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July 12, 2017

Removal of Coatings from PCBs

What the best way to remove coatings from assembled PCBs. Can you point me to some type of guide?

C.M.

Experts Comments

It will depend on the coating type for the right approach and if you want specific areas removed or total board removal. Dynaloy Corp. makes a variety of chemical stripping agents.
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Terry Munson
President/Senior Technical Consultant
Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
The short answer to this question is "use the materials and process recommended by the coating manufacturer." It truly is always best to look at the coating material manufacturer's data and select chemistry based on their recommendations. That said, in general acrylic coatings can be removed fairly easily without damage to the PWB.

Silicones are a little tougher to deal with, but usually can be removed. Urethane coatings can be extremely difficult to remove, and the chemistry used to strip them will leave ionic residues that are very hard to remove. I recommend removing this type of coating only in small areas, not across an entire board.

In fact, it is a good general rule to avoid wholesale removal of any coating where possible. In some cases, rework can be performed without any removal (burn through the coating). If you do this, use very good ventilation that draws away from the operator.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
IPC-7711/7721B has a multitude of information including how to identify and remove coatings from a circuit board.  To begin, the IPC-7711/7721B has a procedure for identification of the coating and the best method for removal. (See attached Procedure 2.3.1)

Once you have identified the coating, there are 5 different methods for removing the coating. Then there are 2 methods of coating replacement!

See: http://www.circuitnet.com/pdf/IPC-7711B-7721B_Coating_Identification.pdf
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Kris Roberson
Manager of Assembly Technology
IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
The best way to remove conformal coating depends on the coating that is to be removed, how hard that coating is, how thick that coating is, and whether or not you are removing it from a small area of the board for rework purposes, or trying to globally strip the coating from the assembly.  

The coating manufacturer can recommend the best chemical stripping agent and the optimal parameters. Companies that make chemical strippers, such as Dynalloy, may also be able to provide guidance.  

Mechanical means of stripping, using specialized media blasting such as Crystal Mark's Swamblaster or CCRD are good for localized removal of coating, especially the more rigid kinds like Parylene and Urethanes.  

The various methods are covered in greater depth in the IPC-HDBK-830, Rev A, Conformal Coat Handbook. Available from the IPC. A valuable reference tool for anyone who has to work with conformal coatings.
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Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
Coating removal from a PCB depends on several factors: 1) type of coating, 2) type of substrate, 3) type of materials on the board.

Generally, your Type of Coating is going to be a) water soluble, b) solvent soluble, or c) impossible to remove by chemistry. Your Type of Substrate will probably be either a) G10 epoxy-based, b) polyimide, or c) ceramic. The Type of Materials on the Board are is very critical assuming that you actually wish to reuse the board.  Be careful of sensitive plastics such as polycarbonate or Lexan, connector inserts (ZZR765 red rubber silicone, or others) ... So, in order to guide you more info is needed; there is not a simple guide.

If you are simply looking for information on "what dissolves what", you should get a copy of this book by Allan Barton and learn about Hildebrand Parameters, " Handbook of Solubility Parameters and Other Chohesion Parameters" CRC Press.   It will help you to understand what solvents dissolve which polymers.
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Rick Perkins
Chemical Engineer / Owner
Chemical Logic Inc.
Rick Perkins is a chemical engineer with more than 25 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
There are several good guides for conformal coating removal. This includes the IPCs Conformal Coating Handbook which is very comprehensive.   

However, for free information you can try the following:  

SCH Technologies FAQs section on  http://www.conformalcoating.co.uk/Conformal-Coating-Defects-Finishing-and-Repair-FAQs.php

Nexus Conformal Coating Consultancy have a good section on the various methods of coating removal including mechanical abrasion, chemical stripping and soldering through. http://www.conformalcoatingconsultancy.com/cms/default.asp?iID=MILDHE#Rework%20and%20Repair
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Lee Hitchens
Owner
SCH Technologies
Lee has worked within the conformal coating and electronics industry for over 18 years. His work includes scientific research into long term reliability of electronics, technical sales of conformal coating materials and equipment, owner of SCH Technologies, a conformal coating service in the UK, a member of the Diamond Coating Solutions Group, a global liquid conformal coating and Parylene coating service solutions provider, a founding member of Nexus3c, Conformal Coating Centre and a partner of Thin Film Partners.
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