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.

PDN mode replaces the single driven pair with a whole power rail: any number of supplies (Thévenin sources with configurable output resistance and open-circuit voltage) and loads with prescribed current draws on one net — set up from marker rectangles in an editable dialog or a JSON config — reporting the IR-drop map, per-supply current sharing and per-load contact voltages in absolute volts.

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+.

Platform support

Platform	Status	Verified by
Windows	✅	development platform, full suite before every release
macOS	✅	field-tested in KiCad 10 (last on plugin v1.3)
NixOS	✅	field-tested in KiCad 10 (last on plugin v1.3, setup)
Debian 12	✅	CI test suite in container
Ubuntu 24.04	✅	CI test suite in container
Fedora (latest)	✅	CI test suite in container
Arch (latest)	✅	CI test suite in container

CI (.gitea/workflows/ci.yml) runs the full pytest suite — solver, rasterizer, and the platform-fallback regressions — headless against the real pip wheels of each Linux row, including the PySide6.QtWidgets import probe that decides the matplotlib backend. What CI cannot do is launch KiCad itself, so "runs inside KiCad" remains field-tested (Windows continuously; macOS and NixOS last field-tested with plugin v1.3 — later versions are covered there by the test suites only).

Setup (one-time)

The plugin is developed and tested on Windows; macOS works (field-tested on KiCad 10 after a round of mac-specific fixes, last with plugin v1.3), and Linux works (field-tested on NixOS — the hardest Linux to run pip wheels on, last with plugin v1.3; mainstream FHS distributions should be no harder, reports welcome). KiCad builds the plugin a private Python venv from requirements.txt on every platform, from pre-built wheels only, no compiler needed. Steps 1–4 are the same everywhere; OS specifics are spelled out per step and in Platform notes below.

1. Enable the API server: KiCad → Preferences → Plugins → check Enable KiCad API.
2. Check the interpreter path on the same page (after a 9→10 upgrade it can still point at KiCad 9):
   • Windows: KiCad's own Python, C:\Program Files\KiCad\10.0\bin\pythonw.exe;
   • macOS: the Python bundled inside the app, /Applications/KiCad/KiCad.app/Contents/Frameworks/Python.framework/Versions/Current/bin/python3;
   • Linux: the first python3 on PATH — needs Python ≥ 3.9 with the venv module (Debian/Ubuntu: sudo apt install python3-venv).
3. Deploy (dev checkout; end users install the PCM zip instead, see Packaging / publishing). Windows:

   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
   

   Plugin directory: Documents/KiCad/10.0/plugins on Windows and macOS, ~/.local/share/kicad/10.0/plugins on Linux.
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 the plugin's venv and restart KiCad —
   • Windows: %LOCALAPPDATA%\kicad\10.0\python-environments\th.co.b4l.fill-resistance
   • macOS: ~/Library/Caches/kicad/10.0/python-environments/th.co.b4l.fill-resistance
   • Linux: ~/.cache/kicad/10.0/python-environments/th.co.b4l.fill-resistance

Platform notes

• Windows is the development and test platform — everything in this README was exercised here. KiCad's bundled Python is 3.13, so the venv gets the current dependency stack.
• macOS — works (field-tested on KiCad 10, last with plugin v1.3). Requires macOS 12+ (KiCad's own minimum; Intel and Apple Silicon — the dmg is universal). KiCad's bundled Python is 3.9, so pip resolves an older stack (numpy 2.0, scipy 1.13, matplotlib 3.9, PySide6 6.9/6.10); the plugin code is kept 3.9-compatible (guarded by a test) and the suite is also run against that older stack. Plot and dialog windows may open behind the KiCad window (they are raised best-effort) — check the Dock if nothing seems to appear after a solve.
• Linux — works (field-tested on NixOS, KiCad 10, last with plugin v1.3; mainstream distributions are audited but not yet field-tested). The venv uses the system Python (3.9+), so the stack matches your distribution. On ARM64 (aarch64) there are no pyamg wheels — requirements.txt skips pyamg there and the solver falls back to Jacobi-CG: same results, noticeably slower on large grids. NixOS: works (field-tested on NixOS 26.05, Plasma 6). pip's Linux wheels link against standard FHS library paths, which NixOS does not provide — PySide6 fails with libgthread-2.0.so.0: cannot open shared object file. The plugin cannot fix this from inside its venv (KiCad installs wheels only); run KiCad inside an FHS environment built with buildFHSEnv, and — on KDE Plasma — unset QT_PLUGIN_PATH, which otherwise poisons the wheel's bundled Qt with the system's Qt plugins. The tested wrapper (exact package list incl. the non-obvious zstd.out and xcb-util family), a steam-run quick test, and a debugging guide are in docs/NIXOS.md.

