Test Coupons & Witness Boards

Conformal coating test coupons and witness boards let you validate coverage, edge definition and thickness before you risk production assemblies. They are essential for proving that your spray, dip or Parylene process is stable, repeatable and compliant with your design rules.

Every coating process has natural variation — viscosity drift, gun pattern changes, masking differences, fixture effects and chamber loading. Conformal coating test coupons and witness boards help you:

• Verify coverage and edge definition under UV inspection.
• Measure thickness using pads, steps or dedicated gauge areas.
• Capture evidence before and after process adjustments
• Build SPC trends for thickness and coverage stability.
• Qualify new materials, parameters or equipment safely.

Good coupon strategy works alongside design-for-coating rules such as keep-outs, component orientation and edge coverage & drain paths.

• Test coupons – small, engineered PCB sections designed to include challenging geometries that let you
  consistently evaluate coating quality and thickness.
• Witness boards – scrap, surrogate or representative PCBs that run with the job and “witness” the real
  coating conditions of each batch.

Most coating lines use both: test coupons for measurement and repeatability, and witness boards to capture real-world production behaviour.

A good coupon is small, consistent and representative. When specifying conformal coating test coupons, ensure they include:

• The same laminate, finish and soldermask as the main PCB.
• Representative pad sizes, track widths and gaps.
• Critical coverage challenges such as tall parts, under-component gaps and vertical edges.
• Flat test pads sized for your thickness gauge or interferometer.
• Clear UV inspection areas for meniscus and edge definition checks.
• Unique ID markings for batch traceability.

Wherever possible, place coupons in the panel rails or add them as break-off sections.

Coverage issues usually appear in predictable places. Make sure your coupons include:

• Vertical edges for meniscus height checks.
• Slots, vias and apertures to observe flow and wetting.
• Fine-pitch IC footprints for bridging and shadowing checks.
• Dense passive arrays to test masking access and de-masking risk.
• Connector-like geometries to validate keep-outs and masking strategy.

Conformal coating test coupons are ideal for consistent thickness measurement. Methods include:

• Dedicated copper pads for eddy-current or magnetic thickness gauges.
• Raised steps or fins for optical measurement (ideal for Parylene).
• Microsectioning sacrificial coupons instead of product boards.
• Correlation mapping between coupon readings and accessible features on the main PCB.

Use the defined thickness targets from your process specification and record coupon data for every batch.

Correct placement is essential to get meaningful results:

• Spray coating: place coupons at leading/trailing edges and low-angle zones.
• Dip coating: orient coupons the same as production items to duplicate meniscus behaviour.
• Parylene: distribute coupons across top, middle and bottom of the chamber to track uniformity.
• Use a consistent minimum quantity per batch or per carrier.

To gain real value, link coupon and witness board results to your quality system:

• Define visual coverage acceptance criteria (with UV reference photos).
• Record thickness measurements against batch numbers and rack positions.
• Ensure every coupon has traceability: job number, PCB revision, date, operator.
• Use AQL or SPC charts to track process drift over time.
• Store reference coupons from important qualifications.

Combined with rules for clearance & creepage and PCB layout, test coupons and witness boards provide strong evidence that your coating process is controlled, stable and predictable.

Why Choose SCH Services?

Partnering with SCH Services means more than just outsourcing — you gain a complete, integrated platform for Conformal Coating, Parylene & ProShieldESD Solutions, alongside equipment, materials, and training, all backed by decades of hands-on expertise.

• ✈️ 25+ Years of Expertise – Specialists in coating technologies trusted worldwide.
• 🛠️ End-to-End Support – Selection of chemistry/process, masking strategies, inspection, and ProShieldESD integration.
• 📈 Scalable Solutions – From prototypes to high-volume production.
• 🌍 Global Reach – Responsive support across Europe, North America, and Asia.
• ✅ Proven Reliability – Consistent results across services, equipment, and materials.

📞 Call: +44 (0)1226 249019
✉ Email: sales@schservices.com
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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.