Simulating EMI Radiation Hotspots at PCB Edges

1. Radiation Mechanisms

• Radiation Sources:

  • Common-Mode Currents: Unshielded edge traces act as monopole Antennas ($E\propto I_{CM}\times L\times f^2$)

  • Edge Cavity Resonance: λ/4 waveguide modes ($f_{res}=\frac{c}{4W\sqrt{{\epsilon }_r}}$)

• Key Parameters:

  Parameter	Radiation Impact
  Trace-to-edge dist. (d)	$\propto 1\mathrm{/}{d}^2$
  Rise time (t<sub>r</sub>)	$\propto 1\mathrm{/}{t}_r^2$
  Reference layer gaps	+20~40dB

2. SIMulation Modeling

2.1 Geometry Requirements

• Edge Treatment:

  • Chamfer radius ≥0.5mm

  • 100pF capacitors across reference layer splits

• 3D Model Details:

  Component	Requirement
  Connectors	Include pin parasitics (1~5nH)
  Heatsinks	Model poor grounding (>1kΩ)
  Stackup	Precise Dk (±0.1)

2.2 Excitation Setup

• Noise Sources:

  • Digital signals: IBIS models (0.5~2ns rise time)

  • Switching power: SPICE models (di/dt>1A/ns)

• Frequency Sweep:

  • Base: 30MHz~1GHz (10MHz step)

  • Critical bands: ±20% of resonance (1MHz step)

3. Hybrid Simulation Flow

4. Algorithm Comparison

5. Hotspot Diagnosis

5.1 Frequency-Domain Analysis

5.2 Spatial Localization

• Near-Field Scan Simulation:

  • Virtual probe @1mm from edge (1mm resolution)

  3mA/m @30MHz = hotspot

• Current Distribution:

  0.1A/m current density = radiation path

6. Optimization Validation

1. Implementation:

   • Ground vias along edge (spacing <λ/20)

   • Critical traces indented 3H (H=board thickness)

2. Re-simulation:

   • Recalculate far-field (3m virtual chamber)

3. Compliance Criteria:

   Standard	Limit (30MHz~1GHz)
   CISPR 25	24~50dBμV/m
   FCC Part 15	40~60dBμV/m
   Internal	-6dB below spec

7. Measurement-Simulation Correlation