Managing the conformal coating process begins with a precise definition of coverage required. Pre-process negotiations between the client, the coating provider, and end-user, clarify coating requirements. They include agreement about whether:
- coating is needed on metallic or hermetically-sealed regions of the assembly, as well as
- coverage below/beneath components, or on the backside of electronic leads.
Understanding these conditions prior to initiating a coating assignment delineates project specifications clearly, generating the basics of a reliable coating strategy.
Equally important is coordinating the project with the appropriate coating material. While each material will provide a coating, each also has specific properties, making it most applicable for specific coating purposes. It is necessary to recognize the properties of various coating types, as well as their interactions with the components they cover. Many coating processes are replicable, regardless of the material, but selecting the proper coating material minimizes performance failure.
Selection of the Appropriate Application Method
Improper management of the preparation, application, and drying stages of the conformal coating process causes flaws in the deposited film that compromise its function and, ultimately, unit performance.
Most conformally coated electronic assemblies are PCBs or electronics related to them. Their various components respond differently to coating types. Each liquid coating – acrylic, epoxy, silicone and urethane – has a rather specified range of performance properties; they work best when their use is limited to providing conformal film for those operational conditions. The same is true for CVD parylene, which can be uniquely problematic because it coats any place exposed to contact with air, including within microscopic openings. Knowledge of the conformal coating process appropriate to the particular coating assignment and careful material selection based on this information can help to mitigate potential defects to the conformal film and the assembly’s function. Because definite conditions prevail for wet coatings, methodological options demand recognition of these standards. For instance, because heat-curing is commonplace for single-part epoxy coatings, thermally-sensitive components are better addressed by use of non-heat cured two-part epoxy, or another coating material altogether. Also, silicone films are often applied in thicker coats than other materials, making them an unsuitable option for MEMS/nanotechnology assignments, or coating for any component spaces with tight tolerances. Parylene CVD limits adherence to metallic substrates like gold or stainless steel; pre-treatment with A-174 silane can eliminate this problem and generate excellent adhesion for the parylene film.
These methodological differences suggest the range of conditions requiring assessment, to assure successful and truly conformal coating. The objective is to limit the development of coating defects prior to initiating the coating process. Application methods must first reflect the substrate’s susceptibility to the coating material; selection of an appropriate coating technique – brush, dip, spray for liquid coatings or parylene CVD – proceeds from there.
Masking is necessary to assure the parts specified keep-out areas, such as electrical components like capacitors, diodes, resistors, or transistors, are not covered during conformal film application. Doing so negates their capacity to provide their expected electronic performance. Assuring this process is correctly implemented is the first step in simplifying the masking assignment and limiting subsequent need for touch-up.
As with conformal coating selection, choosing the appropriate masking material helps ensure lower incidence of touch-up. Masking tapes and dots work well in many cases, effectively blocking coating incursion into keep-out areas, and are easy to remove after the film has dried. Masking boots, covers and plugs work well for larger areas.
For liquid coatings, management of masking processes should further focus on such procedural issues as:
- Appropriately matching masking material/device with job requirements.
- Generating completely smooth seals along all masking borders to the component/PCB surfaces.
- Eliminating any holes, breaches, gaps or fissures to the mask to prevent leakage and material flow into keep-out locations.
Masking for parylene CVD is considerably more complex than liquid masking. The process’s vacuum nature requires assurance that all components are hermetically-sealed – airtight and enclosed – prior to coating. Mechanical parts, tooling holes and PCB topography beyond designated keep-out areas may require masking.
Whatever method and materials are used, simplified masking reduces material build-up, which can limit mask sealing-capacity and flexibility. These residues need to be cleaned away after masks have been removed; higher-strength masks may require manual scoring to effect removal.
Most touch-up processes are largely dependent upon the type of coating used; solvent-based coatings can be re-touched to mend feathering; Making certain the area is clean and dry is essential, as is ensuring the replacement coating overlaps the old coating by at least 1/8 inch. In general, touch-up procedures need to address:
- an appropriate match between conformal material and PCB function, and
- the best means of applying touch-up materials.
Good touch-up requires a reliable confluence of material, process and board. Ensuring this condition minimizes the development of future failure mechanisms within the PCB.
Applying a conformal coating isn't a simple process. Choosing the right coating for the material, applying it correctly, and curing it requires experience and skill.
To learn more about the conformal coating process and how to improve it, download our whitepaper now: