kicad-zone-resistance/README.md

Fill Resistance — KiCad 10 plugin

Computes the DC resistance of copper zone fills and traces between two contacts, single- or multi-layer: the chosen net's fills (teardrops included) and tracks on the selected copper layers are solved as coupled finite-difference sheets linked by the net's via and through-hole-pad barrels (18 µm plating, configurable). Shows per-layer rasterized maps, potential, current density, and power density, and reports per-via currents (via ampacity!) and total dissipation at a selectable test current. PNGs + a text summary are saved per run. An optional skin-effect correction (exact 1D foil/barrel solution at a user-set frequency) estimates the resistive skin rise only — it is not an AC impedance simulation (no proximity effect, no inductance; see Model & limits).

Real output on a synthetic two-layer net: current from a soldered THT-pad contact (V+, injected at the drill-wall ring) squeezes past a notch in the F.Cu pour, transfers through the stitching-via field into the B.Cu pour and leaves at the V− lug. Per-via currents and the hottest via are reported.

The matching potential map with equipotential contour lines: they bunch where the field is strongest — nearly the whole 8.7 mV drop happens around the notch on F.Cu.

Uses the KiCad IPC API (kicad-python / kipy), not the deprecated SWIG API. Requires KiCad 10.0.1+.

Setup (one-time)

1. Enable the API server: KiCad → Preferences → Plugins → check Enable KiCad API.
2. Check the interpreter path on the same page: should point at the KiCad 10 Python, e.g. C:\Program Files\KiCad\10.0\bin\pythonw.exe on Windows or /usr/bin/python3 on Linux (after a 9→10 upgrade it can point at KiCad 9).
3. Deploy (dev checkout; end users install the PCM zip instead, see Packaging / publishing):

   powershell -ExecutionPolicy Bypass -File deploy.ps1        # junction (dev)
   powershell -ExecutionPolicy Bypass -File deploy.ps1 -Mode Copy
   

   Linux / macOS (also works on Windows with developer mode):

   python3 tools/deploy.py            # symlink (dev)
   python3 tools/deploy.py --copy
   

4. Restart KiCad; first load builds the plugin venv (numpy, scipy, matplotlib, PySide6 — takes minutes; the Ω button appears when done). If stuck: in the PCB editor, Preferences → PCB Editor → Action Plugins, right-click the plugin's row → Recreate Plugin Environment (context menu only — there is no button). Manual equivalent: delete %LOCALAPPDATA%\kicad\10.0\python-environments\th.co.b4l.fill-resistance and restart KiCad.

Usage

1. Mark the current-injection terminals. Each terminal may have multiple parts (all merged into one externally bonded contact):
   • V+ rectangles on User.1, V− rectangles on User.2 (marker layers, configurable via ELECTRODE_POS_LAYER / ELECTRODE_NEG_LAYER), any number per side, axis-aligned;
   • pads and vias (SMD pad: real copper shape on its own layer; through-hole pads and vias become barrel contacts: the current enters at the drill wall on every spanned layer, see below). Selected pads/vias fill the side that has no rectangles, so mixing both kinds is the everyday workflow: e.g. select one rectangle on User.1 (V+) plus any number of pads / THT holes (Ctrl-click) — the pads together form the V− terminal (a connector's pin group, a via cluster, …). All selected pads/vias go to that one side; if both marker layers already provide rectangles, selecting pads on top is an error;
   • legacy: exactly 2 selected contacts with no marker rectangles still works; empty selection scans the whole board's marker layers.
2. Select the contacts, click the Fill Resistance Ω button.
3. In the dialog, pick the net (defaults to the selected pad's net), check the layers to include, set each contact's layer scope ("All selected layers" = bolted-lug/through contact), the test current, and optionally a grid cell size. Multiple layers are coupled through the net's via/pad barrels automatically.
4. Wait for the solve. Depending on board size, included layers, cell size and your hardware it can take considerable time — large multi-layer pours at fine cell sizes may run for minutes (on our test setup a typical real-board run finishes in ≈ 8 s). Then read R / voltage drop / total power in the figure titles and status bar. Outputs land in <board dir>\fill_res_results\<timestamp>\: per-layer 1_raster_map / 2_potential / 3_current_density / 4_power_density PNGs, summary.txt (incl. the busiest vias with per-via current and dissipation, and the current through each injection area — computed flux with the equipotential model, prescribed area share with the uniform model), geometry_dump.json.
5. Experimental — overlays inside KiCad (dialog checkbox, default off; KiCad ≥ 10.0.1): after the solve, the per-layer |J| heatmaps are pushed into the open board as unlocked reference images on User.9…User.12 (OVERLAY_LAYERS; enable them in Board Setup), copper layers mapped in stackup order, top first. Toggle them in the Appearance panel like any layer; opaque over copper, transparent elsewhere, cold end lifted so it stays visible on the dark canvas. Reference images never plot to gerbers. Every push replaces all reference images on those layers, so don't store unrelated images there. Also available headless: python tools/kicad_heatmap_overlay.py --net X --amps 10.

