How to Remove Conformal Coatings

Appropriately selected and applied, conformal coatings provide essential protection for printed circuit boards (PCBs). However, removal of conformal coatings is necessary if the wrong coating material is selected relative to the PCB’s functional requirements, inadequately supporting its operating environment. Poor coating application can trigger failure mechanisms within the assembly which also necessitate removal and re-application.

Effective Coating Removal Processes

The type of coating material used directly affects the ability to remove it or rework and repair it. Therefore, the possibility of film removal needs to be addressed upfront when deciding on the coating material and process. Several removal methods can be used depending on the coating:

• Chemical solvents are used more frequently for liquid coating removal than any other process. However, no single solvent exists to remove all coatings. Coating material is the main factor in determining the right solvent. The solvent can brushed onto a controlled area, or the assembly can be immersed into the solvent for quicker results. Butyrolactone is an effective solvent for the chemical removal of most acrylic coatings. Methylene chloride or hydrocarbon-based solvents are recommended for silicone coatings. Solvents made of activators of ethylene glycol ether-base/alkaline or methanol-base/alkaline remove urethane conformal coatings. Generally, chemical solvents are ineffective for epoxy coatings. Tetrahydrofuran (THF) is the only chemical solvent that can be used to remove Parylene coatings.
• Micro-abrasion recommended for the removal of acrylic, epoxy, silicone, urethane and Parylene coatings. Abrasive material and inert gas or dry air are blasted at the coated surface by a tiny nozzle attached to a stylus. Electrostatic potential is dispelled by grounding devices and filtration is used to remove coating debris from the substrate. An entire PCB can be treated or it can be used on small targeted areas such as an individual test node. The method is inexpensive and environmentally friendly
• Oxygen-based plasma is commonly used to remove coating from specific components. It can be used to strip an entire PCB or a specific area. This method is effective for the removal of Parylene removal and liquid coatings.
• Cost-effective laser ablation provides very precise results for challenging removal assignments in assembly regions of a single micron or smaller. Unfortunately, since each laser pulse separates only a minute segment of the existing film’s material thickness, processing is slow. Very effective for Parylene, the laser converts the solid coating into gas or plasma.
• Mechanical/peeling methods require time-consuming cut, sand or scrape processes to remove films from substrates. Less reliable than other methods, thorough masking of non-removal surfaces is required in all cases. Without precision control, mechanical removal damages components. Used only under specific conditions (i.e. removing thickly-applied silicone coating), peeling applies a dull knife/blade to cut the film into sections that can be manually-peeled from the PCB. Acrylic, epoxy and silicone films respond to mechanical removal methods. Mechanical scraping is sometimes the only reliable method for difficult-to-remove Parylene.
• Difficult to manage thermal methods rely on very high-temperatures generated by a soldering iron, which can delaminate/discolor/overheat components, weaken solder joints and leave surface residue. Thermal methods should be limited to spot-removal; toxic fumes can result from careless thermal application. Thermal treatment has been successful for acrylic, epoxy and silicone coating removal. It also has some utility for spot-removal of Parylene, but the process is difficult to control.

Removal strategies are an integral element of design, based largely on the assembly’s specific coating material requirements and operational function. Matching removal methodology with type of coating material used, its thickness after application and the PCB’s purpose is mandatory. There is no one general rule or approach. For instance, solvent removal suitable for liquid coatings is generally useless for Parylene. This kind of procedural inconsistency prevails according to film material and the assembly’s operational requirements; project-specific removal processes are always necessary. Determining the appropriate removal procedure assures optimal removal without interfering with PCB function after it has been recoated.