emc

EMC Pre-Compliance Skill

Automated EMC risk analysis for KiCad PCB designs. Identifies the most common causes of EMC test failures using geometric rule checks, analytical emission formulas, and optional SPICE simulation.

This is a risk analyzer, not a compliance predictor. It catches ~70% of common EMC design mistakes before fabrication. It cannot guarantee FCC/CISPR compliance — only a calibrated measurement in an accredited lab can do that. But it can reduce the first-spin failure rate from ~50% toward ~20-30%, potentially saving $5K-$50K per avoided board respin.

Related Skills

Skill	Purpose
kicad	Schematic/PCB analysis — produces the analyzer JSON this skill consumes
kicad (thermal)	Thermal hotspot analysis — MLCC derating and ferrite/inductor overheating findings can amplify EMC decoupling and filter issues (an over-stressed MLCC degrades; a hot ferrite drifts impedance). Worth cross-checking when EMC flags DC-001/DC-002 or EF-001/EF-002.
spice	SPICE simulation — provides simulator backend for SPICE-enhanced PDN/filter checks

Handoff guidance: Run the kicad skill's analyze_schematic.py and analyze_pcb.py first — this skill consumes their JSON output. Use --full on the PCB analyzer for best results (enables per-track coordinates for ground plane crossing, edge proximity, and return path checks). During a design review, run EMC analysis after the schematic/PCB analyzers, SPICE simulation, and thermal analysis, then incorporate EMC findings into the report.

Requirements

• Python 3.10+ — stdlib only, no pip dependencies
• Schematic analyzer JSON — from analyze_schematic.py --output
• PCB analyzer JSON — from analyze_pcb.py --full --output (recommended with --full)
• SPICE simulator (optional) — ngspice, LTspice, or Xyce for SPICE-enhanced PDN/filter checks. Auto-detected. Without one, analytical models run unchanged.

Workflow

Step 1: Run the analyzers

python3 <kicad-skill-path>/scripts/analyze_schematic.py design.kicad_sch --analysis-dir analysis/
python3 <kicad-skill-path>/scripts/analyze_pcb.py design.kicad_pcb --full --analysis-dir analysis/

Step 2: Run EMC analysis

Pass --analysis-dir analysis/ — the script auto-resolves schematic.json and pcb.json from the manifest's current run, and writes emc.json into the same folder so the manifest tracks it.

# Recommended: auto-resolve inputs from the current run
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/

# Equivalent — explicit paths still accepted (and required if you want to
# point at a non-current run or override one input)
python3 <skill-path>/scripts/analyze_emc.py \
    --schematic analysis/<run_id>/schematic.json \
    --pcb analysis/<run_id>/pcb.json \
    --analysis-dir analysis/

# One-off JSON (bypasses the cache)
python3 <skill-path>/scripts/analyze_emc.py --schematic schematic.json --pcb pcb.json --output emc.json

# SPICE-enhanced (improved PDN and filter accuracy)
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/ --spice-enhanced

# Select target standard
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/ --standard cispr-class-b

# Select target market (sets all applicable standards)
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/ --market eu

# Filter by severity
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/ --severity high

# Human-readable text output
python3 <skill-path>/scripts/analyze_emc.py --analysis-dir analysis/ --text

Step 3: Interpret results

Read the JSON report and incorporate findings into the design review. Each finding has a severity, rule ID, description, and actionable recommendation. See "Interpreting Results" below.

What Gets Checked

44 rule IDs across 18 categories. Each rule has a specific threshold, rationale, and source citation — see references/pcb-emc-rules.md for full details.

Category	Rules	What it detects
Ground plane	GP-001 to GP-005	Signal crossing voids, zone fragmentation, missing ground planes, low fill ratio, multiple ground domains
Decoupling	DC-001 to DC-003	Cap too far from IC, IC with no decoupling cap, cap too far from via
I/O filtering	IO-001, IO-002	Connector without filtering, insufficient ground pins
Switching EMC	SW-001 to SW-003	Harmonic overlap, switching node copper area, input cap loop area
Clock routing	CK-001 to CK-003	Clock on outer layer, long trace, clock near connector
Via stitching	VS-001	Ground via spacing exceeds λ/20 at highest frequency
Stackup	SU-001 to SU-003	Adjacent signal layers, signal far from reference plane, thin interplane capacitance
Diff pair	DP-001 to DP-004	Intra-pair skew vs protocol limits, CM radiation, reference plane change, outer layer routing
Board edge	BE-001 to BE-003	Signal near edge, incomplete ground pour ring, connector area stitching
PDN impedance	PD-001 to PD-004	Anti-resonance peaks, distributed rail impedance at IC load points, cross-rail coupling from downstream switching regulators
Return path	RP-001	Layer transition via without nearby ground stitching via
Crosstalk	XT-001	3H spacing violation, aggressor-victim pairs
EMI filter	EF-001, EF-002	Filter cutoff too close to switching frequency (analytical or SPICE insertion loss)
ESD path	ES-001, ES-002	TVS too far from connector, insufficient ground vias near TVS
Thermal-EMC	TH-001, TH-002	MLCC DC bias derating (SRF shift), ferrite near heat source
Shielding	SH-001	Connector aperture slot resonance near emission source
Emission estimates	EE-001, EE-002	Board cavity resonance, switching harmonic envelope

