Sometimes problems with conformal coating are too complicated or difficult to repair. This can occur when bubbles develop in the coating during the application process; bubbles cause voids in the coating that defeat its protective, insulating purpose, suggesting the need for removal. Other situations that lead to inadequate coverage, and may favor coating removal, rather than repair include:
- Coating application that’s either too thick or thin for the project’s purposes.
- Component surface finishes that adapt poorly to the conformal material chosen for coverage.
- Disparities in surface tension/surface energy.
- Gravity issues that negatively impact application of liquid coating.
- Improper mixture of two-part materials.
- Inadequate fixturing or placement of assembly components in the coating area.
- Inadequate masking implementation.
- Incorrect interpretation of coating requirements.
- Residue on the coating surface during coating application.
- Poor, uneven coating application.
Overly thick film application or use of coating equipment/materials unsuited to the assignment are major causes of coating problems. In these cases, complete or partial removal of the conformal film from the PCB may be the best solution.
Thus, it is important before beginning any conformal coating assignment for designers and users to recognize the various types of conformal coatings and their interactions with the parts/materials they cover, to protect the products in their respective end-use environments, for the expected design-life of each component.
When removal is the best option for your coating problem, it is advisable to consult prevailing industry standards for appropriate process guidelines. For instance, IPC-7711/7721 delineates recommended procedures for conformal coating removal from, and replacement onto, PCBs. IPC-A-610 is a widely-held standard for electronic assemblies, offering users limited but valuable criteria for conformal coating applications. Designed and constructed with the intent of obtaining maximum confidence in the materials with minimum test redundancy, IPC-CC-830B qualifies the definition, use and conformance of all conformal coatings types for PCBs. In most cases, coating removal is required when assemblies don’t meet the requirements of IPC-CC-830, concerning overall quality conformance of each
The Logistics of Coating Removal
The logistics of coating removal are largely dependent on the type of coating material, its position on the PCB, and the board’s components. Proper identification of the coating material, and the methods used for its original application, are essential to correct determination of the removal method. Once these have been identified, determination of the appropriate removal method can be achieved.
In many cases, chemical strippers can dissolve conformal coatings from PCBs. Acrylic films are typically removed easily by soaking in a solution of stripping fluid, followed by mild mechanical abrasion if necessary. These two processes also work for coatings such as epoxy, silicone and urethane; however, since these substances have higher levels of chemical resistance than acrylic, complete coating removal is more difficult and time-consuming. In all cases, the stripping solution’s compatibility with the PCB’s components needs to be verified to minimize potential damage during the removal process.
Chemical removal does the least damage to PCBs; it is effective for the liquid coatings -- acrylic, epoxy, silicon and urethane. Chemical methods work less well for parylene films, since the substance is chemical inert. Abrasion, laser, mechanical, plasmatic and thermal removal methods are more successful for parylene films; they also work for liquid coatings in many cases.
Recently applied coating is more easily detached from substrate surfaces than older coatings, regardless of the material, unless the coating itself has begun to decay with age. Larger areas of the board respond best to complete submersion in a tank of stripping fluid. Gentle abrasion using a soft bristle brush will also eradicate coatings.
Please remember that the removal of conformal coating generally requires use of exceptionally caustic and potentially dangerous chemicals; the safety of process operators, the product being treated and the immediate environment can be jeopardized by use of inappropriate removal materials and methods. Consultation with a certified conformal coating specialist is highly recommended prior to removing conformal coating. To this end, the professionals at Diamond MT are eminently qualified, and would be glad to be of assistance.
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