Suppressing Dog-bone Effect in Ultra-high Aspect Ratio (>20:1) Through-hole Electroplating

2025-03-30

Dog-bone Effect refers to the phenomenon where the plating thICkness at the via entrance and exit is significantly greater than the middle region during high-aspect-ratio through-hole electroplating. This arises from limited electrolyte mass transfer and uneven current distribution. Below are strategies to suppress it:

1. Electrolyte Formulation Optimization

Additive Control:
- Suppressors (e.g., PEG, imidazole derivatives) adsorb at high-current-density areas (via entrance) to slow deposition (20–30% reduction).
- Accelerators (e.g., SPS) enhance metal ion reduction in low-current-density regions (via interior).
- Levelers (e.g., Janus Green B) dynamically balance deposition rates.
- Optimal Ratios: DoE-guided concentrations (e.g., PEG: 50–200 ppm, SPS: 5–20 ppm).
Electrolyte Parameters:
- High CuSO₄ concentration (200–300 g/L) to improve mass transfer.
- Low H₂SO₄ (10–50 g/L) and controlled Cl⁻ (50–100 ppm) for additive synergy.

2. Process Parameter Tuning

Current Modulation:
- Pulse Reverse Plating (PRC): Forward current ( $J_{forward} = 2-5 A/dm^{2}$ ) with periodic reverse dissolution ( $J_{reverse} = 0.5-1 A/dm^{2}$ ).
- Periodic Pulse Reversal (PPR) to redistribute ions.
Mass Transfer Enhancement:
- Jet flow (>2 m/s) or ultrasonic agitation (20–40 kHz) to reduce diffusion layer thickness.
- Temperature control (25–35°C).
Staged Plating: Initial low current ( $J = 0.5-1 A/dm^{2}$ ) for bottom nucleation, followed by higher current ( $J = 2-4 A/dm^{2}$ ).

3. Equipment & Auxiliary Design

Auxiliary Anodes:
- Micro IrO₂/Ti anodes near via bottoms to boost local current density by 10–20%.
Cathode Shielding:
- Insulating masks at via entrances to reduce edge field concentration.
Rotating Cathode:
- 10–30 RPM rotation for uniform electrolyte penetration.

4. Via Pretreatment

Wall Activation:
- Electroless Pd or sputtered Au layers to reduce inteRFacial resistance.
Surface Roughening:
- Plasma etching or H₂SO₄-H₂O₂ micro-etching to enhance adhesion.

5. SIMulation & Monitoring

Multiphysics Modeling:
- COMSOL-based current-mass transfer-deposition models to predict plating profiles.
In-situ Sensors:
- Four-point probes for real-time thickness feedback.

6. Validation Metrics

Uniformity: FIB-SEM-measured thickness variation <10%.
Reliability: No cracks/delamination after 1000 thermal cycles (-55–125°C); resistance drift <5%.