Keep-Outs & Masking-Friendly Layout

Designing coating-free zones and access for efficient masking

A masking-friendly PCB design reduces labour time, cuts rework, and improves conformal coating quality. This guide explains how to define coating-free areas, create effective keep-outs, and ensure mask access for both liquid conformal coatings and Parylene. It is written for designers preparing boards for high-reliability industries where coating is mandatory.

Infographic showing four principles of conformal coating masking-friendly PCB design: coating-free areas, masking access, grouped features, and spacing for masking boots.

1. Why masking-friendly design matters

Masking is one of the slowest, most labour-intensive parts of the conformal coating process. A poorly optimised layout
leads to:

  • Higher masking costs and longer takt times
  • Increased risk of coating leakage into coating-free zones
  • More touch-up and rework on coated assemblies
  • Reduced yield and higher defect rates

Smart masking-friendly PCB design eliminates many of these issues before the board reaches production. By designing with conformal coating keep-outs and mask access in mind, you make coating a repeatable, controlled step
instead of a bottleneck.

2. Define clear coating-free zones (keep-outs)

Coating-free areas protect components or surfaces that must not be coated, such as:

  • Connectors and card-edge fingers
  • Test points and probe pads
  • High-voltage isolation paths and creepage distances
  • Switches, buttons, and adjustable components
  • Mechanical interfaces (screws, studs, metal frames)

Best practice rules for keep-outs

  • Use a clearly defined coating keep-out zone on the mechanical or design rule layer.
  • Allow typically 0.5–1.0 mm clearance around the protected feature for mask placement.
  • Avoid placing small passives tight up against connector shells or card edges.
  • Use copper keep-out, silkscreen, or mechanical outlines where possible for clarity.

What this prevents

  • Masking tape lifting or leaking around critical features
  • Parylene penetrating into connectors and mating surfaces
  • Coating bridging across isolation gaps and creepage distances
  • Operator-to-operator variation in masking

3. Provide adequate masking access

The number one cause of slow masking is poor physical access around features that require protection.

Allow working space around:

  • Headers and I/O connectors
  • Card-edge connectors and fingers
  • Jumpers and links
  • Tall mechanical features and brackets
  • Potentiometers and trimmers
  • Battery contacts

Minimum recommended access clearance

  • 3–4 mm for manual tape masking
  • 6+ mm for reusable masking boots
  • ≥ 10 mm around tall or awkward connectors

Good access is a core part of masking access design guidelines and a major labour saver.

4. Group masking features together

Dispersed masking points increase takt time. Grouping keep-outs improves:

  • Masking speed
  • Consistency of coated edges
  • Reduced tape usage
  • Easier inspection

Cluster connectors, test points, and masked features into defined masked zones whenever practical.

5. Avoid masking nightmares in layout

Certain layout patterns repeatedly cause coating defects. Avoid:

❌ Tall components surrounded by small passives

Almost impossible to tape cleanly; promotes leakage.

❌ Deep, narrow cavities

Cause pooling, voids, and trapped air.

❌ No edge clearance around connectors

Boots cannot seal and tape cannot be applied flat.

❌ Vias too close to masking edges

Enables wicking under tape or boots.

❌ Isolation gaps hidden behind tall parts

Shadowing for spray, inconsistent Parylene coverage.

Avoiding these improves both coating quality and long-term reliability.

6. Plan for reusable masking boots

Masking boots dramatically reduce labour time — but only if the PCB is designed to accept them.

To enable boot use:

  • Provide at least 6 mm clearance around connectors.
  • Avoid components tight to connector sides.
  • Ensure unobstructed access for boot removal.
  • Ensure tall components do not block boot insertion.

Boot-friendly designs cut masking time by 50–80%.

7. Consider coating process type (spray vs Parylene)

Design rules differ based on coating method.

Liquid spray coatings

  • Require larger keep-outs
  • Suffer from shadowing
  • Need access for solvent wipe-offs

Parylene (vapour deposition)

  • Penetrates every gap — seals must be perfect
  • No shadowing issues, but unforgiving of poor masking
  • Tolerates tight spacing but increases masking demand

8. Summary – principles of masking-friendly PCB design

  • Define clear coating-free zones
  • Provide mask access
  • Group masked areas
  • Avoid masking traps
  • Enable boot use
  • Align layout with coating type

Applying these principles reduces takt time and improves coating reliability.

Need help making your PCB layout masking-friendly?

SCH’s coating specialists can review your designs, highlight masking risks, and recommend practical changes that reduce labour, rework, and defects.

Learn more about our conformal coating consulting services or contact us.

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Conformal Coating, Parylene & ProShieldESD Solutions, alongside equipment, materials, and training, all backed by decades of hands-on expertise.

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