Conformal Coating Over No-Clean Flux

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

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

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.

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.

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.

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.

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.

As unanimously stated, cleanliness matters. Anionic cleanliness would be a good practice.

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.

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)