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. The Mode selector at the top switches to PDN mode — per-terminal currents instead of one driven pair — when rectangles exist on both marker layers or a config file provides the terminal set.
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>/ (if the board directory is not writable — e.g. a demo project opened straight from the mounted installer image — a temp directory is used instead and its path printed to the Messages panel): per-layer 1_raster_map / 2_potential / 3_current_density / 4_power_density PNGs (PDN runs add 5_source_sink_pairs, the pair table as a figure), 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.
6. Experimental — low-current copper marking (dialog checkbox, default off): after the solve, the copper whose |J| is below a threshold is outlined as filled graphic polygons on user layers. The threshold is dialog-settable in one of two units (selector next to the field): relative — % of the mean |J| over all solved copper (default, 10 %) — or absolute in A/mm²; since |J| scales with the test current, the absolute variant is meant to be used with the real operating current entered as test current. Polygons land on User.5…User.8 (TRIM_LAYERS in fill_resistance/config.py; enable them in Board Setup), copper layers mapped in stackup order, top first. Marked specks under TRIM_MIN_AREA_MM2 (0.5 mm²) are dropped. Each region is one selectable polygon — use KiCad's Edit → Convert to turn one into a rule area or zone cutout by hand. Per-layer areas are printed to the Messages panel and the polygons also land in low_current_copper.json next to the PNGs. Every push replaces all graphic polygons on those layers (one undo step). This is a suggestion, not a safe cut list: copper carries little current because the rest carries it — removing copper redistributes the current and raises |J| everywhere else, so re-run after any change. The pour may also serve thermal spreading, EMI return paths, or plane capacitance, which this DC analysis does not see.

Configuration file

The plugin optionally reads a JSON config next to the board file. It can fully specify a run — the shared run parameters, the classic setup (optionally including the terminals themselves, by board reference), or the PDN terminal set.

Named configs: several configs can live side by side as fill_res_config.<name>.json; the config named default loads automatically. Search order on launch: <board stem>.fill_res_config.json (board-specific — several boards can share a directory), then fill_res_config.default.json, then plain fill_res_config.json (the legacy spelling of "default"). Any other config is pulled in per run with the dialog's Load config… button (a file picker starting in the board directory): the dialog re-opens with everything — mode, net, terminals, values — taken from the picked file. Save config… asks for the target file name each time (pre-filled with the loaded config), so writing back and saving a variant under a new fill_res_config.<name>.json are both one click — a name outside the auto-load set prints a reminder that it needs Load config…. The dialog opens even when nothing on the board yields a setup (nothing selected, no marker rectangles, no config): load-only, with both mode radios disabled and their reasons shown, so Load config… can bootstrap the run.

Precedence: config.py constants < config file < dialog edits. The file pre-fills the dialog; what the dialog shows is what runs. A missing file changes nothing; a present-but-invalid file stops the run with a readable error (never a silent fallback). The Save config… button in the dialog writes the current dialog values back to the file (creating it if needed) — in classic mode the run parameters and terminals-by-reference, in PDN mode the whole terminal set — so "run once, tweak, save" is the whole authoring workflow.

Format notes: plain JSON, but full-line comments starting with // are allowed, and keys starting with _ are ignored everywhere ("_comment": "..."). Unknown keys print a warning (typo guard) but don't stop the run. Units are plain SI floats (A, Ω, V, Hz), _mm keys are board millimetres, _um metal micrometres. Any number may also be written as a string with an SI suffix — "50m" = 0.05, "4.7k" = 4700, case separating m (milli) from M (mega) — and the dialog's number fields accept the same suffixes typed directly (50m for a 50 mΩ R_out). freq_hz keeps the frequency grammar ("142k", "1.5M"; a lone m means MHz there).

A classic example (docs/fill_res_config.example.json):

{
    "version": 1,
    "mode": "classic",
    "run": {
        "net": "VOUT+",
        "layers": ["F.Cu", "In1.Cu", "B.Cu"],
        "freq_hz": "142k",
        "adaptive": true,
        "trim": {"enabled": false, "mode": "pct", "value": 10.0}
    },
    "classic": {
        "current_a": 40.0,
        "pos": ["J1.1"],
        "neg": ["J2.1", "J2.2"]
    },
    "physics": {"via_plating_um": 25.0}
}

Every run key is optional and mirrors a dialog field (include_tracks, vias_capped, cap_max_drill_mm, adaptive, cell_um, freq_hz, contact_model, include_buildup, extra_cu_um, push_overlays, v_nominal, trim). classic.pos / classic.neg define the terminals by reference — when present, the board selection / marker-rectangle scan is skipped entirely and run.net is required. The reference grammar (shared with PDN terminals):

form	meaning
"U7"	every pad of footprint U7 that is on the run net
"U7.3"	pad "3" of U7 (split at the first dot — pad numbers are strings and may contain dots)
"rect:NAME"	rectangle on User.3 named by a text item placed inside it
{"rect_mm": [x0, y0, x1, y1], "contact": "F.Cu"}	explicit rectangle, board mm
{"via_mm": [x, y]}	the net's via nearest to (x, y), within 1 mm

physics overrides the constants you'd otherwise hand-edit in config.py (rho_cu_ohm_m, copper_thickness_um, via_plating_um); markers renames the marker layers (pos_layer, neg_layer, pdn_layer). The full schema is documented in fill_resistance/configfile.py.

