Locating Micro-Short Circuits in PCBA Using Lock-in Thermography (LIT)

MICro-shorts (<10Ω) are hidden defects in PCBAs that challenge traditional electrical testing or X-ray inspection. Lock-in Thermography (LIT) detects μW-level heat anomalies via synchronized thermal excitation and phase analysis, achieving micron-scale resolution. This guide details LIT implementation through thermal excitation modes, signal processing, and case studies.

1. LIT Principles and System Setup

1.1 Thermal Excitation and Modulation

• Current excitation: Apply AC current (0.1-10Hz, 50-200mA) to induce Joule heating at shorts (ΔT≈0.01-0.1℃);

• External heating: Use IR laser (980nm) to exploit thermal conductivity variations.

1.2 Lock-in Detection (Figure 1)

Thermal images are cross-correlated with reference signals to extract amplitude (A) and phase (φ) maps:

Phase images suppress noise, achieving SNR>20dB.

2. Key Procedures and Parameter Optimization

2.1 Workflow

1. Prescan: Low resolution (640×480) to locate hotspots;

2. Lock-in acquisition: High resolution (1024×768) over 100-200 cycles;

3. Phase demodulation: FFT extracts target frequency components.

2.2 Parameter Tuning

• Frequency selection: Based on thermal diffusion time (), typically 1-5Hz;

• Integration: ≥10ms/frame, ≥10s total to reduce noise;

• Filtering: Bandpass (±0.1Hz) to eliminate environmental noise.

3. Feature Extraction and Localization

3.1 Thermal Signatures

• Amplitude threshold: Target A ≥3× background std. dev.;

• Phase lag: Shorts show 10°-30° phase delay vs. normal areas .

3.2 Multi-Physics Correlation

• Electro-thermal analysis: Validate with IV curve slope (dI/dV);

• 3D tomography: Reconstruct defect depth (±50μm) via multi-angle imaging.

4. Case Studies

4.1 Smartphone Motherboard Micro-Short

• Defect: Tin whisker causing 5.2Ω short between BGA balls;

• LIT setup:

  • Excitation: 100mA@2Hz sine wave;

  • Imaging: FLIR X8580sc (NETD<20mK), 5min integration;

• Result: SNR=28dB in phase map, <20μm localization error.

4.2 Automotive ECU Board Short

• Challenge: Inner-layer short at 0.8mm depth;

• Solution: Laser excitation (2W, 0.5Hz), 0.3mm² phase anomaly;

• Verification: Cross-section confirmed Cu residue between L3-L4.

5. Limitations and Future Work

1. Depth limit: ~2mm max; combine with terahertz waves;

2. Material impacts: Use pulse-compression encoding for high-thermal-conductivity substrates;

3. Automation: Integrate AI (e.g., U-Net) for real-time defect classification.

Conclusion

LIT enables high-sensitivity, non-destructive micro-short localization in PCBAs through time-frequency analysis and multi-physics correlation, revolutionizing fault diagnosis efficiency.