Specific to adhesion, there are a couple of likely candidates:

• contamination of the board with silicone
• contamination of the product with silicone (via the equipment etc used)
• use of 'dirty' solvents when cleaning the boards
• not allowing the boards to dry completely after cleaning prior to coating
• applying too much coating in a single coating on rougher matte-finish solder mas surfaces
• not allowing sufficient time for solvents to evaporate between coats.
  
  

My question would be where is the silicone coming from in the first place? The first thing would be to check how clean the boards are either through a solvent extract test or doing some SIR testing to determine the exact composition of the residues on the laminate surfaces.

Checking for finger prints and other contaminants could also be conducted but an audit of the manufacturing site looking for hand creams could be a good exercise to go through as this is definitely a place where silicone can come from and be deposited on the laminate surfaces.

Adhesion problems are usually related to contaminants or residues present on the board prior to coating. By using a suitable cleaning procedure most residues are removed, solving any problems with adhesion.

Silicone contamination can be very difficult to remove and may require multiple cleaning stages depending on what you are using. Cleaning processes can therefore vary depending on the residues or contamination present and so it is difficult for me to offer any advice on your cleaning process without any further information.

If the coating is adhering to the board itself but not to certain components then this is usually a different matter. Commonly, this is due to the components having an "in-built" release agent from the manufacturing process. It is difficult to get anything to adhere to the surface in this case. The legs or any joints of the components however should be unaffected by this matter and will require protection.

I can possibly help further if you can tell me more about your process and what cleaning agents you have already trialled. Do you know if there is a possibility of silicone contamination or are you just trying to avoid this matter? Is the adhesion to the entire board affected?

With regards to the conformal coating you should check whether it is being diluted with any materials and if so is the dilution ratio as specified by the manufacturer? You may have problems with adhesion if the coating is too thick or too thin but this will be dependent on the PCB layout and environmental conditions it is exposed to.

If you need further help on this matter please feel free to contact me direct to discuss in more detail.

The heart of your issue about urethane conformal coatings is the cleanliness of the substrate you are putting the coating on. Normally the issue is found with light oils as the residue; in my experience fingerprints are the biggest offender. But, for some reason, you have a suspicion that silicone residues are at the heart of the problem. If that's true, you got a REAL problem because (as you have seen) urethane won't stick to silicones and they're a bear to clean...

My first question is, why do you think silicones are the cause?

Secondly, how are you trying to clean the boards? Normal aqueous cleaners cannot remove silicone residues, so if you're running them through your aqueous or semi-aqueous cleaning system it won't get the job done.

May I suggest a gentle cleaning in special solvent made by Dow Corning specifically for removing silicone residues. The OS fluids from Dow Corning are medium-strength, medium-drying, medium-priced, and have excellent toxicity and environmental characteristics. They leave no residues and work extremely well. Available in bulk or in aerosol packaging from MicroCare or Dow Corning directly, it just might do the trick.

A product specification for the MicroCare version of this product is attached; it is available from authorized Microcare distributors globally.