generally happens with solvent based conformal coatings as 100% solids coatings tend to be higher in viscosity.

Solvent based coatings can wick up into the connector body through capillary action.

We solve this problem by producing a gel form of each conformal coating type.

The gel can be applied around the connector base before coating with your standard viscosity conformal coating preventing entry into the connector.

It is important that the coating gel used is compatible your conformal coating.

The approach to this problem is the correct application selection, and second the viscosity of the material selected. Selective coating systems are capable to coat a board without coating the connectors at all, leaving the edge of the wet coatings just a few dozens of mils away from the connector side thus preventing the risk of the capillary action to take place.

Secondly, if the base of a connector has to be coated as well then the viscosity of the coating plays a key role. The higher the viscosity the less likely will be the seeping on the connector.

An alternative approach would be to actually use a two product combination, to use a thyrotrophic material of the same nature as the coating, to surround the base of the connector: this eliminates the seeping completely while allows full coverage of the connector. It is important the material to be of the same nature than the coating to have same cure schedules and adhesion so they behave like a single block of material; silicone-silicone or acrylic-acrylic, etc.

If condensation due to high humidity is penetrating at only the interface(s) of your conformal coating and the board, you can increase the adhesion of the conformal coating to the board by using a suitable gas plasma treatment. The increased adhesion may be enough to keep the condensation out. This kind of plasma treatment is done to the entire surface of the board inside a low-pressure plasma system, just before your conformal coating step.

Argon gas is a popular choice for this process, because it does not cause oxidation of the board material. The plasma treatment will activate the surface (increase the surface energy) of the board due to ion and free-radical interaction with the surface of the board, resulting in increased bondability to the conformal coating.

However, if the condensation is actually penetrating the conformal coating itself, you may need to try a different type of conformal coating, one that is inherently less permeable to liquids. The problem with high-density conformal coatings is they’re usually more difficult to bond to the board. As above, this kind of adhesion issue can be solved by using an appropriate gas plasma treatment. Gas plasma treatments are becoming increasingly popular for board manufacturing processes such as these.

You can contact plasma system suppliers to see if they will run samples for you—most will do so at no charge in order to demonstrate the effectiveness of their gas plasma treatments.