Ask the Experts
May 17, 2024 - Updated
July 13, 2022 - Originally Posted

Polyurethane Conformal and Acrylic Coating on the Same PCB

We were using a polyurethane conformal coating. Now our entire process has been changed to acrylic coating. What about field return PCBs manufactured with polyurethane conformal coating?

After rework, can we touch up with the Acrylic coating at the areas where polyurethane coating was removed during the rework of components?


Expert Panel Responses

Like most engineering question, the answer is, it depends.

In theory the answer is probably yes, but It depends on the condition of the urethane you are repairing and the inter-coat adhesion between the urethane and your new acrylic.

If your new acrylic has good adhesion to the aged urethane, then you will probably be OK, but you need to test this and document/demonstrate the repair is as good as the original coat. On the plus side, many urethane materials do not adhere well to themselves, so you might find you have a more reliable result than previously. If the adhesion is not so good, you may require a primer between the layers.

Keep in mind that the acrylic will likely not withstand the same chemical exposure (solvent splashes etc.) so you may need to factor that into your re-work evaluation and testing.

Acrylic materials have often been used to repair parylene coated boards that have been reworked, so there is precedent, but once again, you need to test the combination.

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

We have successfully touched up acrylic coating with urethane. If there are compatible in that direction, I'm sure it will be fine in the reverse direction.

Garry McGuire
Sr. Engineer
NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.

I have never tried that. My concern is the chemical reaction and the long term reliability of the area in question.

Can you hand apply polyurethane to the rework areas using spray cans? If not, I would remove the polyurethane and coat the whole board with acrylic.

Larry Harman
SMT Engineer
Mr. Harman is an SMT Engineer at ACDi and worked at Oven Industries as a Manufacturing Engineer. H's worked at Philips manufacturing ultrasound probes/circuitry. He holds three patents, one for an ultrasound probe design, and two for innovative rat/mouse zapper circuits. He attended Pennsylvania State University for Electrical Engineering.

Common sense tells me that the requirements the original product was built to remain in place. Because your original product was built using polyurethane coating, you should be touching up rework on returned units with small amounts of the same material so there are no incompatibility issues. If you "decided" as a company to no longer use polyurethane and switched to acrylic, and there was never a customer requirement to use any specific coating, you should simply touch up the reworked areas with the original coating already on the assembly. Coating touchup does not require any special equipment, just small applicator brushes.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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