Optical Thickness Measurement for Parylene
Parylene optical thickness measurement provides fast, non-contact verification of film build for CVD coatings. This guide shows how to set up reflectometry systems, calibrate correctly, and run repeatable plans with coupons and SPC to meet customer and IPC requirements.
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Why Use Optical Measurement for Parylene?
- Non-destructive, rapid readings at multiple points—ideal for load mapping and process tuning.
- High precision at thin builds (5–25 µm typical) with traceable calibration.
- Supports formal verification against IPC-CC-830 and acceptance criteria aligned with IPC-A-610.
Equipment: see our KR-FM20 optical thickness system and complementary eddy-current gauges.
Methods: Reflectometry, Spectral & Ellipsometry
- Normal-incidence reflectometry: Measures interference fringes from the air/Parylene and Parylene/substrate interfaces to compute thickness.
- Spectral reflectance (broadband): Captures a spectrum and fits a model to determine thickness over wider ranges with better robustness.
- Ellipsometry: Analyses polarisation change; highest accuracy for R&D and thin films, but more setup time and modelling skill.
Background reading: NIST metrology resources · thin-film interference overview.
System Setup & Calibration for Parylene Optical Thickness Measurement
- Substrate prep: Clean, dry, and handle with gloves; avoid fingerprints and residues that skew optics.
- Reference standards: Zero with a reflective standard; validate with certified thin-film standards (e.g., SiO2 on Si) near your target range.
- Model selection: Choose refractive index (n,k) for Parylene grade (N/C/D/F). Use vendor defaults, then refine with coupon fits.
- Spot size & focus: Set a consistent spot; confirm focus on flat pads or dedicated witness coupons.
- Backside control: For translucent substrates, use black backing or tape to suppress back reflections.
- Environmental control: Stabilise temperature and lights; shield stray UV to avoid tracer fluorescence artefacts.
Mapping Plans, Coupons & SPC
- Point plan: Define positions (centre, corners, shadow zones) per assembly; keep consistent across lots.
- Witness coupons: Coat coupons with each load for calibration checks and destructive cross-sections when needed.
- SPC charts: Log results to X-bar/R or Individuals charts; act on rule breaks before drift causes non-conformance.
- Acceptance: Set pass bands (e.g., 10–25 µm) aligned with drawing and IPC class; tie to your thickness verification plan.
Need a template? See AQL, coupons & SPC.
Limitations & Cross-Checks
- Requires reflective, reasonably smooth surfaces; rough solder mask and matte plastics reduce accuracy.
- Opaque substrates or heavy texture may need eddy-current gauges like the Positector 6000 or destructive cut-back for correlation.
- Tracer dyes and tinted films can shift fits; validate model on your materials.
Cross-reference results during PFMEA and PPAP to demonstrate method capability (MSA).
Troubleshooting & Tips
- Inconsistent readings: Re-focus, stabilise part, clean surface, and re-zero on the reference.
- Noisy spectra: Increase integration time, shield ambient light, and improve surface contact.
- Model mismatch: Update n,k for the specific Parylene grade and thickness range; re-fit using coupon data.
- Edge effects: Avoid measuring directly on sharp meniscus lines—move 1–2 mm into the field.
Tools that help: digital microscopes for site targeting and edge verification.
Why Choose SCH Services?
Partnering with SCH means gaining a complete, integrated platform for Conformal Coating, Parylene & ProShieldESD with equipment, materials, and training, backed by decades of expertise.
- ✈️ 25+ Years – trusted worldwide
- 🛠️ End-to-End Support – coating, masking, application, inspection
- 📈 Scalable Solutions – prototypes to high-volume
- 🌍 Global Reach – support in EU, NA, Asia
- ✅ Proven Reliability – consistent quality & service
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