contact us
Leave Your Message

Controlling Flow Front Uniformity in Underfill Process

2025-07-02

Underfill.jpg

1. Causes of Flow Non-Uniformity

Factor Mechanism Contribution
Temperature gradient Local viscosity variation >30% 40%~50%
Bump array shadowing Flow resistance difference >5x 20%~35%
Substrate suRFace energy Contact angle fluctuation ±15° 15%~25%
Pre-gelation Viscosity increase >100% 10%~20%

2. Material Optimization

2.1 Rheology Control

  • Viscosity-Temperature:
    Select low activation energy resin (Ea<60kJ/mol), ↓40% sensitivity

  • ThixotropIC Index: 3.0~4.0 (anti-sagging, easy flow)

  • Filler Size: Max diameter ≤1/3 gap size (e.g., Dmax≤16μm for 50μm gap)

2.2 Key Parameters
Parameter Target Test Method
Surface tension 28±2 mN/m ASTM D1331
Contact angle (pad) 15°±3° High-speed camera
Gel time @110℃ 180±20s ISO 2535

3. Dispensing Process Control

3.1 Path Planning

  • Deceleration Strategy:

    • Adaptive Dispensing:

      Zone Speed Bead Width
      Chip center 8mm/s 0.8mm
      Bump dense area 3mm/s 1.2mm
      Edge capillary 1mm/s 1.5mm
  • 3.2 Thermal Management

    • Gradient Preheat:

    • Local Control:

      • IR heating under chip (0.5W/cm²)

      • Peltier cooling at edges (ΔT=-5℃)

    4. Design Optimization

    Strategy Implementation Improvement
    Flow Channels Laser-etched microgrooves (50μmD×100μmW) Uniformity↑45%
    Bump Array Peripheral pitch ↑1.5x Resistance↓30%
    Dam Structures Solder mask dam (20μmH) Overflow↓90%
    Hydrophilic Coating Plasma SiO₂ coating (50nm) Contact angle↓8°

    5. Real-Time Monitoring

    5.1 In-line Inspection
    Technology Resolution Function
    IR thermal imaging 0.1℃ Temperature uniformity
    UV fluorescence 0.5mm Flow front visualization
    Dielectric sensor array 2mm Cure degree mapping
    5.2 Closed-loop Control

    • Adjustment Logic

    6. Defect Prevention

    Defect Root Cause Corrective Action
    Flow stagnation Local cold spot Add IR auxiliary heating
    Side bleeding Surface energy imbalance Apply hydrophobic barrier (contact angle>90°)
    Internal voids Poor venting Step vacuum (-80kPa→-50kPa)
    Flow front bifurcation Asymmetric bump array Adaptive dispensing path

    7. Validation Standards

    Test Method Acceptance Criteria
    Flow front deviation Fluorescence imaging ≤15% length difference
    Filling uniformity X-ray tomography Density variation ≤5%
    Shear strength Shear test @260℃ >40MPa (JESD22-B117)
    Thermal cycle reliability -55~125℃, 1000 cycles Crack propagation <50μm