Maintaining Rz<1.5μm in Ultra-Low Profile (VLP) Copper Foil Processing

1. Definition & Importance of VLP Copper Foil

Ultra-Low Profile (VLP) copper foil requires surface roughness (Rz) <1.5μm for high-frequency (5G/mmWave) and HDI PCBs. Key benefits include:

• Reduced signal loss: Lower surface resistance and scattering loss (0.2 dB/cm improvement @10 GHz);

• Improved impedance control: Minimized dielectric thickness variation (±5% tolerance);

• Enhanced adhesion: Smooth surfaces increase resin mechanical anchoring (20–30% peel strength gain).

2. Critical Process Controls

2.1 Foil Selection & Pretreatment

• Foil types:

  • Electrodeposited: Nano-grained VLP foil (e.g., Mitsui ML-VLP, Rz=1.0–1.2μm);

  • Rolled: Multi-pass rolling (>90% reduction) + annealing (Rz=0.8–1.0μm).

• Surface cleaning:

  • Alkaline degreasing (NaOH 5%, 50°C);

  • Acid pickling (H₂SO₄ 10%, 30s).

2.2 Chemical Etching Optimization

• Etchant formulation:

  • H₂O₂ (8–12%) + H₂SO₄ (5–10%) with 0.1–0.3% BTA;

  • Temperature: 30±2°C.

• Dynamic control:

  • Real-time etch rate monitoring (25–35 μm/min);

  • ORP sensor-controlled H₂O₂ dosing (ORP≥800 mV);

  • Spray pressure: 0.3–0.5 MPa.

2.3 Surface Microetching & Planarization

• Microetching:

  • Na₂S₂O₈ (5%) + H₂SO₄ (3%), 30–60s (Rz improvement >30%);

  • Rate: 0.5–1.0 μm/min.

• Mechanical polishing:

  • Diamond slurry (0.1–0.5μm), 0.1–0.3 MPa, 200–500 rpm;

  • Post-polish Rz: 0.5–0.8μm, Ra<0.1μm.

2.4 Electroplating & Coating

• Copper plating:

  • Pulse plating (2 ASD forward, 20% reverse duty cycle);

  • Brighteners (PEG) + levelers (Janus Green B).

• Anti-oxidation:

  • Electroless Ni (0.1–0.2μm, 7–9% P) or OSP coating.

3. Inspection & Process Control

• Roughness measurement:

  • Laser confocal microscopy (Keyence VK-X3000, ±0.01μm);

  • AFM for Ra<0.1μm.

• Real-time monitoring:

  • In-line white-light interferometry (5–10 points/roll);

  • SPC charts (UCL=1.5μm, LCL=0.8μm).

4. Case Study & Results

Validation: A high-frequency PCB manufacturer achieved:

• Rz reduced from 1.8μm to 0.9±0.2μm;

• 10 GHz insertion loss decreased by 0.25 dB/cm;

• Lamination strength increased to 1.5 N/mm (IPC-TM-650 2.4.8).