|Ask the Experts|
October 6, 2020
Removal of Coatings from PCBs
What the best way to remove coatings from assembled PCBs. Can you point me to some type of guide?
|Expert Panel Responses|
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.
President/Senior Technical Consultant
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.
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. 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!
Manager of Assembly Technology
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.
Principal Materials and Process Engineer
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.
How about laser ablation? You can use a laser beam to remove the coating in some areas whilst keeping the coating in others. There is no residue and one can also avoid damage to the layers underneath. Other components won't be impacted. The spatial resolution is in the 10s of micro-meter range.Dirk Mueller, Coherent
Mike Jones, retired Microcare Vice President, produced an excellent guide on removing conformal coatings. It takes into account the factors that the experts mention including substrate and coating material. You can find the information here. https://bit.ly/3d7Xq6c.Sheri Pear, MicroCare, LLC
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