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July 26, 2018

Cleaning Conformal Coated Boards After Rework

We need to rework conformal coated circuit board assemblies. After the rework is completed, can we send the board through our aqueous DI wash system? Are there any concerns or risk?

B. P.

Experts Comments

I agree with the response posted earlier by Ms. Jade Bridges at Electrolube, Ltd. In a follow up response I would like to add;

My experience has shown if you are using acrylic conformal coatings then after your rework soldering step when using only water soluble fluxes care must be taken in a heated aqueous environment. Acrylic coatings strength is mostly as a barrier against environmental elements, dirt, dust, smoke and atmospheric humidity for example.

However DI water alone at high wash temperatures combined with high wash pressures can impact the surface adhesion of acrylic and it may start flaking off.

Surface degradation will be highly evident in areas where flux has wicked underneath the acrylic coating and any locations with pin-holes, scratches etc. as a result of time in the field and rough handling for example. These affected areas can show up away from the rework site.

Polyurethane, silicon and lastly parylene conformal coating as being the most "bullet proof" will stand up well to heated DI water and even a wash process using aqueous based chemistry required for the removal of rosin and no-clean flux.

If the rework requirement is only several boards a day for example, then setting up a simple "soak station" with an appropriate safe aqueous chemistry to remove all the coating first then followed up with the rework soldering and post cleaning is the best way to proceed. This practice is common with several military accounts I have managed.

Please do not hesitate to contact me for further assistance in this matter.

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Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

If you are cleaning reworked boards where all the coating has been removed then it should not be an issue, however standard cleanliness checks after cleaning are still advised.

If you are cleaning conformally coated boards where only a section of the coating has been removed then there may be an issue. It will depend on the cleaning chemistry you are using and whether it attacks the remaining coating or not.

In addition, if you are using an aqueous based wash system then the water resistance of the coating is also important. If the coating is affected by immersion in water for short periods of time then it could affect the insulation resistance of the coating and overall protection level.

In the latter case it may be best to speak to the coating manufacturer to determine the feasibility of cleaning the coated boards.

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Jade Bridges
European Technical Support Specialist
Electrolube Ltd
Jade Bridges is the European Technical Support Specialist for Electrolube. She is responsible for technical support within Europe, offering assistance to customers with product selection, implementation and after sales support across the range of Electrolube products. Her expertise is carried over from her position as R&D Manager for Electrolube, where she was responsible for the new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication and electronic cleaning solutions.

It really depends on the chemical formulation and the solubility of the coating material in water at specific wash temperature. If the conformal coating absorbs the moisture, this may cause reliability problems in the future.

Client needs to make sure the coating is water insoluble and the coating on the reworked area is fully cured before it goes through the wash process.

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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".

There is no need to put a reworked conformaly coated PCB through an aqueous wash system after rework.

In fact this may harm the board as conformal coatings are designed to protect from high humidity and not direct contact with water and potentially cleaning chemistries.

To rework, remove the coating in a local area with the recommended stripper, rework the device, rinse the local area with the recommended rinse agent, re-coat the area with conformal coating.

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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.
Reader Comment
I strongly agree with Chris Palin of Humiseal. There is no need to wash the entire board to clean a reworked area. Washing the entire board will potentially add additional problems. It very important that the reworked area be cleaned and rinsed thoroughly prior to recoating.
Rick Ramirez, Specialized Coating Services
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