Uneven Conformal Coating

We are experiencing and issue with our conformal coating process. We chemically wash prior to coating and spray the material in two coats. On matte finish boards the conformal coating is migrating away from some areas, and building up in others. We are getting coverage, but this is causing our boards to look like a topographical map with hills and valleys. We do not have an issue with adhesion, just uneven finish. Any help on this would be greatly appreciated.

R. H.

Expert Panel Responses

I am surprised that this is happening only on the matte finnish boards! If the issue were related to surface energy you would probably also see some areas of de-wetting. What I think you are seeing is capillary action of the coating material which is caused by too low viscosity. I suggest that you Increase the viscosity of the coating, I would suggest about 25cps for hand spraying and 65cps for selective coating systems. This can vary depending on the nozzle type. Temperature also effects viscosity so if your production area is not controlled the volume of coating that you apply can vary with viscosity. A faster evaporating solvent, if this is solvent based, will also help to reduce this effect. You coating supplier will be able to help with the solvent and they may also offer preblended coatings at set viscosities for various application methods.

Chris Palin
European Manager
HumiSeal

Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

It may be something as simple as the surface tension of your matte finish boards, The fact that you already clean hopefully means that you are presenting a chemically consistent clean surface for the conformal coating to bond to. The fact that the material moves around may be indicative of a surface tension issue, so bonding will be good just a cosmetic issue. You can test the relative surface tension of your board using Dyne test pens, if you google Dyne pens you should find a number of sites, a Dyne is the measure of surface tension and if you draw a line with the various test pens in a kit it will tell you what the relative surface tension of your board is. This may not necessarily help with the coating as you may not be able to vary its ability to wet to different surface tension areas but it may indicate if different parts of your board have different surface tensions. This may indicate an issue with your cleaning process or the PWB manufacture anyway it should indicate if the surface is consistent or not.

Richard Boyle
Global Product Champion
Henkel Electronics

Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

I believe there is a potential residue problem on the substrate. The source can be a contaminant that was not removed by your wash process (flux, mask, etc) or depending on the type of system, it could be some of the wash chemistry that is not being adequately removed by rinse or other means (surfactant, saponifier, etc). From your comments, it sounds like the coating is de-wetting around an area of residue/contaminant and is being forced into an already-coated area. This results in "blanks" adjacent to areas of too much coating. Most coatings have UV indicator formulated into them, so a quick inspection under a long-wave black light should give you a good reference of one area versus the other. To eliminate a board defect or lack of cleanliness in a particular lot, coat an unprocessed one from that batch and compare results. It's recommended that all boards start out clean in a process -- some board manufacturers clean prior to shipment and others do not. One possibility comes to mind. If you are using "tape" mask, please check with the manufacturer. Some use silicone-based adhesives. Residues from these products are difficult to remove, tend to migrate all over the substrate, and can cause coating to act in the manner you describe. Good luck.

Pierce Pillon
Laboratory Mgr.
Techspray

Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.

Washing the boards should get rid of any incidental contamination in the surface so the initial cleanliness may not be the problem. Matt finish boards have a special treatment in order to provide the finish. It will be interesting to verify, using a lap shear test, the compatibility of the coating selected. The most likely culprit for the finish is the two step process. Having a two step process suggest me the coating selected is solvent based and if so, the time between coatings is extremely important. Even if the first layer is tack free, it may not be free of the solvents and they will indeed interact with the fresh coating in top. An acrylic coating will be very much affected since the new layer of rich solvent coating will re-solvate the initial layer. A urethane layer may be mid affected and a silicone layer the least affected. For multiple coating layers the time between coatings is key and has to be as long as possible to ensure this kind of interaction, in one end some acrylics can range from 8 to 24 hours the time required between coats (And not by dipping, only spraying). Contact the current supplier for the recommended time between layers or change for a product with that given its viscosity does not require a second coat. By definition a conformal coating should be applied only once and thus the importance of selecting the correct solid content, the initial viscosity and the coating method. Multiple coats, extra dilution, or long processes are symptoms of an inefficient coating selection. Krayden with its vast experience can help you select the right coating for the right application.

Wayne Wagner
President
Krayden Inc.

Wayne Wagner has over 25 years in the conformal coating industry and is the president of Krayden Inc., a leading distributor of engineered materials.

Is the same batch of conformal coating working on other boards? I not the product is faulty. If yes then it looks like you have contamination on your substrate causing your board to "dewet".

Steve Cook
Director of Product Technology
Techspray

Steve Cook is the Director of Product Technology at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.

In regards to the below customer issue; there was another customer who had a similar issue during our conformal coating presentation last year. I informed him that if they are having good adhesion, there is a possibility that the flux residues are completely removed. But, improper coverage could occur due to below reasons:

Low viscosity of the conformal coating
Increased amount of coating applied
Low surface energy of the substrate
High surface tension of the conformal coating

In most cases, poor coverage of the conformal coating is due to improper surface tension of the applied conformal coating. If it doesn't match the surface energy of the solder mask, such issues could occur. In order to resolve this issue, customer might need to consider following steps:

Increasing viscosity of the conformal coating
Reduce the thickness of the coating layer
Cleaning the substrate with appropriate chemistry whereby surface energy of the matte finish substrates could be increased
Heat board prior to coating

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".

I've seen already all good recommendation for your problem so instead of repeating, I will add:

are you performing a ionic contamination test too after cleaning to make sure that all chemistry gets washed off?
is the surface perfectly horizontal while processing? (spraying manually or automatically)
is there any source of aggressive air flow around the surface of the board while processing or immediately after?

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

As has been suggested the mask may be absorbingsome of the cleaning chemistry which does also happen with fluxes. A mattsurface is more prone to different refection of light from its surface. Bakingthe boards after cleaning may help. See example of coating defects in the newfree guide I am producing for a exhibition feature in April see http://coatingguide.smartgroup.org/ Try a test by cleaning half a blank board. Waitfor a period then coat as normal and see if you still see the coating separate.We are running a FREE - Hands on Conformal Coating and Cleaning Experience inEurope for three days if you want to come along see http://www.new-expo.co.uk/newuk/conformalcoating/ Chris Palin & Richard Boyle from theExperts Panel should be there too supporting IPC, NPL & SMART Group!

Bob Willis

Reader Comment

Steve, your comment on heating the board prior to coating has worked great for us over the years but is now being rejected by Nadcap auditors indicating assemblies should be at room temperature. Is there any documented papers known to support this?

Dennis Fowler, Sanmina-sci

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