{
  "name": "uniformly heated slab",
  "ambientC": 20,
  "pitchMm": 1,
  "assumptions": ["Only the top face exchanges heat with air"],
  "bodies": [{
    "name": "slab",
    "boxesMm": [[0,0,0,10,10,10]],
    "conductivityWMk": 10,
    "convectionWM2k": [0,0,0,0,0,100],
    "watts": 1,
    "provenance": "Analytical test fixture, not a product rating"
  }],
  "contacts": []
}

python3 tools/thermal3d_glb.py solved/thermal3d.json solved/thermal3d.glb --color-min 20 --color-max 100

# Thermal fields in solids beyond the PCB

`adom-fields thermal3d --spec assembly.json --out solved`

This independent mode solves steady 3D conduction through axis-aligned solid volumes, including heatsink fins, interface layers and package contact surfaces. It does not reuse PCB layer geometry or automatically assign materials to GLB meshes. Existing board `analyze`, `serve` and `ui` commands are unchanged. This mode is not yet integrated with the board viewer/SSE commands.

Example (millimetres, watts, degrees Celsius):

```json
{
  "name": "uniformly heated slab",
  "ambientC": 20,
  "pitchMm": 1,
  "assumptions": ["Only the top face exchanges heat with air"],
  "bodies": [{
    "name": "slab",
    "boxesMm": [[0,0,0,10,10,10]],
    "conductivityWMk": 10,
    "convectionWM2k": [0,0,0,0,0,100],
    "watts": 1,
    "provenance": "Analytical test fixture, not a product rating"
  }],
  "contacts": []
}
```

A body's boxes form a union; different bodies may touch but cannot overlap. Every touching pair of different bodies requires one contact declaration, e.g. `{"bodies":["case","isolator"],"resistanceM2kW":0.00002,"provenance":"Measured interface at specified pressure"}`. Zero resistance explicitly means ideal contact. A declared contact with no shared face is rejected. Contact resistance is area-normalized, not K/W; a uniform patch has R=R''/area.

`convectionWM2k` orders faces -X,+X,-Y,+Y,-Z,+Z. It applies only to exposed faces; zero insulates. Air is not meshed. Each source's watts are distributed uniformly over its union volume. To model a localized die, create its explicit body/contact rather than heating an entire heatsink. A lumped junction-to-case estimate may be added separately to a suitable package case boundary; never count the same resistance twice.

The grid includes every box boundary and subdivides intervals to at most `pitchMm`, preserving thin layers instead of rounding them away. Neighbour conductance includes both half-cell material resistances and any interface resistance. Boundary convection includes half-cell conduction. The solver rejects unsupported fields, negative/nonfinite inputs, overlaps, missing contacts, insulated islands, excessive grids and failed convergence/energy balance. Limit: two million Cartesian grid cells, including empty cells.

`thermal3d.json` contains the input and its SHA-256, cell bounds and temperatures, signed heat transfer and flux on conductive faces, exposed-face heat rejection, body extrema, PCG residual and energy balance. Temperature is cell-centred, not an exact surface measurement. Check mesh convergence near thin contacts and local sources. Float colours are for visualization; numerical extrema remain in JSON.

Export a Z-up, metre-unit GLB with named bodies and solved cell colours:

```
python3 tools/thermal3d_glb.py solved/thermal3d.json solved/thermal3d.glb --color-min 20 --color-max 100
```

Use the canonical Adom 3D viewer with Z-up enabled. All scenarios must use the same colour limits for comparison. Mesh faces at material interfaces are retained so hiding a heatsink reveals the underlying contact layer. Export is piecewise constant per cell, with no invented smooth temperature gradient. Above/below colour limits saturates; include the numeric maximum and range in the viewer. No automatic GLB material/thermal-property inference, arbitrary triangle mesh import, fan animation, CFD, radiation, phase change, transient capacity, or heat-pipe transport is implemented. Prescribed moving-air h is not measured installed fan performance. Thermal ceilings from these assumptions are conditional estimates, never qualified device ratings.