Conformal Coating Removal Standards

If problems in a conformal coating defy reliable repair, coating removal may be the only solution. Removing conformal coating from PCBs requires matching removal methodology with coating type and the component’s function. A variety of factors – bubbles/voids, coating thickness, component failure, inadequate masking, poor adhesion, surface finish, among many others – can necessitate coating removal.

Recognizing the various types of conformal coatings and their interactions with the parts/materials they cover is as critical to their removal as it is to original covering process. To accurately determine correct procedural guidelines, consulting prevailing industry standards is recommended. IPC Standards 7711/7721 delineate recommended procedures for conformal coating removal from, and replacement onto, PCBs. Their objectives:

• Deliver standardized training developed by the industry, recognized, approved and traceable to enhance understanding and appropriate application of removal criteria.
• Designate accept/reject criteria for these processes that encourage consistent and correct application of benchmarked activities, pertinent to conformal coating removal.
• Teach methods and processes to assure acceptable workmanship.
• Communicate methods and techniques that support application of professionally certified standards for coating removal.

IPC-7711/7721         

Coatings must be removed in almost all instances of assembly repair. In some cases, the films themselves are defective, unable to generate the protective properties that is their purpose. In others, components covered by coating are inoperative for some reason; conformal films must be removed before repair can begin.

Successful removal of conformal films largely depends on the levels of operator skill and experience. Titled Rework, Modification, and Repair of Electronic Assemblies, IPC Standard 7711/7721 details standards for appropriate removal of conformal coatings, describing “the procedural requirements, tools, materials and methods to be used in the modification, rework, repair, overhaul or restoration of electronic products.”

The standard aims to cultivate and ensure the mastery of workmanship skills required for the safe removal of faulty conformal films from substrate surfaces. Like every other IPC training and certification program, 7711/7721 was “developed by and through consensus of industry [leaders and] … representatives from academia, government agencies, original equipment manufacturers (OEMs), electronic manufacturing service companies (EMSs) and circuit board manufacturing companies, and are used worldwide.”

A multi-step identification and treatment procedure is mandated by IPC 7711/7721, prior to actual coating removal. Essential procedural activities include:

• Product classifications, skill levels, tools and materials
• Basic surface mount and through hole component removal
• Land preparation and component installation
• Primary heating methods: conductive, convective and others
• Handling electronic assemblies
• Wire splicing procedures
• Through hole component removal and installation
• Chip and MELF rework procedures
• SOIC/SOT, J-lead and QFP rework
• Printed wiring board circuit and laminate repair
• Conformal coating removal

Coating Removal                  

Coating Replacement   

Major Removal Methods        

Depending upon the PCB’s operational requirements, each of the wet coatings – acrylic, epoxy, silicone, urethane – responds best to a particular removal treatment; vapor-deposited Parylene similarly reacts best to removal techniques suitably adapted to its use. IPC-7711/7721 standards detail coating removal methods determined by their impact on the film, its thickness, and effect on the substrate.

• Used most frequently for removal of conformal films, the selection of the appropriate chemical solvent is dependent on the coating material.  Most effective for liquid coatings, no single solvent will remove every coating material with the same degree of efficiency; solvents are ineffective for chemically-inert Parylene(2.3.2).
• Peeling is limited to specific removal conditions, such as those where coating application has been overly thick. A dull knife/blade slits the film, which is then peeled-off the PCB by hand(2.3.3).
• The very high temperatures required for successful thermal removal can generate delamination/discoloration, overheat temperature-sensitive components, damage solder joints, and leave surface residue(2.3.4).
• Mechanical removal techniques include grinding, scraping, cutting, or sanding coating from the surface. They require thorough masking of non-removal surfaces. Poor mechanical removal processing can accelerate damage to the coating/PCB(2.3.5).
• Cheap, environmentally friendly abrasion (micro-abrasive blasting) uses a tiny nozzle to direct project-specific formulas of abrasive media at the coated surface, via automated or human handheld-methods. The process can be focused onto targeted areas as minute as an individual test node or those as large as an entire PCB(2.3.6).

Selecting the right removal method based on the coating material, its age, thickness and the board’s function is the key insight from IPC-7711/7721.