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tscircuit Essentials

Using ViaGrid / Prefabricated Vias with the Autorouter

Use autorouter="laser_prefab" when your board process gives you a fixed set of prefabricated vias and you want the autorouter to claim those vias instead of placing arbitrary new ones.

The key is to add vias with netIsAssignable={true}. These vias start without a net, then the laser_prefab autorouter can assign each via to a signal when it needs a layer change.

A small ViaGrid example​

This example connects a top-layer SMD IC on the left to a bottom-layer SMD IC on the right. Because both endpoints are surface-mount pads, the route needs a layer change; there are no plated through-hole pads that would already connect the layers.

import {
controllerPins,
peripheralPins,
PrefabViaField,
} from "./via-grid-helpers"

export default () => (
<board
  width="60mm"
  height="30mm"
  layers={2}
  autorouter="laser_prefab"
  defaultTraceWidth="0.1mm"
  minTraceWidth="0.08mm"
>
  <chip
    name="U_CTRL"
    footprint="soic8"
    pinLabels={controllerPins}
    pcbX={-18}
    pcbY={0}
  />
  <chip
    name="U_RIGHT"
    footprint="soic8"
    pinLabels={peripheralPins}
    layer="bottom"
    pcbX={18}
    pcbY={0}
    pcbRotation={180}
  />

  <PrefabViaField />

  <trace from=".U_CTRL .VCC" to=".U_RIGHT .VCC" />
  <trace from=".U_CTRL .GND" to=".U_RIGHT .GND" />
  <trace from=".U_CTRL .CS" to=".U_RIGHT .CS" />
  <trace from=".U_CTRL .SCK" to=".U_RIGHT .SCK" />
</board>
)
PCB Circuit Preview

What makes the vias prefabricated​

The important part is the netIsAssignable via field:

<via
  fromLayer="top"
  toLayer="bottom"
  outerDiameter="0.42mm"
  holeDiameter="0.18mm"
  netIsAssignable={true}
  pcbX={x}
  pcbY={y}
/>

Do not connect these vias to net.GND or another fixed net unless you want them to be stitching vias. For prefab routing, leave them assignable so the router can reserve each via for whichever trace needs it.

Tuning the grid​

  • Keep the grid pitch matched to your fabrication process.
  • Leave keepout space around dense footprints so the generated vias do not sit under pads.
  • Arrange SMD pins and component rotations so the example demonstrates prefab via use without unnecessary crossings.
  • Use autorouterEffortLevel="5x" or higher when the grid is dense or many traces are competing for the same vias.
  • Verify the snippet in an isolated playground before publishing it:
bunx tsci build your-file.circuit.tsx