Model & limits

• Sheet model per layer: R□ = ρ/t, ρ = 1.68e-8 Ωm (20 °C), t from the board's physical stackup. Layer z-positions from the stackup drive the barrel lengths.

• Via/pad barrels: thin-wall annulus, R = ρ·L/(π·d·t_plating), VIA_PLATING_UM = 18 in fill_resistance/config.py. Vias are always plated. Each via also contributes its ring/pad copper (a full-thickness disc of the pad diameter on every spanned layer) and its drill mouth, area-weighted per cell: with the "vias filled + capped" checkbox (default on, VIAS_CAPPED) the mouth carries a thin copper cap (CAP_PLATING_UM = 15, fab spec) on the outer layers and is an open hole on inner layers; unchecked, mouths are open holes everywhere. The fab caps only small vias: drills above the dialog's "capped up to drill" threshold (default CAP_MAX_DRILL_MM = 0.5) keep open mouths even with capping selected. Layer-to-layer the cap never matters at DC (it is in parallel with the annular-ring contact, not in series), so the checkbox only affects in-plane conduction across outer-layer mouths. Sub-cell mouths scale their cells' sheet conductance by the true covered fraction (4×4 supersampling), so coarse grids see the correct small perturbation instead of a whole-cell hole. Barrels are gathered in single-layer runs too (drill mouths perforate a lone plane). THT pads are fully modeled: their exact copper shapes (incl. oblong pads, fetched from KiCad; the outer shape stands in for inner rings) are stamped onto every included layer, and every populated pad carries its full soldered joint on its SOLDER side (opposite the component; the component-side pad face stays bare). The hole holds the component lead (a cylinder of drill − THT_LEAD_CLEARANCE_MM, resistivity THT_LEAD_RHO_OHM_M, copper by default; raise it for brass/steel leads) plus solder in the remaining annulus, both in parallel with the plating. The filled hole also conducts in-plane on every spanned layer (component side and inner layers included): the mouth keeps its copper and additionally carries the plug — lead disc plus solder bore — as conduction-equivalent copper of the full hole depth (the pin continues beyond both mouths, so each layer sees the whole plug cross-section). The joint is side-symmetric except for the solder: coat and cone on the solder side only. On the raster map these mouths render in a darker tin color. The pad face gets the average-thickness solder coat (exact pad shape) and the protruding-lead cone (see barrel contacts below; on oblong pads the cone tapers to the pad's short dimension). Slotted (oval) holes keep their true stadium shape: the barrel wall, drill mouth, contact ring and lead cone all follow the slot (rotated with the pad), and the barrel conducts over the slot's real perimeter/bore area — not a circle of the slot's long dimension. Whether a hole is a via or a THT pad, the owning footprint's side, and its Do not populate flag are all read from KiCad. DNP pads get an open hole and a plating-only barrel, no joint. At f > 0 the thickness scaling is applied multiplicatively to the skin-corrected sheet conductance (approximation). Per layer a barrel attaches to the fill cell under it, or to the nearest copper cell within the pad footprint plus one grid cell — fills joined by thermal-relief spokes still connect; wider antipads do not, and the barrel bridges the layers above/below with the full barrel length. Barrels that reach fill on fewer than two layers carry no current and are reported.

  The four hole types: capped small via, open large via, populated THT pad with its full solder joint (lead ∥ solder ∥ plating in the hole, one-sided pad coat, protruding-lead solder cone), and a DNP THT pad.

• The net's traces (straight and arc tracks, exact outline polygons incl. rounded ends) conduct together with the fills — dialog checkbox, on by default (INCLUDE_TRACKS). Traces narrower than TRACK_1D_FACTOR (3) grid cells are modeled as exact 1D resistor chains along their centerline — true arc length per link, so their series resistance carries no discretization error and no cell-size tuning is needed for thin traces. 1D-modeled traces show potential, power density, and |J| (the true in-trace density from the link currents, |ΔV|/(ρ·Δl)). Pad copper is part of the conductor: THT pad shapes are stamped on every included layer (see above), SMD pad shapes on their own layer (INCLUDE_SMD_PADS) — pads are the junctions where traces and thermal-relief spokes actually meet, so without them a multi-track junction necks down to the accidental overlap of the track ends. Dead-end pads (component terminals) are dropped with the other copper not connected to both contacts.

