Dewetting of Acrylic Conformal Coating

We are using a no clean, alcohol based flux for wave soldering. After soldering we are coating the assemblies with an acrylic conformal coating, but are experiencing dewetting of the coating. Would it help to switch to a water based flux? Any other suggestions?

M. P.

Expert Panel Responses

Factors that influence dewetting usually involve ionic and non-ionic contamination such as:

Residues from board manufacture including silicone surfactants from solder resist & HASL rinse contamination
Component residues like mold release agents
Silicone oil from adhesives in production.
Soldering process residues
Cleaning bath contamination where rinsing has failed
Operator handling that adds contaminants

The minimum cleanliness for coating processes is typically referenced through the established J-STD 001E, which is held to be the most used industrial standard for the cleanliness qualification of assemblies. The following methods are required for complete qualification according to J-STD 001E:

Visual qualification 20x or 40x magnification (according to IPC A610E)
Quantity of rosin on the board (< 40 ug/cm2 for Class 3 assemblies)
Measurement of ionic contamination (<1.56 ug/cm2 NaCl)
Proof of other organic impurities
SIR measurement after or during climatic storage

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".

Using a water-based flux is more likely to create problems than solve them, both with respect to coating tendency to dewet, and longer term reliability issues, due to the higher solids contents, greater activity and inherent water-solubility of the flux acids used in water-based fluxes. Is this a new issue, or is this something that you have always seen. Some older flux technologies, were formulated with fluorosurfactants, to leave a low surface energy film that prevented bridging with the transition to lead-free. The solder couldn't wet the residues, neither can the coating. Speak to your coating vendor who should be able to ask sufficiently intelligent questions to help diagnose your issue... and offer advice on how to overcome it. Not really enough information in the question to make an informed suggestion.

Phil Kinner
Global Business Director conformal coatings division
Electrolube

Phil Kinner - Electrolube - Global Business Director conformal coatings division.

There are a few factors that need to be addressed to narrow the problem down.

Is there any cleaning of the post-solder residues prior to coating? If not, I would suggest re-evaluating the process. Just because it is a no clean flux, doesn't mean that problems don't arise by leaving the residues. "No clean" generally refers to the remaining residue in that it doesn't promote dendritic growth etc associated with the residue. Other problems that can and do occur include coating issues, which are evident here. If the assembly is fairly small, a quick use of an aerosol defluxer followed by coating should give an indication if the flux residues are causing the dewetting. If so, then a thorough review of the "to clean or not to clean" question is in order.
Are you using any temporary masking tape? If so, do the dewetted areas coincide? Some of these tapes use a silicone-based adhesive which will cause dewetting if not properly cleaned off.
Is the board assembly cleaned by the manufacturer as you get it in the door? The contaminant may already be there prior to your soldering operation. Something to look into.

The easiest step is just to clean an area of dewetting and try to recoat. At least that would show whether or not the contaminant is removable and go from there. I don't think the switch from alcohol to water-based flux in and of itself will cause a difference. However, just by switching fluxes sometimes leads to residues that are more compatible with the coating process you use.

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.

Dewetting is directly related to contamination. Many boards are made with a release coating that needs to be removed prior to coating. In some cases the contaminant may or may not be soluble. Switching to a water base will only help if the no clean flux currently being used is causing the issue.

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.

You seem to be missing one process step... cleaning. You should remove all contamination from the assembly after reflow prior to coating the assembly. By not removing the no-clean flux residue, you are also allowing other process residues (from board fab, component fab, and assembly processes) to remain on the assembly. While switching to a water soluble (OA) flux is an option, it will still require a cleaning process. My recommendation is to not change your flux (unless there is another compelling reason to do so) but rather implement a post-reflow cleaning process.

Mike Konrad
President
Aqueous Technologies

Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.

It doesn't sound like a flux issue but more like a problem with un cured solder resist. Try and check the resist cure.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Even though you are using "no clean" fluxes, best practice is still to clean the board. Any residue (even trace amounts in nanograms) on the surface of the board is a surface contamination for all subsequent coating or printing operations, unless it is removed by cleaning with a solvent. The "no clean" marketing label is a disservice to us, in my opinion.

Jim Williams
Chairman
Polyonics, Inc.

Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.

Test the Solder Resist with a good Solvent Stencil Cleaner and see if you get any discolouration off the wipe when rubbed. IF you get discolouration I would not clean the PCB but reject back for recuring. Using a Semi Aqeuos solution on poorly cured resist could cause a bleaching effect and will absorb generally conductive cleaning residues and any other undesirable chemicals used in PCB manufacture. Once coated over could lead to faster field failures being observed.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Reader Comment

One more point to discuss is the misconception about using No Clean flux, and then applying an acrylic polymer coating to waterproof, and protect the PCB. Acrylics are hydroscopic so water can, over time, migrate to the surface, and can induce dendritic growth, which I have seen in the past. When coating boards, cleaning is mandatory.

Joseph Fabian, OSI Electronics, USA

Reader Comment

There should be no flux residue at all on the CCA prior to conformal coating application.

David Blanchard, Arizona Procoat LLC, USA

Reader Comment

Prior to any test or root cause research, check the tension surface compatibility between the PCB (or the material where the coating is needed) and the comformal coating.

Julien Manfredi, Sagem

Reader Comment

Effectively one solution could be to increase the surface tension of the substract, by activation atmospheric air plasma systems. This technology can increase the surface tension of the substract by adding some Nm/m and then improve the adhesion process by developping Van der vaals forces.

Gonsolin Mathieu, Exelsius

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