Preventing Sidewall Serration in Low-Profile Copper (HVLP) Etching

Hyper Very Low Profile (HVLP) copper foil (Rz<3μm) is critICal for high-frequency PCBs, where etching quality directly impacts signal integrity. Sidewall serration—periodic edge roughness (>0.5μm)—causes additional 0.2dB/cm loss at mmWave frequencies. 

1. Serration Formation Mechanisms

1.1 Micro-Etching Dynamics (Figure 1)

Copper etching anisotropy:

Cu+2FeCl3→CuCl2+2FeCl2Cu+2FeCl3​→CuCl2​+2FeCl2​

Serration arises from competing processes:

• Vertical etching: Grain boundary preferential dissolution;

• Lateral undercut: Chemical seepage at resist edges.

1.2 Critical Factors

• Resist slope angle: <70° increases undercut risk by 30%;

• SuRFace tension: >40mN/m causes non-uniform wetting;

• Crystal orientation: <100> etches 1.8x faster than <111>.

2. Etchant Formulation Optimization

2.1 Advanced Chemistry

2.2 Parameter Monitoring

• Specific gravity: 1.32±0.02g/cm³ (refractometry);

• ORP: 450-500mV (vs. Ag/AgCl);

• Fe²+: <0.15mol/L (electrolytic regeneration if exceeded).

3. Process Parameter Tuning

3.1 Spray & Transport

• Pressure: 0.8-1.2bar (high pressure damages resist);

• Nozzle angle: 15° tilt for laminar flow (turbulence <5%);

• Speed: 1.2-1.5m/min (etch factor 2.5-3.0).

3.2 Temperature & Time

• Temperature: 50±1℃ (>2℃ fluctuation triggers orientation-selective etching);

• Multi-stage etching:

  1. Pre-etch: 30s for oxide removal;

  2. Main etch: 90s for vertical profile;

  3. Finishing: 20s with PEG for sidewall smoothing.

4. Equipment Engineering

4.1 Spray Chamber Design

• Zoned flow: Independent upper/lower nozzles to compensate gravity effects;

• Vortex suppressor: Honeycomb baffles reduce Reynolds number <2000.

4.2 Resist Lamination

• Vacuum chuck: Porous ceramic (10μm pores), -60kPa pressure;

• Pre-bake: 75℃×5min to eliminate resist-copper gaps (<0.1μm).

5. Defect Inspection & Validation

5.1 Sidewall Characterization

• Laser confocal microscopy: 3D profile analysis of serration period/amplitude (Figure 2);

• SEM: Grain boundary inspection to evaluate BIA performance.

5.2 Electrical Testing

• Impedance consistency: ±1.5% intra-board variation (10GHz);

• Insertion loss: 0.18dB/cm reduction at 28GHz.

6. Case Studies & Data

6.1 77GHz Automotive Radar

• Copper: HVLP Rz=2.1μm;

• Parameters:

  • Etchant: FeCl₃ 2.3mol/L + PEG-600 0.4g/L;

  • Spray: 1.0bar, 52℃;

• Results:

  Metric	Before	After
  Serration (μm)	0.72	0.15
  Impedance error	±3.8%	±1.2%
  Etch factor	1.8	3.1

6.2 Reliability Tests

• Thermal shock: -40℃~125℃, 500 cycles (no micro-cracks);

• Humidity aging: 85℃/85%RH, 1000h (<0.5% impedance drift).