All conformal coatings can be removed if the assembly in question requires minor or isolated repair/rework. There are a number of removal methods and they include solvent stripping, thermal degradation, micro-abrasive removal, mechanical removal, excimer laser and plasma stripping.
It is common to use a combination of procedures but it is critical that the coating manufacturer be contacted for the best possible method(s). Regardless of the procedure, it is critical that the affected area be rinsed with a chemically neutral material such as deionized water. Remaining residues can contain halides or other corrosive agents which can cause electrical failures if they are not removed.
Conformal Coating Removal
To determine the coating removal procedure, the coating first must be identified. Commonly, the type of coating is unknown. As a result, IPC has provided a general decision tree to coating classification (see Figure 1 on the following page). It should be clear that this procedure provides only general coating classification and not specific commercial identification. A coating's characteristics will affect the type of removal method. Although most conformal coatings are soluble in solvents, compatibility with the PWB and its components must be determined. Tables 1 and 2 provide general characteristics of the five major coatings and ranks the removal procedures (in ascending order) to use for various coating types.
Removal should be done carefully and slowly during mechanical grinding, blasting, and/or abrading, especially when removing coatings which have an opaque appearance as components can be easily damaged. With thermal removal applications, IPC recommends testing the coating with a dull thermal parting device. If the coating becomes fluid or gums up, the temperature is too hot or the coating is not suitable for thermal removal. Most conformal coatings are soluble in solvents but, as mentioned earlier, compatibility with the PWB and its components must be determined. IPC states that "PBAs should not be immersed in harsh solvents"1 and recommends isopropyl alcohol be used unless stated otherwise.
The detailed procedures for each technique can be found in IPC 7711 2.4.2 through 2.4.6 "Rework of Electronic Assemblies, Coating Removal" (sections on Solvent Method, Peeling Method, Thermal Method, Grinding/Scraping Method, and Micro Blasting Method, respectively).
In addition, along with courses relating to conformal coatings, the EMPF offers the following certification course: IPC-7711/7721 "Rework, Repair and Modification of Printed Boards and Electronic Assemblies (Operator)".
1.IPC 7711 2.4.1 "Rework of Electronic Assemblies, Coating Removal, Identification of Conformal Coating"; February 1998