Component Selection & Orientation – shadowing, venting, standoffs

The way you select, place and orient components has a huge impact on how easily a conformal coating can reach all component orientations and how effectively Parylene can access the surfaces that need protection. Tall parts, shields, tight gaps and boxed-in areas all create shadowing, voids and trapped material that are difficult to see or fix later.

This guide explains how to design coating-friendly component selection and orientation – using simple ideas such as venting, standoffs, drain paths and grouping – so coating coverage is predictable and repeatable rather than left to luck at the end of the process.

Fast read – what matters most

• Think line-of-sight – if you can’t “see” it, you probably can’t coat it.
• Use vent holes and drain paths in shields, housings and cavities.
• Specify standoffs under components to avoid uncoated interfaces and trapped pools.
• Group tall parts together so they don’t cast random coating shadows.
• Choose coating-friendly component styles early, not after layout is frozen.

1. Why component orientation matters for coating access

Most liquid conformal coating processes (spray, selective, dip) rely on a degree of line-of-sight. If a surface is hidden behind a tall component or boxed inside a cavity with no venting, the coating will either:

• Struggle to enter at all – leaving uncoated areas and localised reliability risks.
• Enter slowly and trap bubbles or pools that take far longer to dry.

Parylene coating is less line-of-sight dependent because it’s a vapour deposition process, but orientation still matters. Deep blind pockets or closed cavities with no venting can block monomer flow, and very tight interfaces can act like “contact areas” with minimal coating thickness.

Good conformal coating component orientation is about ensuring that:

• The coating can reach all critical surfaces with adequate thickness.
• Excess material can drain away under gravity instead of pooling.
• Inspection teams can see what’s happening with UV light or magnification.

2. Managing PCB shadowing from tall components

PCB shadowing and conformal coating component orientation become a problem wherever tall parts sit in front of smaller features. Common offenders include:

• Electrolytic capacitors and tall film caps
• Large connectors and headers
• Transformers, inductors and tall relays
• Metal can oscillators and shields

When the coating is sprayed (either manually or selectively), these components create “wind shadows” where the atomised coating struggles to reach behind the part. Even with dip coating, crowded tall parts can trap air and stop coating from flowing properly around leads and pads.

Design tips to reduce shadowing:

• Group tall parts together in “forests” instead of scattering them randomly across the PCB.
• Keep tall components away from critical creepage paths or high-voltage features that must be coated.
• Allow a small “spray corridor” so the coating head can approach key areas from at least two directions.
• Where possible, choose a lower profile component if it removes a serious shadowing risk.

During design reviews, it helps to ask: “From a spray head sitting above the board, which areas of this PCB can’t be seen because a tall part is in the way?” Those are the places where coating access will be compromised.

3. Venting and drain paths in shields, cans and housings

Any time you place components inside a shield can, enclosure or mechanical housing, you need to think about how coating (and cleaning fluids) will get in and out. Without venting for conformal coating, cavities behave like sealed pockets.

Coating-friendly venting principles:

• Provide at least two openings – one for air to escape and one for coating to enter.
• Use vent holes near the top of a cavity so trapped air can escape as coating flows in.
• Add drain paths near the bottom so excess coating can run out under gravity.
• Keep vent/drain holes large enough that surface tension doesn’t block them (typically > 1 mm as a starting point – process dependent).

For Parylene component orientation, vent holes are just as important. The monomer needs a pathway into and out of cavities, otherwise you risk reduced thickness or partial coverage inside “sealed” areas.

As a rule of thumb, if you design a box or shield where cleaning fluid can’t flow freely, then coating is unlikely to behave well either. Aim to design venting and drain paths that work for both cleaning and coating.

4. Using standoffs to improve coverage under components

Many reliability issues arise at the interface between the component body and the PCB. If a part sits completely flush with no gap, the result can be:

• Uncoated interfaces where moisture can track.
• Trapped solvent pools or highly variable thickness.
• Difficulty cleaning flux residues before coating.

Specifying component standoffs for coating gives cleaning fluids and coating a controlled gap to work with. Even a small standoff height can make a big difference in coverage and drying behaviour.

Where standoffs help most:

• Chip resistors and capacitors in critical high-voltage or high-impedance circuits.
• Transformers, inductors and relays with large flat bodies against the board.
• Bottom-terminated components (BTCs) and power devices where feasible, or via dedicated keep-outs and underfill rules.
• Connectors and headers where coating must flow under to protect hidden pads or tracks.

The “right” standoff height is process dependent (coating type, viscosity, film build, cleanliness needs), but the design goal is consistent: create a controlled, inspectable gap so coating can flow, drain and cure predictably.

5. Choosing coating-friendly component styles

Component selection for conformal coating is easiest at the BoM definition stage. Once layout is fixed, your options are far more limited. When possible, favour components that:

• Have accessible leads and pads rather than hidden interfaces.
• Offer versions with moulded bodies and standoffs rather than fully flush seating.
• Allow a lower profile or slimmer package to reduce spray shadowing.
• Provide vented shields or open frames instead of fully enclosed metal cans.

For dense assemblies and coating access, it can be worth defining an internal guideline that flags “coating-sensitive components” (for example, high-voltage parts, sensitive analogue nodes, safety-related circuits). These get an extra check during design-for-coating reviews to confirm that orientation, spacing and component style are compatible with your chosen coating process.

6. Practical design checklist

Use this high-level checklist when reviewing conformal coating component selection and orientation for a programme:

• Have we considered line-of-sight for spray or selective coating heads?
• Are tall components grouped and kept away from key coated areas where possible?
• Do all shields, housings and cavities have vent holes and drain paths sized for coating and cleaning?
• Have we specified standoffs for parts where under-body coverage is important?
• Are there any critical features hidden beneath components that might remain uncoated?
• Does the component choice allow for inspection and rework if needed?
• Is the design compatible with both our cleaning process and our chosen conformal coating method (liquid or Parylene)?

Building these questions into your design process keeps coating-friendly component orientation as a deliberate choice, rather than an afterthought once hardware is already in production.

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Note: This article provides general design guidance for conformal coating and Parylene. It does not replace product-specific standards, OEM requirements or safety approvals. Always validate final layouts and electrical spacings against the relevant IEC/IPC standards, customer specifications and safety agency rules for your application.