How does traditional plating address the “dog bone effect” and void defects when the aspect ratio of the interlayer vias exceeds 10:1?
Technical Principles and Process Challenges
Structural Characteristics of Blind and Buried Vias
- Definition Differences
- Blind Via: Connects outer layers to inner layers, typically 50-200μm deep
- Buried Via: Completely embedded within inner layers, aspect ratios can reach 15:1 (e.g., 0.1mm diameter / 1.5mm depth)
- Plating Challenges Matrix
Issue Physical Cause Consequence Layer buildup at via openings Uneven current density distribution (edge effect) Bridge breakage in solder mask (gap < 25μm) Thin copper in the middle of the via Insufficient ion diffusion rate Impedance fluctuation (±8%) Bottom voids Hydrogen bubble entrapment Thermal cycling failure (cracking after >300 cycles)
Electrochemical Kinetics Model described by the Butler-Volmer equation:
i = i_0 \left[ \exp\left(\frac{\alpha nFη}{RT}\right) - \exp\left(-\frac{(1-\alpha)nFη}{RT}\right) \right]
Where:
- (i_0) Exchange current density: Typical value for acidic copper plating solution is 0.5-2 A/dm²
- (α) Transfer coefficient: 0.3-0.7 (depends on additive system)
Case Validation: A 6-layer HDI board using pulse reverse plating improved the copper thickness uniformity from ±35% to ±8% (measured by SEM cross-section)
Core Process Solutions
Horizontal Pulse Plating System (HPS)
- Three-Stage Current Control
- Forward Pulse (50ms On, 10A/dm²): Initiates deposition at the bottom of deep vias
- Reverse Stripping (5ms Off, -3A/dm²): Dissolves excessively thick copper layers at the via openings
- Relaxation Period (20ms): Promotes ion diffusion (Cu²+ concentration gradient reduced by 62%)
- Titanium basket anode (containing 0.03% phosphorus, uniformity improved by 40%)
- Jet system (3m/s flow rate to flush bubbles from within the vias)
Additive Chemical System
- Functional Component Ratios
Component Concentration Range Mechanism Supplier Example Sodium Polysulfide (SPS) 2-5ppm Accelerator (reduces polarization resistance) Atotech Cupracid Polyethyleneimine (PEI) 50-100ppm Inhibitor (preferentially adsorbs in high current areas) Enthone ViaForm Chloride Ions 40-60ppm Complexing agent (stabilizes Cu⁺ intermediate state) MacDermid MCM
Experimental Data: In blind vias with an aspect ratio of 12:1, the optimal additive combination improved coating ductility from 15% to 28% (ASTM E8 standard test)
Process Monitoring and Defect Prevention
Online Monitoring Technology
- Hall Cell Testing (hourly sampling):
- Monitors decomposition products of additives (e.g., SPS → MPS conversion rate >20% requires replenishment)
- Constant Potential Meter (LPR)
- Real-time measurement of polarization resistance (abnormal fluctuations >5% trigger alarms)
Typical Defect Solutions
- “Volcano” Phenomenon
- Cause: Residual photoresist leads to local insulation
- Countermeasure: Plasma de-gluing (O₂/N₂ mixed gas, 200W RF power)
- Dendritic Crystallization
- Cause: Excess chloride ions (>80ppm)
- Countermeasure: Activated carbon filtration (adsorption capacity ≥0.5g/g)
Industrial Application Cases
Automotive Radar PCB (77GHz)
- Requirements
- 20-layer board, buried via aspect ratio 10:1
- High-frequency loss < 0.02dB/cm @ 76.5GHz
- Solution
- Adopted semi-additive process (mSAP):
- Chemical copper plating (0.3μm seed layer)
- Pattern plating (5μm thick, uniformity ±3%)
- Flash etching to remove excess copper
- Impedance control accuracy ±2Ω (original ±5Ω)
- Production yield increased from 82% to 96%
Server Motherboard (PCIe 5.0)
- Innovations
- Nanowire diameter 80nm, length-to-diameter ratio >1000
- Conductive resistance reduced by 45%
- Copper nanowire filling technology (conductive first, then plating):
Technical Economic Analysis
| Item | Traditional DC Plating | Pulse Filling Plating | Difference |
|---|---|---|---|
| Copper Consumption (kg/m²) | 1.2 | 0.8 | -33% |
| Wastewater Treatment Cost | ￥15/piece | ￥9/piece | -40% |
| Equipment Investment | ￥8 million | ￥12 million | +50% |
| Total Cost (24 months) | ￥2.3 million | ￥1.8 million | -22% |
Interactive Topics:
- When PCB routing density exceeds 10μm line width, should filling plating introduce atomic layer deposition (ALD) as a seed layer?
- In high-frequency signal transmission, how can the effect of copper grain orientation (111) on insertion loss be quantified?
- In vertical stacking packaging (3D IC), what are the essential differences between TSV plating and blind/buried via processes?
- What are the feasibility boundaries of cyanide-free plating solutions in high-speed copper plating?
- Can artificial intelligence achieve zero-defect plating through real-time adjustment of pulse parameters?
Topic Tags:#HighDensityInterconnect #PulsePlating #PCBManufacturing #NanoCopperFilling #ElectrochemicalEngineering
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