How to Mask Areas During Conformal Coating

We have PCBs with contact pads for switches. We apply conformal coating to the entire board using a simple hand held spray application but are having problems efficiently masking the contact pads. Can you suggest options for us to evaluate?

S. S.

Expert Panel Responses

There are basically two options open to you if you are hand spraying with a solvent based conformal coating.

Use a pealable masking compound, probably latex based. This is applied allowed to dry and you then coat over the masking. The pealable mask is later peeled off exposing the contact pads.
Masking dots, these are basically small stickers provided on a roll that are simply stuck over the pad before coating therefore eliminating the drying time of a pealable mask. It is important to use silicone free masking dots as the silicone can cause de wetting of the coating.

In both cases above it is a good idea to remove the pealable mask or dot soon after the coating is dry to prevent tearing the coating during removal. One more good tip for clean removal of masking is to use a little thinner on a cotton bud to dampen the area at the edge of the masking. This will soften the coating eliminating tears.

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.

For masking contact pads, there are two widely used solutions. Firstly, the application of a liquid peelable masking material, which can be either air/heat dried, or UV cured. These materials are designed to peel easily from the contacts after coating, through the conformal coating layer, thus removing the coating from the keep-out area. The second method is the use of a tape... polyester or Kapton tape are common, and the process is largely the same. Key thing to note, ensure the adhesive in the tape is non-silicone based, or else you may encounter dewetting and lack of coating adhesion. For both methods, it is usually best to demask as soon as the coating is dry to the touch. The drier or more cured the coating, the harder it can be to remove the masking material through the coating and the more likely you are to damage the coating, requiring touch-up to give the desired cosmetic finish. Depending upon the volumes of boards you have to coat, a selective robot may be a better long-term choice, thereby largely eliminating the need to mask. Hope this helps.

Phil Kinner
Global Business Director conformal coatings division
Electrolube

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

Conformal coating is more an art than science, in its application. Even the automated spray systems require some technique to control the wetting properties. Assuming you are using Acrylic, you might use some simple process changes with hand application. Your contact pads are ENIG or Conductive Rubber? If they are ENIG, you may want to remove solder resist in an area that matches or exceeds your masking tape. If your conformal coating is migrating under masking, you can also apply silkscreen print to hinder the capillary action of the conformal coating under the adhesive tape. This is a good technique to help control wetting properties. If you are applying this coating or any coating with a spray method, we recommend a laminar flow to outline the attribute or area on the boad, then follow it with a spray which is boarding the laminar flowed area. This method will help the first applied coating to wick toward the second pattern applied. This is effective especially around components which are not sealed or have tendency to collect coating where you don't want it.

Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems

Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Masking is still the best option. But the masking can also be done with precision placement of a UV cured lacquer over the contact pads. The cured lacquer will have a surface tension different from the conformal coating, and thereby will not bond with the conformal coating as it is applied. Think of it as precise location of "incompatible surfaces" ... sort of like lithographic printing processes.

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.

From my experience, for the switches pads masking is the most effective using Kapton tape "dots" (you can find them available in different diameters). The down side of this is the fact that it is a manual process and time consuming.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

Henkel and LPMS have developed a hotmelt peelable mask. The Henkel material is called AS8998 and can be applied with the LPMS USA DISP4410 hotmelt dispensing unit. The material is simply dispensed on the components or areas that you want to mask from the conformal coating. The material solidifies instantly when touching the component. After coating the material is easily removed. There are videos of this on the Henkel website.

Brian Betti, LPMS USA

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