The Board Critic

The Board Critic is circsim's read-only, pre-fabrication audit of the layout you brought it. Where the live bench answers "does it work?", the Critic answers "will it survive fabrication and physics?" Those are the risks that live in the copper, not the schematic.

It runs automatically when you open a board (for the checks that need no simulation) and again after each operating-point solve (for the checks that need to know the currents).

Findings are risks to check, not verdicts

This framing matters, so it's worth stating plainly: every Critic finding is a risk to check, not a defect it's certain about. A board layout is full of intentional choices: an exposed pad that's genuinely a no-connect, a wide-tolerance spacing that's fine for your fab. The Critic can't know your intent, so it flags things worth a second look and hands you the numbers and assumptions behind each one, so you can judge.

Every finding carries:

• a title with the specific parts, nets, and numbers involved;
• a detail explaining what it measured and why it matters;
• an "Assumes: …" line stating the key assumption (copper weight, an IPC formula's temperature rise, a heuristic's coarseness), so you can see whether the finding applies to your situation;
• a suggestion: advice only, never auto-applied.

That "Assumes" line is the honesty mechanism. The Critic never over-claims; it shows its work.

Strictly read-only

The Critic never touches your design files, and never edits the board: not the copper, not the components, not a single track. It only reads the layout and adds its own markers to a separate overlay. When you click a finding, circsim flies the camera to it and highlights the involved net or part using the same read-only highlight as hovering. Nothing in your board changes.

You act on a finding by reading it, deciding whether it applies, and (if it does) fixing it in your PCB tool. circsim reports; you decide; your CAD tool edits.

This is more than politeness. It's the same principle that keeps circsim from designing the board it grades: a trustworthy validator has no hand in the artifact.

Static vs. simulation-informed checks

The checks split into two groups by what they need.

Static checks run the moment you open a board (no power, no simulation):

• Floating / dangling connectivity: pads on no net, likely-unconnected exposed pads, nets that reach only one pad.
• Copper clearance: different-net tracks whose copper (track width included) is too close together or too near the board edge. Pads, vias and zones are not assessed.
• Decoupling proximity: IC power pins whose nearest bypass capacitor is missing or too far away.
• Loop area: a coarse estimate of how much area high-speed signal nets enclose against their ground return.

Simulation-informed checks need an operating point first, because they depend on the currents circsim just measured:

• Trace ampacity: is each trace wide enough for the current the solve puts through it?
• IR-drop / rail sag: how much voltage does the copper's own resistance drop between the supply entry and the load, counting copper pours and the ground return?
• Thermal proximity: a first-order relative look at where heat concentrates.

Before you energize, these appear in the panel as "needs simulation" so you know what an operating-point solve would add. Press Energize and they run against the branch currents of the solve: LEDs, resistors and bench sources are measured, and every other part gets the current Kirchhoff's current law leaves on its pads. If the solve cannot work out a part's current, or the copper model cannot reach a pad that carries current, the panel says so ("not assessed" or "partly assessed") instead of reporting a clean pass.

A note on the thermal check

The thermal check is a relative heat-spread proxy in arbitrary units. It tells you which parts sit at the hot end and which hot parts crowd each other, never an absolute temperature in °C. It doesn't model copper pour, layer stack, airflow, or thermal vias. In this version it also needs per-part power data that circsim doesn't compute yet, so it does not run: the Critic panel lists it as "thermal: not assessed" rather than counting it as checked. Treat the other six as the working set today. Once power data is available and it does fire, read it strictly as "these are relatively hotter," not "this reaches N degrees."

Two limits are worth knowing before you lean on the copper-carrying checks:

• Copper weight is assumed to be 1 oz everywhere. It's a fixed default, not read from your board. Ampacity and IR-drop are computed against that, so they're conservative on heavier copper and optimistic on lighter.
• Loop area can't run without ground copper. It measures distance to a ground plane. On a board with high-speed nets but no ground plane at all, rather than silently reporting nothing (which would read as "clean"), the panel shows an explicit "loop area: not assessed (no ground copper)" line, so you know it couldn't check the return path.

The checks reference states every threshold and assumption in full.

For the exact check thresholds, formulas, and the verbatim messages, see the Board Critic checks reference.

How to read the panel

The Board Critic panel (right dock) shows a summary line (N error, N warn, N info) and the findings grouped by severity. Severity is about how likely the risk is to bite, not how certain the Critic is:

• error (red) is a strong signal: touching tracks, a rail sagging past 5 %, an IC with no decoupling at all.
• warn (amber) is worth attention: tight-but-not-touching clearance, a cap a bit too far, a rail sagging 2-5 %.
• info (grey) is a heads-up: a single-pad net, the warmest part.

Click any finding to fly the 3D view to it and light up the net or part involved. Read the detail and the "Assumes" line, decide whether it's real for your board, and if so, go fix it in your layout tool.

An empty panel reads "No risks flagged. Findings are checks, not verdicts." That's the good outcome, stated with the same humility as everything else here.

Related

• Run the Board Critic audit: the hands-on walkthrough.
• Board Critic checks reference: every check in detail.
• The validation bench: why the Critic only ever audits boards you brought.