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98 SMT007 MAGAZINE I JULY 2020 chemistry is not cleaned off thoroughly, thereby affecting the reliability of the assembly. Acrylic conformal coatings are the most sensitive to solvents, hence their easy removal using this technique. Silicone and urethane coatings are the least sensitive to removal solvents. In gen- eral, solvent removal techniques for epoxies, as well as parylene, are ineffective. Some conformal coatings can be removed by simply peeling or scraping them from the PCB and component surfaces. A dental pick, wooden stick, or sharp knife can be used to remove these soft coatings. This mechanical removal method may be used in conjunction with thermal or solvent removal techniques. Care needs to be exercised to make sure compo- nents, and the laminate are not damaged during the removal process. This removal technique is often employed when removing soft silicone- based or other flexible conformal coatings. Another coating removal technique involves using a heat source to soften up or break down the coating. A hot air gun or soldering iron is usually employed as the heat source. After softening the coating, it can be removed by the application of gentle pressure with a dental tool or wooden stick. This removal method can work well with most types of conformal coat- ing. Applying heat requires extreme care not to damage the underlying laminate or neighbor- ing components. Acrylic, epoxy, and silicone coatings can be removed using this technique. Microabraision coating removal uses a vari- ety of soft abrasives accelerated through a small inert gas-propelled nozzle to break down the conformal coating. Walnut shells, glass, plastic beads, and various powder mixtures are pro- pelled to the surface to chip away at the coat- ing. The air pressure, hardness of the removal media, and nozzle diameter all have an impact on the efficacy of this removal process. An ion- ized air source is typically used to neutralize the static charge generated during this process. Proper masking of the board around the coat- ing removal area protects the board and com- ponents. Typical conformal coatings where this process is used as a removal method include parylene, urethane, and epoxy-based coatings. Laser sources are used in cases where pre- cision conformal coating removal is required. The pulsed energy density of the laser gradually removes or ablates the coating material. A laser source with the correct energy level, frequency, and number of laser source passes needs to be established so that it only ablates the coating and does not damage the underlying or sur- rounding materials. Laser areas as small as a few microns can be selectively ablated. Parylene coatings can be removed in this manner. Visual inspection determines if the conformal coating has been removed in the proper area. While there are analytical methods for deter- mining whether the coating has been entirely removed, the UV tracer material as part of the coating formulation allows the coating to show up visibly under "black" lighting. The removal of conformal coating in prepa- ration for rework and repair on a PCB requires care as incidental damage to the laminate and components can occur during the process. The choice of method requires careful consider- ation, experience, and operator skill. SMT007 Further Reading • IPC-HDBK-830: Conformal Coating Handbook Guide- lines for Design, Selection, and Application of Conformal Coatings. • IPC-7711/IPC-7721: Rework, Modification, and Repair of Electronic Assemblies. • G. Caswell, "Coatings and Pottings: Issues and Chal- lenges," IMAPS, May 2014. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact info@solder.net. To read past columns or contact Wettermann, click here. In general, solvent removal techniques for epoxies, as well as parylene, are ineffective.