Doing this without the proper control is very dangerous. The controls are necessary to prevent encapsulating ionic/conductive materials under the coating, not just flux, but also finger soils, plating salts solderballs etc. If you are going to be successful, you must have a flux that is compatible with your selected conformal coating and you must also start with a clean board and keep it clean through the assembly process. You may need to take a serious look at your handling procedures. Cleaning prior to conformal coating is critical if you can not be certain the assembly is clean of all ionic soils. I recommend clean prior to coat.
Steve Stach
President, Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
The short answer is yes. The majority of circuit boards that are coated are no clean. However it is a good idea to get the combination of no clean flux and coating compatibility tested. Most coating suppliers offer this service.
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.
I would not recommend that any coating be applied over any surface, without that surface being cleaned to remove possible contamination. The adhesion of the conformal coating is directly proportional to the adhesive forces between the coated layer and the coated surface. ANY particulate or chemical contamination, even at microscopic or trace levels, will weaken these forces, which, in turn, will weaken the continuity and effectiveness of the conformal coating. It is possible that delamination can occur in these weakened areas, depending on the environmental stresses the board will experience. As an experienced coatings chemist, my recommendation is to CLEAN the board before conformal coating. "No-clean" flux is more of a marketing catch-all phrase than a practical reality, for high performance requirements
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.
Many people do this the feasibility of doing so depend on both the conformal coating and the flux. There are two main areas of concern, one is inhibition of cure of the coating which is typically limited to thermal cure silicones. The other is adhesion to the flux reside, poor adhesion to the flux residue can allow pooling of moisture under the coating, this is difficult to predict and will need to be tested.
Neil Poole
Senior Applications Chemist, Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
It should not have any issues. If the coating of selection is not adhering most likely it will be a harmful excess of no clean residue that is not supposed to be there in any case. A standard average no clean flux residue is normally acceptable.
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.
Coating over no-clean flux residues is a standard practice, but as with any engineering question, the answer is it depends... On the type of coating chemistry you are using and the specific nature of the flux residue. Some coatings are more tolerant of flux residues, especially those with very high solvent contents. Others such as water-based materials are very sensitive to flux residues, with specific defects including dewetting (the coating won't completely coat the residue -- it will pull back away from the residue), poor adhesion, CTE mismatch between the coating and flux residue causing cracking during thermal cycles/shock, or more insidious cases of electrochemical corrosion... This last defect/failure mode is generally believed to be caused by the coating interacting with the residue to liberate the excess activators from their resin matrix which during high humidity conditions can lead to the formation of corrosion products. First point of call, I'd check in with your coating and flux suppliers to see if they have any experience or data regarding the 'compatibility' of the materials. This data whilst useful to you, is unlikely to replicate your exact process (thermal profiles etc) so you would want to make an internal evaluation (flux/coating suppliers should be able to help you verify your process) to ensure you are not solving one problem and inadvertently causing another. Aside from the flux residues, general cleanliness of the assembly (residues from handling, bare board fab etc) and the cure conditions of the solder mask will have a big impact on the overall level of reliability you achieve. This is why cleaning would generally be considered the defacto standard for high reliability applications, but even cleaning has its own pitfalls beyond the scope of your question. I hope this helps for now, please don't hesitate to contact us if we can assist you in any way.
Phil Kinner
Global Business Director conformal coatings division, Electrolube
Phil Kinner - Electrolube - Global Business Director conformal coatings division.
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