Printed circuit boards (PCBs) and related electrical assemblies benefit from the protection of conformal coatings. However, because the films are insulative when dry, they can disrupt operation of the assemblies’ electrical components, items like capacitors, connector contacts, diodes, operational amplifiers, resistors, or transistors. Conformal coating masking protects specified regions of PCBs and related assemblies from being conformally coated during film application. These components must remain uncoated to function as designed. Consisting of masking appliances constructed with appropriate materials, masking systems prevent migration of conformal coatings into designated keep-out areas. Masking processes enacted prior to coating application assure the conformal materials DO NOT invade designated keep-out areas.
Masking boots, dots, tape and liquid latex configurations represent the basic physical equipment used when protecting assembly components from conformal coating. The list of components not normally coated by conformal films is extensive and includes:
- actuators and similar electromechanical components,
- EMI shields
- glass-bodied diodes,
- grounding points,
- mating pins/sockets,
- mounting holes/surfaces/hardware,
- photodiodes, sensors, and other optical devices,
- potentiometers and variable capacitors,
- RF boards/filters or related components sensitive to the additional capacitance of conformal coating,
- spacers and fasteners,
- switches/relays and other unsealed components, and
- test points.
The key issue is recognizing what masking systems are best applicable to the component and its operational environment. Improper selection and use can lead to masking failures and compromised board function. Fortunately, techniques for masking assembly components have been developed and standardized over the years, permitting considerable consistency for masking success.
However, it remains essential to match the correct method/material mix to the specific masking job, to ensure coatings are kept out of designated areas, protecting reliable, longer-term assembly performance.
Masking equipment is optimally effective when manufactured with materials appropriate to the masking task at hand. Common masking materials include:
- seldom recommended water soluble masks,
- latex peel-able masks, wherein ammonia emissions are a concern for the operator,
- hot-melt masks,
- frequently used paper, polyimide, or polyester masking tapes, and
- polymeric covers/plugs; typically quick and easy to use, they peel off accumulated coating after application.
Application of these materials must be monitored for various outputs occurring during manufacturing process, including:
- Electrostatic discharge (ESD), a buildup of static electricity, wherein the dielectric between differently-charged objects breaks down; visible sparks develop. For control, an ionized air blower is suggested for all masking and demasking procedures, to limit ESD generation.
- Fixturing, can also an issue that requires management.
- Residue from masking materials; development of undesirable residues can contribute to either coating dewetting during application or loss of coating adhesion during thermal cycling.
- Timing; improperly timed production/coating processes can generate demasking.
Several variables are involved in the masking process. Most prominent are the type of coating being applied, the PCB’s surface geometry, the part to-be-masked, and overall production volume. Monitoring of masking processes should assess these outcome variables for various masking materials, tested in relation to masking processes/outcomes.
Optimal management of the masking process identifies differences between conformal coating materials, before masking is initiated. Liquid conformal coatings – acrylic, epoxy, silicone and urethane – are applied by fluid methods, where the wet-coating substance is brushed, dipped or sprayed onto the substrate. Unless otherwise specified on the drawing or specification, what follows are general guidelines for applying the masking. Centering of the masking materials over the specified keep-out areas is essential, regardless of its size or location; masking should extend no more than 1/8 inch beyond its outer limits. As important, all component edges must be entirely masked; smooth seals throughout ensure the PCB’s board and shell surfaces are completely encapsulated and tear-free. The objective is to remove any surface breaches, eliminating coating leakage into the component.
Because it is applied differently than liquid coatings, masking for parylene requires a separate approach. Parylene’s chemical vapor deposition (CVD) uses gaseous, rather than liquid, application processes, a significantly more complicated technique necessitating specialized masking procedures. While coatings are uniform, pinhole free and durable, there is a need to mask more than just typical keep-out locations.
Ultraviolet curable masking compounds are increasingly common. Before use, the operator must determine if the selected compound will leave a harmful residue and is compatible with the proposed solvent system. Ion chromatography is helpful. Surface insulation resistance (SIR) testing checks the electrical resistance of an insulating material between a pair of contacts, conductors, or grounding devices. Determining a component’s capacity for resisting current leakage or dendritic growth (electrical short) is also recommended.
Masking for conformal coating differs according to the type of assembly, the type of coating, and the materials comprising the masking appliance. Costly and time-consuming, a wide range of custom reusable boots, tapes and dots accommodate virtually any masking job. Explicit operator care is necessary to make appropriate materials’ selections and assure processes are correctly implemented. A confirmed confluence between masking appliance and material type assures the optimal level of system performance.
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