Touch Glass Surface Roughness Control

Topic: Ra & Rz indicators, double-sided polishing, flatness TTV tolerance, edge CNC finishing, surface defect limits for capacitive touch screens

Overview

Capacitive touch panel glass requires ultra-smooth planar surfaces to guarantee uniform capacitive coupling between the finger and underlying ITO electrodes. Excessive surface roughness creates uneven air gaps, leading to inconsistent touch sensitivity, dead zones and poor yield during photolithography & ITO sputtering. Felix Glass implements double-sided fine polishing, tight TTV flatness control and strict edge grinding to meet electronic-grade touch panel tolerance requirements.

Key Roughness Parameters: Ra vs Rz

• Ra (Arithmetic Mean Roughness): The primary acceptance index for touch glass. For polished clear touch panels, Ra ≤ 0.02 μm (20 nm) across the active area; no local peaks exceeding 30 nm.
• Rz (Peak-to-Valley Height): Controls micro peaks that cause short circuits during ITO etching. Electronic touch grade requires Rz ≤ 0.15 μm.
• Measurement method: Contact profilometer or white-light interferometer, grid sampling across the whole clear aperture.

Polishing Process & Flatness (TTV) Tolerance

1. Double-Sided Fine Polishing Two-step polishing: First coarse grinding to remove cutting ripples, then chemo-mechanical polishing (CMP) with cerium oxide slurry to achieve ultra-low Ra without surface distortion. Single-side polishing easily causes warpage and uneven thickness variation.
2. Total Thickness Variation (TTV) Mobile touch grade: TTV ≤ 0.005 mm (5 μm) over the whole panel; industrial HMI grade: TTV ≤ 0.01 mm. Excessive TTV creates uneven OCA bonding gaps and optical Newton rings after lamination.
3. Global Flatness Max warp ≤ 0.2 mm per meter length; no local dents or bulges that break touch uniformity.

Dimensional & Edge Finishing Specifications

• Dimensional Tolerance High-precision capacitive touch panels: Length & width tolerance ±0.1 mm; standard industrial panels: ±0.2 mm. Mounting holes and cutouts: ±0.15 mm with fully polished inner walls.
• Edge Treatment Standard (Critical for Strength)
  • Basic requirement: CNC ground + polished C-chamfer (0.2~0.3 mm) to eliminate cutting microcracks.
  • High-end 2.5D touch glass: R0.5~R1.0 mm full rounded pencil edge, no sharp corners to avoid chipping during chemical strengthening.
  • Defect rule: No edge chips larger than 0.1 mm; all edge roughness Ra ≤ 0.03 μm.

Roughness & Flatness Defects That Ruin Touch Performance

1. Local high Ra peaks: Causes uneven ITO film thickness and inconsistent sheet resistance, leading to random touch failure. 2. Poor flatness / high TTV: Creates uneven air gaps between cover glass and sensor glass; capacitive signal drifts across the screen. 3. Residual polishing ripples: Visible optical distortion after display lamination, plus uneven AF coating coverage. 4. Unpolished sharp edges: Edge cracks propagate during ion exchange tempering, greatly reducing drop resistance.

Standard Inspection Checklist for RFQ

1. Define Ra and Rz values on the active touch area, not just the whole sheet. 2. Specify TTV total thickness variation and global warp limit to prevent bonding problems. 3. Clearly state edge profile: chamfer size or rounded radius plus edge roughness requirement. 4. Add cosmetic limits: No scratches longer than 1 mm, no pits larger than 0.05 mm in the touch active area.