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Achieving Precision Plating Resist in Selective ENIG Process for 50μm-Pitch Areas

2025-05-20

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Selective Electroless Nickel Immersion Gold (ENIG) in advanced IC substrates and Hdi Pcbs requires defect-free plating resist patterning within 50μm pitch. The micron-scale accuracy and chemical resistance of the resist layer determine pad-line insulation reliability. 

1. Plating Resist Materials and Coating

1.1 High-Resolution Resist Materials

Material Resolution Limit Chemical Resistance (80℃×10min) Thickness
Liquid Photo Resist (LPR) 20μm Slight swelling (<5%) 8-10μm
Dry Film Resist (DFR) 35μm No swelling 15-20μm
Laser Thermal Layer (LTHC) 5μm Fully inert 2-3μm

1.2 Coating Process Optimization

  • Spin coating (LPR):

    • Step 1: 500rpm×10s (spread);

    • Step 2: 2000rpm×30s (uniform film), thickness CV≤3%;

  • Lamination (DFR):

    • Temperature: 70±2℃;

    • Pressure: 0.4MPa;

    • Speed: 0.8m/min (bubble-free).

2. High-Precision Patterning

2.1 Exposure & Development

  • Exposure energy:

    • i-line (365nm): 120mJ/cm² (LPR);

    • Laser direct write (405nm): 25mW×50μm spot (LTHC);

  • Development:

    • 1wt% Na₂CO₃, 0.15MPa spray, 60s;

    • Linewidth loss <5%, sidewall angle 85°±2° (Figure 1).

2.2 Edge Enhancement

  • Plasma ashing: O₂/N₂ (3:1), 300W×30s removes residues;

  • Micro-etching: H₂SO₄-Na₂S₂O₈ (pH=2), 0.2-0.5μm depth for adhesion.

3. Selective ENIG Process

3.1 Electroless Ni/Au Parameters

Parameter Ni Layer (EN) Au Layer (IG)
Temperature 85±0.5℃ 88±0.5℃
pH 4.5-5.0 6.0-6.5
Deposition Rate 15μm/h 0.05μm/min
Additives Stabilizer (thiourea) Complexant (citrate)

3.2 Anti-Overplating Measures

  • Electric shielding: Auxiliary cathodes reduce edge current density (Figure 2);

  • Pulse plating: 30% duty cycle, 100Hz suppresses dendrites;

  • Real-time monitoring: Hull cell tests maintain ±5% Ni²⁺ stability.

4. Inspection & Defect Analysis

4.1 In-line Monitoring

  • AOI: 5μm resolution detects overplating/pinholes;

  • XRF thickness: Ni (2-5μm), Au (0.05-0.1μm), ±0.01μm accuracy.

4.2 Defect Remediation

  • Overplating: Laser ablation (355nm, 0.5J/cm²);

  • Non-uniformity: Adjust spray angle to 30°, flow rate to 2m/s.

5. Case Studies & Validation

5.1 5G mmWave Antenna Module

  • Design:

    • Pitch: 50μm;

    • Pad size: 80×80μm²;

  • Results:

    Metric Before After
    Overplating Rate 12% 0.3%
    Insulation (1GHz) 10⁸Ω 10¹²Ω
    Thickness Uniformity ±15% ±5%

5.2 Reliability Tests

  • Thermal stress: 260℃ reflow ×3 cycles, no delamination;

  • Chemical resistance: 15min ENIG bath, linewidth change <0.5μm.