Masking Boots for Conformal Coating

With proper equipment and professional expertise, liquid conformal coatings and vapor-deposited Parylene readily provide printed circuit boards (PCBs) and similar electronics a durable overlay of protective, insulative film. Parylene’s chemical vapor deposition (CVD) processes ensure the coating actually penetrates the substrate surface, generating further device security.

However, the very properties that help conformal coatings safeguard PCBs can also negate the performance of critical electrical components such as capacitors, connectors, diodes, resistors or transistors, blocking their ability to transmit or receive electrical signals. Protecting these PCB components from the encapsulating properties of conformal materials, which would suspend their operational capabilities, is imperative. These keep-out areas require masking prior to coating to assure designated PCB components are not covered.

Also called caps or plugs, custom reusable boots surpass masking tapes or dots for assignments demanding an explicit volume of repeat-masking. Efficiently covering PCB’s electronic components, masking boots ensure reliable performance, preventing coating seepage onto and within keep-out regions. In addition, boots do not leak as frequently as tapes or dots, eliminating the need for masking (and coating) re-work. Additionally, they can also be adapted more easily to accommodate the masking needs of a wider range of component shapes and sizes.

Other masking solutions often necessitate costly, labor-intensive and time-consuming processes, delaying project completion. Fastening over often large areas of the PCB, boots provide a labor-saving alternative for masking components prior to coating application and demasking after coatings have dried.

Reduced masking and demasking time can result in lowered operating costs. Moreover, reusable boots do not require constant replacement; some can be used more than 200 times before replacement is necessary. The initial investment of constructing custom-made boots is rapidly recovered through continual reuse.

Different Styles of Masking Boots

Effective masking requires coordinating masking types and materials with the specific masking project. Tapes, dots and contact pads are frequently used when less expansive surface areas require coverage, and covers and plugs are easy to use for specified masking purposes. Masking boots are recommended for larger projects, especially those characterized by irregular board topographies.

Designed to flexibly cover larger PCB components, recyclable masking boots readily accommodate projects requiring repeat masking of high-volume production. The most common boot materials are silicone rubber and similar low- hardness elastomers, which provide:

• Enhanced sealing properties compared to other substances
• Supple strength required to resist repeated applications of conformal coating

Electrostatic discharge (ESD) resistant boots are always available, generally at higher cost. Custom boot sizes and colors are also available but depend on the coating project specifications. A boots’ material composition varies to meet the following requirements:

• Function, shape and size of the component
• Location on the PCB assembly
• Type of conformal coating to be used
• Coating material application method requirements

Once these factors have been determined, criteria for determining the assignment’s proper boot configuration are better defined.

A and C type cups are the two basic format styles of masking boots:

• A cups represent the simplest style. They are used to shield vertical connectors and components and often take the form of masking covers for d-type and molex connectors.
• Typically more complex in design, C cups mask horizontal connectors, attaching to and encasing the component from both sides. Their most prominent advantage is customization, housing asymmetrical or multifaceted assembly edges and surfaces.
• Both A and C cup boots generate dependable security for the static-sensitive components they cover. These boot styles are customizable and can be tailored to the precise topographical requirements of PCB surfaces, regardless of components’ size, shape or existing irregularities — depressions or indentations — along component surfaces; the result is exceptional production versatility.
• A and C cups are also available in standardized versions, according to such component-specific factors as the assembly’s size, shape and differences in external and internal dimensions.

While boots are not recommended for dip applications, they are very efficient for spray coatings and other liquid-application methods; production turn-around is fast and reliable. The same is true for most Parylene CVD processes. However, A cup boots applied for spray-shielding purposes may begin to deteriorate over time, causing coating-seepage into the component. Boot replacement is then necessary. These factors largely determine whether standard or customized boots will best meet the masking requirements.