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May 22, 2018

Conformal Coating Over No-Clean Flux

We have a new project and need to apply conformal coating to one circuit board assembly. We have not applied conformal coating before.

Is it normally acceptable to apply conformal coating directly over circuit board assembled using no-clean flux?

D. L.

Experts Comments

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.

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

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.

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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.

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.

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.

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.

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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.
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.
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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.
Reader Comment
Our product reliability needs are such that we fall into the "wash before conformal coating" camp. As has been stated by some above, the adhesion is definitely better after washing, no doubt about it! This has been proven with testing and experience. Whether or not you need better adhesion is each companies call.

We have used HumiSeal 1B73 Acrylic Lacquer with no-clean flux and water washed for decades now. The water wash includes the appropriate "Adders" and should be done quite soon after the soldering process (but after the PCBA has cooled to at least the same temp as your water wash).

We have noted that the longer you wait, the harder it is to remove the no-clean flux residue. And I should mention one very important warning: If your transferring your PCBAs to a different EMS shop and you just simply tell them that you know that you want the boards water washed before conformal coating, they may very well assume that they should use a "Water Soluble Flux" type in the process (if you don't say anything on this topic). This is a very common assumption, and with real incidents of very bad things happening because of this.

Because many of the old Water Soluble fluxes are much more aggressive and corrosive, certain Components can be very badly affected which did not have any problems in the previous "no-clean flux/water wash" process at the previous supplier. That is why when SCM decides to move PCBAs from one EMS to another, a very skilled and experienced team should be put in place to handle the transfer process, and ALL factors should be thoroughly investigated and understood using a well-defined process before moving forward.
Steven McLaughlin, ABB, Switzerland
Reader Comment
As unanimously stated, cleanliness matters. An ionic cleanliness would be a good practice.
Randy Allinson, Ascentech LLC, USA
Reader Comment
This is very important. I'll keep it brief. If you clean, you must do it properly. it is better not to clean than to clean poorly. The cleaning agents required to remove no-clean flux need to be thoroughly rinsed off otherwise you'll be leaving highly ionic material on the board. We use low-clean and a 30% solids 70% solvent acrylic and do not clean. Wetting and adhesion are good despite a strong deposit of flux residue. Other coatings may be less forgiving. You may need to do ROSE testing, or try DYNE pens.
Gordon Smith, Pektron, UK
Reader Comment
This is SOOOOO frustrating. YES / NO / YES / NO. The answer is simple, Why are you coating the board? Protect from moisture or other conductive contaminants  (metallic dust, spiderwebs)?

You want a yellow Ferrari but you buy a red Ferrari and go to Home Depot and pick up a gallon of yellow gloss paint and paint your car yellow.  Seriously? Drive it in the rain at 60MPH for an hour and tell me what happens.

You coat the board to stop moisture from getting to the surface and to stop any dendritic growth (board failure). Dendritic growth between conductive paths is fueled by water and acidic ions. Contrary to what seems to be a popular belief is that this does happen quickly, hours to days.

For a production run, clean, then clean, then clean again until your Ionic contamination level as tested reads below 5 on a typical ionic tester.

In reality, true no clean fluxes cannot be cleaned off enough to provide high levels of protection under a hydroscopic acrylic polymer coating. If that is the requirement, then encapsulate with an epoxy. Costly, difficult to do, almost impossible to repair. 

But for now, change your flux to a cleanable, no-clean flux (they do exist and work well) and start over.

The protective coating has to hold on to something, either a mechanical or a chemical bond. Not a skin coating of a water soluble "paint" on a Red Ferrari.

But if you want to be really cheap, I have seen pure beeswax with a melting point of 160F (not an oil based paraffin candle wax) used on electronics as a moisture protector. It actually seemed to work well, and since it was a wax, it repelled 100% of the water. OK if there is no vibration, but it does encapsulate the board and will not melt when used on commercial grade components (70C)
Joseph Fabian, Cetirus
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