PDN mode

For power-distribution studies the plugin can replace the single driven terminal pair with multiple supplies and loads on one net: each supply is a Thévenin source (open-circuit voltage v_oc behind r_out_ohm), each load draws a prescribed current i_draw_a. The solve then runs in absolute volts: supply currents fall out of the Thevenin split, and you get the IR-drop map, per-supply delivered current and per-load voltage (mean and worst-case) in summary.txt and the figures.

The summary also carries a source-sink pair table: for every supply × load pair, the effective copper resistance between the two contacts (an operating-point-independent board property — source R_out excluded, contact patterns as in the solve, one extra linear solve per terminal, deferred-corrected on the adaptive grid too) and the copper loss attributed to the pair by proportional sharing (f_ij = I_i·I_j / I_loads, per copper island). The attribution is a convention — the pairwise flow split is not unique physics — but it sums exactly to the total copper dissipation, never crosses a copper gap, and pairs without a common copper path report "no path". The same table is also rendered as its own figure (5_source_sink_pairs.png) next to the field maps.

There are two ways to set a PDN run up: the dialog editor (marker rectangles, no JSON needed) or the config file.

The dialog editor

Draw supply rectangles on User.1 and load rectangles on User.2 — the same marker layers as classic mode, but in PDN mode each rectangle is its own terminal (classic merges each layer into one V+/V−). Optionally place a text item inside a rectangle to name it; unnamed rectangles get automatic names (S1, S2… / L1, L2…). Launch the plugin and switch the Mode selector at the top of the dialog to PDN: two tables — Supplies and Loads, each titled with the marker layer its rectangles come from — list one row per rectangle with its role taken from the layer. Only rectangles whose copper belongs to the selected net are shown: switching the net swaps the visible set, a count of hidden rows appears under the tables, and hidden rows take no part in the run (not validated, not solved). They are still saved, though: Save config… writes every rectangle to the file — off-net ones as "active": false, values and comments included — so nothing set up in the dialog is ever lost by a save. Config-backed runs apply the same per-net filter, so such archived terminals are skipped per run, never an error. A read-only Component column identifies each row by the footprint whose pad intersects the contact rectangle (e.g. U5); when no pad touches it, the nearest component is shown as near U5. You type each load's current draw and each supply's output resistance (plus an optional per-supply V_oc; empty = the V nominal field below the tables). Every row also has a Layer combo picking the copper the terminal contacts — All selected layers (a rectangle's natural scope: a bolted-lug/through contact) or one specific layer, exactly like the classic contact scopes. The tables size themselves to their rows and are height-resizable (drag the divider between them, or enlarge the dialog); in PDN mode the dialog opens at ~60% of the screen height so the tables start with room, and when the form outgrows the screen the dialog scrolls, with the buttons always in view. An Active checkbox per row disregards a terminal for what-if runs without deleting anything: an unchecked row takes no part in the solve, may leave its value cells blank, and is still saved (as "active": false) so it can be re-enabled later — the totals line counts disabled rows. A free-text Comment column annotates each terminal and is saved along with it. A Bonded checkbox per row toggles the lug model (the config's bonded key): checked, the terminal's contacts are shorted into one internally joined lug — the total value stays prescribed, the per-contact split is a solve outcome — and a single-contact terminal checked gets an equipotential-lug contact instead of uniform injection. Rectangles that share one name become a single terminal with Bonded seeded checked — one table row, one total current, per-rectangle split solved (a multi-pin package whose pins are joined by internal metal: the total draw is known, which pin carries how much is exactly what the solve determines); uncheck it to fall back to the area-share split. OK solves; Save config… writes the whole setup to a config file whose name you pick per save (pre-filled with the loaded config, else fill_res_config.json) so the values survive between runs — named rectangles are saved as live rect:NAME references (they follow the rectangle wherever it moves), unnamed ones are frozen as rect_mm coordinates, so label your rectangles if the layout is still moving. A saved config provides the terminal set: the next launch opens in PDN mode (the saved mode is only the starting mode — nothing is pinned, the Mode selector and the net stay switchable) with the geometry read-only, everything else editable. Newly drawn rectangles still show up: any marker rectangle the config doesn't reference yet appears as a fresh terminal row (a note under the tables counts them), and Save config… appends it to the file; a rectangle named after an existing rect:NAME terminal instead joins that terminal as another contact part (add "bonded": true in the file if those parts are internally joined). Delete the config's terminals section (or the file) to return to the pure live scan. In a directory where several boards share fill_res_config.json, save to the board-specific <stem>.fill_res_config.json instead. Rectangle names must be unique across User.1/User.2/User.3. The board selection is ignored in PDN mode (terminals are the rectangles; pads/footprints as terminals need the config file).