Advisory outputs (not findings):

• Pre-compliance test plan — frequency band prioritization, interface risk ranking, near-field probe points
• Regulatory coverage — market-to-standards mapping, coverage matrix (what the tool checks vs what requires lab testing)

Output Format

{
  "summary": {
    "total_checks": 42,
    "critical": 2, "high": 5, "medium": 8, "low": 12, "info": 15,
    "emc_risk_score": 73
  },
  "target_standard": "fcc-class-b",
  "findings": [
    {
      "category": "ground_plane",
      "severity": "CRITICAL",
      "rule_id": "GP-001",
      "title": "Signal crosses ground plane void",
      "description": "Net SPI_CLK crosses a 3.2mm gap in GND on In1.Cu",
      "components": ["U3", "U7"],
      "nets": ["SPI_CLK"],
      "recommendation": "Route around the gap, or fill the void"
    }
  ],
  "per_net_scores": [
    {"net": "SPI_CLK", "score": 67, "finding_count": 3, "rules": ["GP-001", "CK-001", "BE-001"]}
  ],
  "test_plan": {
    "frequency_bands": [{"band": "30-88 MHz", "risk_level": "high", "source_count": 12}],
    "interface_risks": [{"connector": "J1", "protocol": "USB", "risk_score": 8}],
    "probe_points": [{"ref": "L1", "x": 45.2, "y": 32.1, "reason": "switching inductor"}]
  },
  "regulatory_coverage": {
    "market": "us",
    "applicable_standards": ["FCC Part 15 Class B"],
    "coverage_matrix": [{"standard": "...", "coverage": "partial", "note": "..."}]
  }
}

Severity Levels

Severity	Meaning	Action
CRITICAL	Almost certain to cause EMC failure	Must fix before fabrication
HIGH	Very likely to cause issues	Strongly recommend fixing
MEDIUM	May cause issues depending on specifics	Review and assess
LOW	Minor risk, good practice	Fix if convenient
INFO	Informational — frequencies, estimates	Useful for lab prep

Risk Score

Each rule ID contributes at most 3 findings to the score (worst severity first). This prevents per-net rules like GP-001 from saturating the score on 2-layer boards. All findings are still reported — only the score is capped.

penalty = sum(worst 3 per rule × severity weight), score = max(0, 100 - penalty). Scores below 50 indicate significant EMC risk.

Interpreting Results

Ground plane findings — Any CRITICAL finding (signal crossing a void) is almost always a real problem. Fix unconditionally.

Decoupling findings — Distance-based findings have moderate false positive rates. A cap at 6mm may be fine for a low-speed IC but problematic for a 100MHz clock buffer. Use frequency context to prioritize.

I/O filtering — Highly relevant for cable-connected products. For board-to-board connections inside an enclosure, the risk is lower.

Diff pair findings — Protocol-specific skew limits are well-defined. USB HS (25ps), PCIe (5ps), Ethernet (50ps). Findings exceeding these limits are real issues.

PDN findings — Anti-resonance peaks are real and cause voltage droop. SPICE-verified findings are more accurate than analytical. If a peak is flagged, add a capacitor with SRF near the peak frequency.

Emission estimates — Order-of-magnitude estimates (±10-20 dB). Use them to prioritize frequency bands for pre-compliance testing, not to predict pass/fail.

EMC Standards

Standard	Flag	Use Case
FCC Part 15 Class B	fcc-class-b	US residential (default)
FCC Part 15 Class A	fcc-class-a	US commercial/industrial
CISPR 32 Class B	cispr-class-b	International (EU CE marking)
CISPR 32 Class A	cispr-class-a	International commercial
CISPR 25 Class 5	cispr-25	Automotive (strictest)
MIL-STD-461G RE102	mil-std-461	Military/defense

The --market flag maps markets to all applicable standards: us, eu, automotive, medical, military.

Limitations

• Cannot predict absolute emission levels better than ±10-20 dB
• Cannot account for enclosure effects (shielding, apertures, seams)
• Cannot predict cable radiation without knowing external cable routing
• Cannot replace full-wave simulation for complex geometries
• Cannot guarantee compliance — only accredited lab measurement can