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Optimizing Pulse Plating Parameters for Uniform Copper Thickness in High-Aspect-Ratio Through-Holes

2025-06-11

In through-holes with aspect ratio>8:1, conventional DC plating shows up to 40% thickness variation (suRFace vs. center). Pulse plating achieves ±10% uniformity via reverse current stripping. This guide details optimization through parametric modeling, bath chemistry, and equipment innovations.

Pulse Plating.png

1. Pulse Plating Mechanism & Modeling

1.1 Mass Transport Equation

Ion concentration distribution:

​

Where:

  • : Ion concentration (mol/cm³)

  • : Diffusion coefficient (6.5×10⁻⁶ cm²/s for Cu²⁺)

  • : Current density (A/cm²)

  • : Electron number (2 for Cu²⁺)

1.2 Key Pulse Parameters

Parameter Symbol Optimal Range Physical Role
Forward CD Jₚ 3-6 ASD Controls deposition rate
Reverse CD Jᵣ 1.5-2.5 Jₚ Strips over-plated surface
Forward Time Tₚ 10-30 ms Governs deposit depth
Reverse Time Tᵣ 0.2-0.5 Tₚ Determines stripping intensity
Off Time Tₒ 1-5 ms Allows ion replenishment

Optimal Ratio: Uniformity peaks at

2. Bath Chemistry & Additives

2.1 Base Solution

Component Concentration Function
CuSO₄ 60-80 g/L Copper ion source
H₂SO₄ 180-220 g/L Enhances conductivity
Cl⁻ 50-70 ppm Promotes anode dissolution

2.2 Additive Synergy

Additive Type Example Dose(mL/L) Mechanism
Suppressor PEG-8000 1.0-1.5 Adsorbs at hole opening
Leveler Janus Green B 0.3-0.5 Prefers high-current areas
Accelerator SPS 2.0-3.0 Boosts bottom deposition

Synergy: 3:1:4 ratio improves throwing power by 120%

3. Equipment & Process Control

3.1 Pulse Power Supply

Parameter Standard DC Optimized Pulse
Rise Time >100 μs <5 μs
Waveform None Trapezoidal (adjustable)
Multi-Channel Sync - ≤1 μs phase error

3.2 Auxiliary Systems

  • Vibration:

    • 30-50Hz, 1-2mm amplitude

    • 25% mass transfer improvement

  • Pulsed Jet:

    • Flow rate 0.5-1.5m/s, pulse cycle 0.5-2s

    • 40% higher ion concentration at hole bottom

4. Validation & Process Window

4.1 Thickness Uniformity Inspection

Method Location Accuracy Metric
XRF Thickness Top/Mid/Bottom ±0.1μm Thickness ratio (Mid/Top)
Cross-section Depth sections ±0.5μm Thickness range
Impedance Test Whole board ±3% Impedance consistency

4.2 Process Window (AR=10:1)

Parameters Uniformity(σ) Throwing Power(T.P%)
Jₚ=4ASD, Jᵣ=8ASD, Tₚ:Tᵣ=10:1 ±8.2% 72%
Jₚ=5ASD, Jᵣ=10ASD, Tₚ:Tᵣ=5:1 ±6.5% 85%
Jₚ=6ASD, Jᵣ=15ASD, Tₚ:Tᵣ=3:1 ±4.7% 93%

5. Case Study

5.1 12-Layer HDI Board (0.2mm hole/2.4mm thickness)

Process Surface(μm) Center(μm) Uniformity
DC Plating 32.5 18.2 44.3%
Optimized Pulse 25.7 23.6 8.2%

5.2 Reliability Tests

  • Thermal Stress (288℃ solder dip):

    • DC holes: Micro-cracks after 5 cycles

    • Pulse holes: No defects @20 cycles

  • Current Loading:

    • 35% higher current capacity (30A for 1h)

Conclusion

Optimized pulse parameters (), additive synergy (PEG-JGB-SPS=3:1:4), and vibration-assisted mass transfer achieve:

  1. ±5% copper thickness uniformity for AR=10:1

  2. 90% throwing power

  3. 4× improvement in thermal reliability

Five Critical Controls:

  1. Reverse Current Law: Jᵣ = 1.8 × J

  2. Golden Time Ratio: T:Tᵣ=5:1 (AR>8:1)

  3. Additive Balance: Real-time suppressor/leveler/accelerator monitoring

  4. Waveform Steepness: Rise/Fall time <5μs

  5. Mass Transfer Enhancement: 30-50Hz vibration + pulsed jet