Thickness Verification Plans (AQL, Coupons, SPC)

Conformal coating thickness verification confirms that films meet design intent, industry standards, and reliability targets. This page outlines how to plan and execute conformal coating thickness verification using AQL sampling, test coupons, and Statistical Process Control (SPC), with practical measurement methods and references.

Infographic showing conformal coating thickness verification methods with AQL sampling, test coupons, SPC monitoring, and measurement tools

Why Thickness Verification Matters

Thickness directly affects coating performance. Too thin, and insulation or barrier protection may fail. Too thick, and problems like cracking or delamination can occur. Verification aligns production to acceptance criteria in IPC-A-610 and material requirements in IPC-CC-830.

AQL Sampling for Conformal Coating Thickness Verification

AQL (Acceptable Quality Level) provides a statistically based plan to check a defined number of boards in each lot. Measure thickness at defined locations across the AQL sample to infer batch conformity without testing every unit.

  • Choose inspection levels and AQL values appropriate to risk (e.g., Class 3 builds).
  • Document locations (edges, leads, vias, shadow zones) and gauge type.
  • Escalate to tightened sampling when trends drift or fails occur.

Test Coupons for Conformal Coating Thickness Verification

Coat test coupons alongside production to enable repeatable measurement without touching customer hardware. Coupons reflect process conditions (viscosity, dwell, cure) and support destructive or non-destructive checks.

  • Use coupon geometries that mimic component density and keep-outs.
  • Capture pre/post-cure readings to verify solvent loss and final film build.
  • Archive coupon data and photos for traceability.

SPC for Conformal Coating Thickness Verification

SPC (Statistical Process Control) tracks thickness over time using X-bar/R or individuals charts. By plotting routine readings, engineers identify drift, special causes, and loss of control before defects escape.

  • Define control limits from stable baseline data; review at set intervals.
  • Log key process inputs (viscosity, temp, line speed, cure) alongside thickness.
  • Trigger corrective actions (maintenance, recipe tweaks, retraining) when rules are violated.

Thickness Measurement Methods

  • Eddy-Current Gauges: Provide fast, non-destructive readings on metallic substrates. Ideal for mapping coating variation across edges and flat areas.
  • Optical Interference / Reflectometry: Delivers high-precision, non-contact film measurement for transparent coatings. Perfect for Parylene and ultra-thin conformal layers.
  • Destructive Cut-Back with Microscopy: Uses cross-sectioning and digital microscopes to measure coating thickness on coupons or test boards when detailed validation is required.

For reliable results, always calibrate instruments to the specific substrate and coating system. Use certified standards and reference coupons before production measurement to ensure accuracy.

Summary

Robust conformal coating thickness verification combines AQL sampling, test coupons, SPC monitoring, and fit-for-purpose measurement tools. With clear plans and external standards, manufacturers prove compliance, reduce variation, and protect PCB reliability.

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 by aerospace, medical, defence, automotive, and electronics industries worldwide.
  • 🛠️ End-to-End Support – Guidance on coating selection, Parylene grades, masking strategies, application methods, inspection, and ProShieldESD integration.
  • 📈 Scalable Solutions – From small prototype batches to full-scale, high-volume production, SCH delivers flexible capacity that grows with your needs.
  • 🌍 Global Reach – Responsive technical support and supply coverage across Europe, North America, and Asia.
  • ✅ Proven Reliability – A strong reputation built on quality, consistency, and customer satisfaction across services, equipment, and materials.

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