Managing the Conformal Coating Process

Managing the conformal coating process begins with a precise definition of coverage required. Pre-process discussions need to take place to clarify coating requirements and agree whether the 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 delineates project specifications clearly, generating the fundamentals of a reliable coating strategy.

Equally important is identifying the appropriate coating material for the project. Each coating type has specific properties, making it more 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

Poor 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, performance.

Most conformally coated electronic assemblies are PCBs or related electronics. Different components react uniquely to coating types, such as acrylic, epoxy, silicone, urethane and Parylene for conformal film in specific operational conditions. Careful process knowledge and material selection are crucial to reduce defects and maintain assembly function.

Wet coatings pose specific conditions that must be considered when choosing the right method. For example, while single-part epoxy coatings commonly use heat-curing, components sensitive to heat are better suited for non-heat cured two-part epoxy or alternative coatings. Silicone films, applied in thicker layers than other materials, may not be suitable for MEMS/nanotechnologies or components with tight tolerances. Parylene CVD has limitations in adhering to metallic substrates like gold or stainless steel; however, pre-treatment with A-174 silane can resolve this issue, ensuring excellent adhesion for the Parylene film.

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.

Simplified Masking

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.

Touch-up

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, contact SCS.