• Solder buildup on mask openings (dialog checkbox, off by default; INCLUDE_MASK_BUILDUP): zones drawn on F.Mask/B.Mask are treated as mask openings that collect SOLDER_THICKNESS_UM (50 µm) of solder on the exposed pour, plus an optional user-defined added copper thickness (dialog field, e.g. a soldered busbar/wire). The sheet conductance there becomes t_Cu/ρ_Cu + t_solder/ρ_solder + t_extra/ρ_Cu (SAC305 ρ = 1.32e-7 Ωm: 50 µm solder ≈ 6.4 µm copper); interface faces use harmonic-mean conductances. Buildup areas render tin-gray on the raster map; |J| in them is referenced to the conductance-equivalent copper thickness.

• Barrel contacts: a selected via or through-hole pad injects at the drill-wall ring on every layer the barrel spans — the current physically enters through the lead/wire soldered into the hole, so the spreading resistance across the pad and surrounding pour is part of the result (both contact models; verified against R = ρ/(π·t)·acosh(d/2a) for two circular contacts on a sheet). Slotted holes inject along the stadium-shaped slot wall. A soldered THT joint additionally assumes the hole is filled with solder (core in parallel with the plating) and the pad face on the solder side carries an average-thickness solder coat (SOLDER_THICKNESS_UM, 50 µm) over the modeled copper under the pad shape — the solder side is the side opposite the component (taken from the owning footprint; assumed B.Cu if it cannot be found), and the component-side pad face stays bare. There the clipped lead protrudes THT_LEAD_PROTRUSION_MM (1.5 mm, 0 = off) and a solder cone wraps it: full protrusion height at the drill wall, tapering linearly to zero at the pad edge, applied as extra conduction-equivalent copper per cell. The tall solder column at the wall pulls the joint vicinity to lead potential — equivalent to extending the barrel wall vertically — while the taper carries the radial spreading. To model a probe pressed onto the pad face instead, draw a marker rectangle over the pad.

• Contact models (dialog / CONTACT_MODEL): default uniform injection — a conductor pressed on top feeds the current orthogonally with uniform surface density, so |J| ramps across the contact area (R = ΔV̄/I from area-averaged terminal potentials); or equipotential — ideal bonded lug (Dirichlet). The two bracket a real contact: R_equipotential ≤ R_real ≤ R_uniform. If the selected fills form several disconnected copper groups that each touch both terminals (e.g. planes joined only through the bolted lugs), only the equipotential model is well-defined; the uniform model stops with an error instead of prescribing an arbitrary split.

  |J| around the same 3×3 mm contact under both models: the ideal bonded lug crowds the current at the contact edges (no in-sheet current inside an equipotential region), the pressed conductor ramps it across the contact area.

• Fields are reported at the dialog's test current; power scales with I².

• Skin effect (f > 0): per-layer effective sheet resistance from the exact 1D foil-diffusion solution Zs = τρ·coth(τt), τ = (1+j)/δ (SKIN_SIDES = 1 in config: plane facing a return plane; 2 = isolated foil), and the analogous correction for the 18 µm barrel wall. Enter one frequency per run (e.g. a switching harmonic, with its RMS amplitude as the test current); suffixes k/M are accepted. Caveat: this is not an AC impedance simulation — skin resistance is only a small part of real AC behavior. Only through-thickness crowding is modeled: lateral (proximity-effect) redistribution needs a magneto-quasistatic solver and is not captured — since the resistance-driven distribution is the minimum-dissipation one, the f > 0 resistance is a rigorous lower bound — and inductance, usually the dominant term of a real AC impedance, is absent entirely. Rule of thumb for 70 µm foil: skin is negligible below ~300 kHz (δ = 173 µm at 142 kHz), ~+11 % at 1 MHz. At f > 0 the |J| maps are referenced to the skin-reduced conduction-equivalent thickness t/(R_AC/R_DC) — the density in the copper that actually conducts — not the geometric foil thickness.

• 5-point FDM per layer on an auto-sized shared grid (~2 M fine cells with the uniform grid; ~8 M with the adaptive grid, whose unknown count no longer scales with the fine-cell count). Direct sparse solve up to 500 k unknowns, AMG-preconditioned CG (pyamg) above (Jacobi-CG if pyamg is missing). Discretization error typically ≲ 2 % at defaults; halve the cell size and compare to judge convergence.

