# Fill Resistance — KiCad 10 plugin

Computes the **DC or AC resistance of copper zone fills** between two
contacts, **single- or multi-layer**: the chosen net's fills 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). At a user-set **frequency** the exact 1D foil/barrel
skin-effect correction is applied (AC results are a rigorous lower
bound — see *Model & limits*). Shows per-layer rasterized maps,
potential, current density, and **power density**, reports **per-via
currents** (via ampacity!) and total dissipation at a **selectable test
current**. PNGs + a text summary are saved per run.

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*):
   ```powershell
   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):
   ```bash
   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: Preferences → Plugins → *Recreate Plugin Environment*.

## 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** (real copper shape; through-hole pad contacts all layers,
     SMD pad its own layer) — selected pads fill a side that has no
     rectangles;
   - 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. 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`.

## 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; capped vs uncapped does not change the layer-to-layer DC path
  (the ≥5 µm cap sits over the hole mouth in parallel with the
  annular-ring contact, not in series). 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.
- Tracks and pad copper (other than the selected contacts) are **not**
  part of the conductor model — zone fills + barrels only.
- **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.
- **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.
- 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` accepted.
  **Caveat:** 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, AC results are a rigorous **lower bound**.
  Rule of thumb for 70 µm foil: skin is negligible below ~300 kHz
  (δ = 173 µm at 142 kHz), ~+11 % at 1 MHz.
- 5-point FDM per layer on an auto-sized shared grid (~2 M cells total
  across layers by default). 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.

## Offline / development

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

```powershell
.venv\Scripts\python.exe -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 (Windows shown; on
Linux/macOS use `.venv/bin/python`):

```powershell
uv venv --python 3.11 .venv
uv pip install --python .venv\Scripts\python.exe kicad-python numpy scipy pyamg matplotlib pytest
.venv\Scripts\python.exe -m pytest tests -q          # incl. exact analytic cases
.venv\Scripts\python.exe tools\api_probe.py          # IPC API probe vs live KiCad
.venv\Scripts\python.exe -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`.

## License

GPL-3.0-or-later — see [LICENSE.txt](LICENSE.txt).

## Troubleshooting

- **No toolbar button**: venv still building (wait), or build failed →
  *Recreate Plugin Environment*; 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.