Thickness Verification Plans (AQL, Coupons, SPC)

How to Control Coating Build Using Sampling Plans, Witness Coupons, and SPC

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

Important: UV (UVA) inspection confirms coating presence and boundaries, but it cannot be used to verify film thickness.

Part of the Inspection & Quality Hub
This article sits within SCH’s Conformal Coating Inspection & Quality Hub, alongside: Standards & Methods, Acceptance Criteria, and UV Inspection.


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 supports defensible acceptance decisions and helps prove process stability (especially where customers require objective evidence).

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).
  • Define measurement locations (edges, leads, vias, shadow zones) and the gauge method used.
  • Escalate to tightened sampling when trends drift or failures occur.

Test Coupons for Conformal Coating Thickness Verification

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

  • Use coupon geometries that mimic component density and typical keep-outs.
  • Capture pre/post-cure readings if solvent loss affects final build.
  • Archive coupon data and photos for traceability and customer evidence packs.

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 inputs (viscosity, temperature, line speed, cure profile) alongside thickness.
  • Trigger corrective actions (maintenance, recipe adjustments, retraining) when rules are violated.
  • SPC data must link to a reaction plan β€” not just be recorded.

Thickness Measurement Methods

  • Eddy-Current Gauges: fast, non-destructive readings on metallic substrates; useful for mapping variation.
  • Optical Interference / Reflectometry: non-contact measurement for transparent coatings; strong option for Parylene and ultra-thin films.
  • Destructive cut-back + microscopy: cross-sectioning on coupons/test pieces when detailed validation is required.

For reliable results, calibrate instruments to the substrate and coating system. Use reference standards/coupons before production measurement to support accuracy and repeatability.

Summary

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

Need Help Setting Up Thickness Control That Holds Up to Audit?

If thickness results vary by operator, coupons aren’t representative, or customers are asking for stronger objective evidence, SCH can help you build a thickness verification plan that is practical, repeatable, and defensible.


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Note: This article provides general technical guidance only. Final design, safety, and compliance decisions must be verified by the product manufacturer and validated against the applicable standards.