The config file

The config file can express everything the editor can, plus terminals made of pads and footprints (reference grammar above). A PDN example (docs/fill_res_config.pdn.example.json):

{
    "version": 1,
    "mode": "pdn",
    "run": {"net": "VCC_3V3", "v_nominal": 3.30, "adaptive": true},
    "terminals": [
        {"name": "buck", "role": "supply",
         "parts": ["U1.SW2", "U1.SW3"], "r_out_ohm": 0.004},
        {"name": "ldo", "role": "supply",
         "parts": ["U2.OUT"], "r_out_ohm": 0.050, "v_oc": 3.28},
        {"name": "mcu", "role": "load", "parts": ["U7"],
         "i_draw_a": 1.8, "bonded": true},
        {"name": "cam", "role": "load", "parts": ["rect:CAM_ZONE"],
         "i_draw_a": 0.35, "contact": "F.Cu"}
    ]
}

• run.net is required; every terminal needs a unique name, a role (supply / load) and non-empty parts (reference grammar above). Active loads need i_draw_a (≥ 0; 0 = voltage probe), active supplies need r_out_ohm (0 = ideal source) and optionally v_oc (default: run.v_nominal, default 3.3 V). With one supply — or all supplies at the same v_oc — that voltage is only the absolute reference (drops and currents don't depend on it); with several supplies at different v_oc, the differences drive the current sharing between them, so per-supply setpoints matter. "active": false keeps a terminal in the file (and in the dialog, unchecked) without it taking part in the run — its value may then be omitted; "comment" is a free-text note shown in the dialog's Comment column.
• "bonded": true shorts all of a terminal's contact cells into one lug (a package with internal metal): the total value stays prescribed, but the per-part/per-cell split becomes a solve outcome and the contact face is equipotential. Without it (the default), a multi-part load splits its draw by area share and a multi-part supply attaches its r_out_ohm per cell. Use bonded for "all pads of U7 draw 1.8 A total, per-pin unknown"; use the default for genuinely distributed draws (a heater area on a plane). A bonded load may even span disconnected copper sheets — the bond is the connection.
• Area terminals (a plane region rather than pads): draw a rectangle on User.3 (markers.pdn_layer) and place a text item inside it — the text names the rectangle for rect:NAME (rectangles have no name of their own in the IPC API). One layer serves supplies and loads alike; the role comes from the config.
• On launch the plugin resolves every reference against the board and shows the terminal tables in the dialog with the geometry read-only (a config file is authoritative for which terminals exist) but everything else editable — the I / R_out / V_oc columns, the Layer scope, the Active checkbox, the Comment, the net, and the Mode selector itself (the file's mode is only where the dialog starts; switching to classic and saving keeps the terminals section intact). Tweak a value, OK runs with it, Save config… writes it back. The Layer combo edits the terminal-level contact key (part-level contacts inside parts keep winning, per the schema); structural edits (adding terminals, changing parts) happen in the file.
• Contact models are fixed in PDN mode: supplies attach through their output resistance (per-cell equal conductances), loads inject uniformly (the classic "uniform" model). The contact-model setting is ignored with a note.
• Current must be able to flow: every load must sit on copper connected to at least one supply (through vias counts), and a load spanning disconnected sheets is an error. Two supplies with unequal v_oc on the same copper exchange a circulating current — that is physics, not a bug.
• power balance in summary.txt is the Tellegen check: source power = copper loss + r_out loss + load power. At freq_hz > 0 all draws are assumed in phase (worst case), same skin-only caveats as classic AC.
• The geometry_dump.json of a PDN run embeds the full terminal set (schema v8), so standalone.py re-solves it offline with no extra flags; older dumps load unchanged.

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 (or physics.via_plating_um in the config file). 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] [--config fill_res_config.json]
    [--v-nominal 3.3]

--config applies a config file's run section as defaults under the explicit flags (CLI wins; the dump already bakes geometry and physics). PDN dumps (schema v8) re-solve their embedded terminal set automatically; --v-nominal overrides the default supply open-circuit voltage there.

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); on Linux make sure python3-venv is installed. Last resort: delete the venv directory by hand (setup 4) and restart.
• "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.