Urethane Conformal Coating Problem

My product line uses Urethane conformal coating for its Circuit Card Assemblies. We have experienced significant difficulty with the coating adhering to the board despite washing and efforts to avoid any silicon contamination. Can you shed some light on what might be causing this issue?

R. S.

Expert Panel Responses

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 have 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 a 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.

Mike Jones
Vice President
Micro Care

Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

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.

Jade Bridges
European Technical Support Specialist
Electrolube Ltd

Jade Bridges is the European Technical Support Specialist for Electrolube. She is responsible for technical support within Europe, offering assistance to customers with product selection, implementation and after sales support across the range of Electrolube products. Her expertise is carried over from her position as R&D Manager for Electrolube, where she was responsible for the new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication and electronic cleaning solutions.

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.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

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.

Doug Dixon
Marketing
360-Biz

Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.

First of all, I suggest confirming that the surface is clean with sufficient surface energy for good conformal coating adhesion based on below criteria: Test Recommended value Ionic Contamination (R.O.S.E. Test) < 2.6 ug NaCl eq./in2 Surface Energy via Dyne Pen or Dyne Ink Test Kit > 40 mN/m (> 40 dyne/cm) Zestron Flux Test Kit Negative Zestron Resin/Rosin Test Kit Negative Tin Organic Compound Test Kit Negative If none of the above criteria is met then we need to investigate optimizing the wash process based on the contamination type we face.

Umut Tosun
Application Technology Manager
Zestron America

Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

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