How Dimer Purity of Parylene Affects Coating Quality
Linking ppm-level impurities to film performance, yield and reliability
Dimer purity of Parylene is a primary driver of coating quality. Trace non-volatile residues (NVR), moisture, particulates and side products can seed pinholes, haze and rate instability—ultimately reducing dielectric strength and long-term reliability. This guide explains how purity translates into visible and latent defects, what to include on your acceptance criteria & COA, and how handling and storage protect quality.
Further reading: Wikipedia – Parylene
Overview
Even with identical deposition hardware, dimer purity of Parylene will shift base pressure, rate stability and film morphology. Four impurity classes matter most:
- NVR/ash – non-sublimable residues that foul crucibles and increase haze/roughness.
- Particulates – seed nodules and pinholes; often introduced during transfer/loading.
- Moisture – raises bubble/foam risk and contributes to local thin spots.
- Side products/solvent traces – can depress dielectric strength and elevate leakage.
Purity Metrics & COA Essentials
Your supplier’s Certificate of Analysis should include the following—align these with your internal spec:
| Metric | What It Shows | Typical Target |
|---|---|---|
| Purity (GC/HPLC) | Main dimer content vs side products | ≥99.8% electronics; ≥99.9% critical builds |
| NVR/Ash (TGA) | Non-sublimable residue at process temp | ≤0.10% (tighten for high-rel) |
| Moisture (Karl Fischer) | Water content driving bubbles/foam | Low ppm, with alarm threshold set |
| Particle Cleanliness | Counts of >10 μm contaminants | Supplier statement + incoming spot checks |
| Identity (DSC/Melt) | Confirms dimer type; flags contamination | Matches reference range per grade |
| Traceability | Batch ID, pack date, packaging type | Mandatory on all lots |
Set your own release, warn and reject bands and align them with product risk classification (e.g., aerospace, medical, safety-critical).
Impurity → Process → Defect
The following summary links impurity classes to process symptoms and the defects you’ll see at inspection when the dimer purity of Parylene is poor:
| Impurity | Process Effect | Typical Defects | Risk |
|---|---|---|---|
| NVR/Ash | Crucible fouling, rate drift | Haze, roughness, orange peel | High |
| Particulates | Contaminated monomer flow | Nodules, inclusions, pinholes | High |
| Moisture | Bubble formation, thin spots | Bubbles/foam, microvoids | Medium-High |
| Side products/solvent | Electrical property drift | Higher leakage, lower breakdown | Medium |
Tip: Place witness coupons in shadowed regions to reveal particulate and dewetting tendencies that normal boards may hide.
Acceptance Criteria & Tests
Define clear release thresholds for the dimer purity of Parylene and verify at incoming inspection. Suggested bands:
- Purity: ≥99.8% (electronics); ≥99.9% (safety-critical/medical).
- NVR/Ash: ≤0.10% typical; tighten where dielectric margin is low.
- Moisture: Low ppm; quarantine above alarm limit.
- Particles: Supplier declaration plus membrane patch spot checks.
| Verification Test | Purpose | Notes |
|---|---|---|
| TGA (Residue) | Confirms non-volatile content | Proxy for haze/orange-peel risk |
| GC-MS/HPLC | Profiles side products | Tracks lot-to-lot variation |
| KF Moisture | Detects water uptake | Correlates with bubbles/foam |
| DSC/Melt | Identity confirmation | Flags contamination or mix-ups |
| Witness Film + SIR | Functional validation | Dielectric & leakage performance |
Handling & Storage
Consistent handling protects purity between COA and deposition:
- Dry & clean transfer: Sieve in a clean hood; use dedicated tools per dimer grade.
- Moisture control: Keep sealed with desiccant; minimise ambient exposure during loads.
- Crucible housekeeping: Remove legacy residue; pre-bake new crucibles.
- Process trending: Track base pressure and rate; investigate drift immediately.
- FIFO & re-test: Rotate stock; re-check moisture on aged lots.
Explore More on Parylene Dimers
Next steps:
Parylene Dimers Hub ·
What is Parylene Dimer? ·
Parylene Dimer Comparison ·
Choosing the Right Dimer
Useful Links
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