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Determining Reflow Profiles for Lead-Free Solder Using Thermomechanical Analysis (TMA)

2025-06-25

1. TMA Principles & Solder Properties

  • Core Mechanism:
    TMA detects phase transitions via dimensional changes (0.1μm resolution):

    • Expansion kink → Solidus temperature (Ts)

    • Sharp contraction → Liquidus temperature (Tl)

2. Test Procedure

Sample Preparation

  • Geometry:

    • Cylindrical sample (Ø5mm×3mm, suRFace Ra<0.2μm)

    • Pre-cleaned (IPA ultrasonic, 10min)

  • Fixture:

    • Ceramic probe (CTE=7ppm/°C) with 0.02N contact force

Temperature Program
Stage Range Ramp Rate Objective
Equilibration 30°C→100°C 5°C/min Eliminate thermal history
Main test 100°C→300°C 10°C/min Capture phase transitions
Cooling 300°C→30°C 20°C/min Record solidification

3. Curve Analysis & Critical Temperatures

  • Key Points:

    • Ts (Solidus): Peak of first derivative (dL/dT max)

    • Tl (Liquidus): Inflection point of contraction curve

  • Wetting Window:

    • Effective Reflow Range = Tl + 25°C ~ Tl + 40°C (DSC-verified)

4. Reflow Profile Design Rules

Stage Temperature Basis Target Parameters
Preheat Below Ts-50°C Slope 1~2°C/s
Soak Ts-30°C → Ts+10°C Duration 60~120s
Reflow Peak = Tl + (25~35)°C TAL (Time Above Liquidus) 40~90s
Cooling Based on TMA contraction Rate 2~4°C/s

5. Validation Testing

  • DOE Matrix:

    Factor Level 1 Level 2 Level 3
    Peak Temp Tl+25°C Tl+30°C Tl+35°C
    TAL Time 40s 65s 90s
    Cooling Rate 2°C/s 3°C/s 4°C/s

  • Metrics:

    • IMC Thickness: Cu₆Sn₅ layer 3~5μm (>8μm embrittlement)

    • Voiding: X-Ray ≤5% (IPC-A-610G Class 3)

    • Shear Strength: ≥35MPa (JIS Z3198)

6. Failure Mode Prevention

TMA Prediction Reflow Risk Corrective Action
Cooling rate>0.4%/s Micro-cracks Reduce cooling to 2°C/s
Secondary peak Component floating Extend soak to 100s
Wide transition zone Cold solder joints Increase peak to Tl+32°C

7. Equipment & Standards

  • TMA Specifications:

    • Temp accuracy: ±0.5°C

    • Displacement resolution: 0.05μm

    • Sample holder heat capacity <0.1J/K

  • Standards:

    • J-STD-020: Moisture sensitivity

    • IPC-7530: Profile guidelines