A three-part deep dive into the root causes, contamination paths, and SQE control measures for fingerprint-prone cover glass without an AF coating.
AF coating (Anti-Fingerprint) is a nano-scale oleophobic/hydrophobic thin film, typically deposited on the cover glass via vacuum evaporation or spray coating. Its core function is to lower the surface energy, so fingerprints, oils and dust are less likely to adhere — and are easier to wipe off.
When the cover glass is uncoated (no AF), the surface is in a high-energy state:
Historical validation: Under identical conditions (same environment, same operator, same peel count), AF-coated cover glass shows no obvious blotchy contamination, while no-AF products show clear contamination. The fingerprint-resistance advantage of AF has been fully proven in practice.
Surface wettability is determined by surface energy. AF coating (fluorosilane compounds) has a very low surface energy (~10–20 mN/m), while bare glass sits around 60–80 mN/m.
💡 Plain-language analogy: AF coating is like a non-stick pan — food (dirt) hardly clings and wipes off easily. Bare glass is like a regular stainless-steel pan — oil and grime stick stubbornly.
Drawing on multiple project complaint records, contamination on no-AF cover glass is generated through the following four paths:
| Path | Description |
|---|---|
| ① | Finger-cot dirt transfer: Operators do not change finger cots every 2 hours; dirt transfers from the cot → protective film → cover glass surface. High surface energy makes it even harder to wipe off on no-AF products. |
| ② | Re-peeling secondary contamination: Re-applying a misaligned protective film, or re-covering an old film after the inspection peel-off, leaves adhesive residue or causes secondary contamination. No-AF products are particularly sensitive. |
| ③ | Static-leather transfer: Inspectors peel the protective sheet completely onto a static-dissipation leather, transferring contamination to the film and then onto the cover glass. Equally severe on no-AF parts. |
| ④ | Environmental static adsorption: Without AF, surface static attraction is stronger, so airborne dust particles deposit more readily. |
Summary of no-AF cover-glass contamination cases across multiple projects and suppliers in the past year:
| Supplier | Defect data | Handling |
|---|---|---|
| Supplier A | 33.6% (121 of 360 pcs contaminated at customer end) In-house stock 10.3% (62 of 600 pcs) | Rework capacity 10.5K, shipped 8/6 |
| Supplier B | — | Alcohol-clean finger cots, added CCD inspection |
| Supplier C | TP surface contamination must be wiped before use; rework leads to re-contamination | Resident engineer assigned for line-side re-inspection |
| Supplier D | Cover glass cannot be used online without wiping | Supplier bears labour; factory assists cleaning |
| Supplier E | Peel-tab too small — inspection finger contact causes contamination | Optimised peel-tab design |
📊 Trend assessment: No-AF cover-glass contamination is not an isolated incident but a systemic problem. It recurs across different suppliers and projects, rooted in the combination of product design (no AF) and insufficient process control. As long as the glass is uncoated, a certain percentage of contamination is unavoidable under the current manufacturing environment.
| # | Category | Specific requirement |
|---|---|---|
| 1 | Mandatory protection | No-AF products must use anti-static bags or inter-layer protection; bare film contact with hard or rough surfaces is prohibited throughout production, inspection and transit. |
| 2 | Finger-cot control | Change finger cots every 2 hours; use different coloured cots by time period for visual audits. |
| 3 | Film-application standard | Apply once and align correctly — re-peeling is forbidden. After cleaning any contamination, replace the protective film; reusing a dirty film is not allowed. |
| 4 | Environment control | Add ionising air snakes in the assembly area to remove static-attached particles. After backlight unpacking, wipe the surface with a clean-room wipe before loading. |
| # | Category | Specific requirement |
|---|---|---|
| 1 | Tightened inspection | Add inspection layers for no-AF products: 100% in-line + 100% OQC + OQA sampling. Remove layers only after capability is proven. |
| 2 | Inspection-method optimisation | Inspect at 45° to the table, rotate 90° up/down/left/right; cover 0–90° viewing angles to expose edge contamination. |
| 3 | Hands-on training & assessment | Use real no-AF contamination samples for inspector training; pass GRR before qualification. |
| 4 | Standard clarification | Align with suppliers on clear acceptance limits for no-AF surface contamination (grey level, size, location, count) to avoid ambiguous standards. |
"No-AF cover-glass contamination" is not a random accident — it is the result of design choices and manufacturing control interacting. Understanding the mechanism, quantifying the risk, and systematising the controls is what moves SQE from "firefighting" to "fire prevention".
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