• Adaptive cells (dialog checkbox, on by default; ADAPTIVE_CELLS): solves on a 2:1-balanced quadtree — fine cells at copper boundaries, electrodes, traces, via mouths and buildup, blocks up to ADAPTIVE_MAX_CELL_UM (1 mm) in plane interiors (ADAPTIVE_GUARD sets the clearance a block needs to grow). The minimum element size is the grid cell size itself (auto / dialog / CELL_UM_OVERRIDE); the uniform limit reproduces the normal grid exactly. Large speed/memory wins on big pours. The raw coarse–fine interface flux bias (~0.5–2 % low) is removed by a deferred-correction pass (ADAPTIVE_CORRECTION_PASSES, default 1: reconstruct leaf gradients, move the tangential term to the RHS, re-solve on the reused factorization/AMG hierarchy) — measured residual deviation from the uniform grid ≲ 0.03 %, with the power-balance identity intact. All fields are expanded back to the fine grid for the maps and reports.

  The raster map of the demo net: quadtree leaves drawn on the copper (fine at boundaries, electrodes, via mouths and pads; coarse blocks in plane interiors), the tin-gray solder coat of the THT-pad contact P1, and the via field with its pad copper.

Measured vs. computed: we tested the plugin on a few real boards against a UT3513+ micro-ohm meter; the measured resistances were within ±20 % of the computed values. We attribute the deviation to imperfections of the testing setup (probe placement and probe contact resistance vs. the ideal modeled contacts) and to manufacturing inaccuracies — actual copper and plating thicknesses routinely deviate from nominal. Relative comparisons between layout variants are accordingly more trustworthy than absolute numbers.

Offline / development

Every run writes geometry_dump.json; re-solve without KiCad:

uv run python -m fill_resistance.standalone dump.json `
    [--current 40] [--cell-um 50] [--layers F.Cu,In1.Cu] [--no-show] `
    [--out DIR] [--force-iterative]

Dev environment, tests, headless extraction — uv manages the venv from pyproject.toml/uv.lock (requirements.txt stays: KiCad builds the plugin's runtime venv from it):

uv sync                                              # one-time env setup
uv run pytest -q                                     # incl. exact analytic cases
uv run python tools/api_probe.py                     # IPC API probe vs live KiCad
uv run python -m fill_resistance.board_io dump.json [NET]  # extract only

Packaging / publishing

python tools/build_package.py builds the PCM addon zip in dist/ (installable right away via Plugin and Content Manager → Install from File) plus dist/metadata-registry.json with the SHA-256 and sizes filled in. To publish: upload the zip to a release, set download_url (and the homepage resource in metadata.json), then submit the registry copy as packages/th.co.b4l.fill-resistance/metadata.json in a merge request to https://gitlab.com/kicad/addons/metadata. Icons are regenerated with python tools/gen_icons.py; the README figures in docs/img/ with uv run python tools/gen_readme_figs.py (real solver output on small synthetic boards, plus the hand-drawn hole cross-section).

License

GPL-3.0-or-later — see LICENSE.

Troubleshooting

• No toolbar button: venv still building (wait), or build failed → Recreate Plugin Environment (right-click the plugin's row in Preferences → PCB Editor → Action Plugins); check the interpreter path (setup 2).
• "Could not connect to KiCad's IPC API": API server not enabled, or KiCad not running (no headless mode in KiCad 10).
• "KiCad is busy": a modal dialog is open in KiCad — close it, rerun.
• Windows don't appear: they may open behind KiCad (raised best-effort); PNGs are always saved regardless.
• Result seems too low/high: remember the model is fills + barrels only, with ideal contacts; measure electrode-to-electrode.

LLM disclaimer

This plugin was developed with an LLM: Anthropic's Claude (Claude Code, model Claude Fable 5). The physics model, solver, tests, tooling and this documentation (including the figures; all but the hand-drawn hole cross-section are generated by the solver itself) were written by the model, feature by feature, under human direction and review (janik / B4L); most commits carry a Co-Authored-By: Claude trailer.

What keeps this honest: the test suite pins the numerics to exact analytic references (strip and annulus resistances, the acosh spreading resistance of two circular contacts, skin-effect limits, power-balance identities) and to convergence/regression checks; run it with uv run pytest. Real boards were measured against a UT3513+ micro-ohm meter (see Measured vs. computed above). Nevertheless, an LLM wrote this: read Model & limits critically, treat surprising numbers with the usual engineering suspicion, and cross-check against a hand estimate before trusting a result with hardware. Bug reports are very welcome.