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Quantifying Water Absorption Rate Changes of Substrate Materials in THB Testing

2025-06-17

Substrate Materials.jpeg

1. Test Standards & Conditions

  • Environmental Parameters:

    • Temperature: 85°C ±2°C

    • Humidity: 85% RH ±5% RH

    • Duration: 168 hours (7 days) per IPC-TM-650 2.6.2

  • Sample Preparation:

    • Cut into 50mm×50mm squares (edges polished)

    • Pre-bake: 125°C for 24 hours to remove moisture

2. Water Absorption Rate Formula

Absorption Rate (%) = $\frac{W_{\text{wet}} - W_{\text{dry}}}{W_{\text{dry}}}} \times 100%$

  • $W_{\text{dry}}$: Constant weight after pre-baking (±0.1mg)

  • $W_{\text{wet}}$: Instant weight after wiping suRFace moisture

3. Key Measurement Steps

Step Procedure Instrument Precision
Pre-bake Weighing Weigh after cooling in desICcator Analytical balance (±0.1mg)
Humidity Exposure Suspend vertically, avoid container contact Chamber (±1°C/±2%RH)
Wet Weight Measurement Wipe & weigh within 5 seconds Anti-static tweezers + lint-free cloth

4. Data Recording & Processing

  • Time-Series Monitoring:

    • Weigh every 24 hours, plot absorption-rate vs. time curve (three phases: rapid→slow→saturation)

  • Saturation Point:

    • Absorption rate change <0.02% for 3 consecutive measurements

5. Material Performance Comparison

Parameter Low-Absorption Material (e.g. PTFE) Standard FR-4
Saturation Rate ≤0.01% 0.1–0.3%
Diffusion Coeff. D 10⁻¹⁴ m²/s 10⁻¹² m²/s
Saturation Time >500 hours 48–120 hours

6. Error Control Measures

  • Environmental Interference:

    • Weighing room: 23°C±1°C / 50%RH±5%

    • Transfer samples via desiccator (<30s exposure)

  • Sample Anomaly Handling:

    • Discard samples with bubbles/cracks (deviation up to 300%)

7. Failure Criteria & Correlation

  • Electrical Performance:

    • Every 0.1% absorption increase: Dk ↑0.3, Df ↑15%

  • Mechanical Strength Loss:

           0.25% absorption causes ≥20% flexural strength reduction