Liquid conformal coatings provide inexpensive, easily-applied insulative protection for printed circuit boards (PCBs) and similar electronics. However, the films’ otherwise desirable insulative qualities can interfere with operation of the assemblies’ electrical components, items like capacitors, connectors, diodes, resistors, or transistors, if these are coated. Liquid coatings are designed to "wick" under components and in between connectors to provide a complete coat, through brushing, dip-immersion, or spraying. These methods work exceptionally well, rendering an overall, dielectric conformal film for the PCB.
They also simultaneously create operational issues if coating wicks into holes and connector pins, creating functional/solderability problems. The solution is masking those keep-out areas whose performance is disrupted by incursion of conformal coating. Masking assures assembly components are NOT covered by the coating during application. Integral to surface preparation, the masking process protects designated assembly components from the coating’s encapsulating effects, which would suspend their operational capacities.
If the requirement is to coat extremely close to connectors, masking assures wet coatings do not penetrate keep-out sectors, leaving them several millimeters away from the connector; capillary action is prevented, so the conformal coating does not wick into the connector. Masking:
- prohibits coating of PCB components distressed by conformal protection,
- so they continue to function as designed,
- accepting an electrical charge and/or
- moving as intended.
Masking Systems, Processes and Materials
Masking processes enacted prior to coating application assure the conformal materials DO NOT invade designated keep-out areas. Basic masking appliances and materials are configured as dots, tape and boots. Among components masking protects from conformal coating are:
- actuators/electromechanical components,
- grounding points,
- mounting holes/surfaces/hardware,
- photodiodes/sensors/other optical devices, and
- switches/relays/other unsealed components.
Masking failures compromise board performance. Recognizing the masking systems best applicable to a PCB’s functional environment is essential.
Good project management identifies masking requirements in consideration of coating materials, their relation to the PCB, the assembly’s function, and size, prior to mask implementation. Unless otherwise identified in a specification, some general rules when applying coating masks:
- Focus masking materials over the center of the targeted keep-out area.
- Do not extend mask material more than 1/8 inch beyond the region’s outer limits.
- Mask all component edges to the finest possible degree.
- Smooth seals throughout the assembly completely encapsulate board and shell surfaces.
- Make sure mask-surfaces are tear-free.
- Test surface insulation resistance (SIR) levels to ascertain a coating’s electrical resistance between contacts, conductors, or grounding devices. (Optional)
- Determine component capacity for resisting current leakage or electrical short (dendritic growth). (Optional)
Effective masking requires coordinating masking types and materials with the specific masking project. Common masking materials/types include:
· frequently used paper, polyimide, or polyester masking tapes,
· masking dots/contact pads of similar materials,
· quick/easy-to-use polymeric covers/plugs that peel-off accumulated coating after application, and
· custom/reusable masking boots for larger projects.
Polyester, polyimide or Kapton backings are commonly used for masking dots and tapes. Dots are small stickers fastened over contacts before coating is initiated. Peel-able latex masking materials effectively prevent coating ingress into the component; they peel easily from contacts after coating
Masking tapes’ standardized masking model effectively protects flat assembly areas like conductive pads or PCB-edges. Dots are optimally used to safeguard smaller, specified keep-out sites like mounting holes. However, both require slow, labor-intensive hand-application, most efficient for low-volume projects.
Designed to flexibly cover larger PCB components, recyclable masking boots are more cost-effective than dots/tapes, and far more efficient for large-batch component masking. Because they can be custom-made to fit a wide spectrum of component alternatives, boots easily accommodate projects requiring repeat masking of high-volume production. Unlike dots/tapes, boots can be tailored to specific to the physical configuration of the component to-be-covered. Utilization is simple, with boots being fastened over the selected components (connectors, plugs, sockets, etc.) needing protection from conformal coating application. They are adaptable to all wet-application techniques but dipping.
In general, dots/tapes are most effective for flatter surfaces and smaller production runs. Reusable masking boots are a more efficient and cost-effective option for larger production volumes, better adapted to a wide range of component shapes and sizes.
While the effectiveness of other masking solutions – liquid latex, tapes and dots – is undeniable for many purposes, masking boots typically save as much as 75% of operating costs. While boots are not recommended for dip applications, they are more efficient for other wet-application procedures, with quicker production turn-around. In addition, boots don’t leak as frequently as tapes or dots, eliminating the need for technical demasking (and coating) re-work.
Without the need to constantly purchase new dot/tape supplies, custom-made boots’ initial construction-expense is quickly recovered through continual reuse. These conditions also hold for small volumes of repeat-processing, saving you time and money in comparison to hand-masking. Boots can be reused as many as 200(+) times.
Remember, adverse reactions between mask and coating causes PCB de-wetting. Assuring compatibility between liquid coating and mask materials assures no unfavorable consequences. Inadequate masking can cause bleeding/leaking during film application. Coating penetration into keep-out areas may necessitate costly, time-consuming repairs.