Biscuit Board Laser Ablation

The laser_prefab autorouter pairs tscircuit with a fabrication process that combines laser ablation and a "biscuit" carrier board. Fabricators pre-drill a matrix of vias into an inexpensive substrate—the biscuit—then laminate thin copper to the top and bottom. A UV or IR laser ablates the copper, defining the traces while leaving the via barrels intact. Because the vias are prefabricated, you route against a known template instead of drilling new holes for every board.

This guide walks through designing for that workflow. You'll learn how to:

1. Prepare a via template that can be reused across designs.
2. Configure your board to route with autorouter="laser_prefab".
3. Mark vias as assignable so the autorouter can claim them dynamically.
4. Validate that the generated routing honors the biscuit's geometry.

1. Build a reusable via template

Create a component for your biscuit template that contains every via location available on the board. Mark each via with netIsAssignable so it is treated as an assignable via when the router runs. The vias can optionally include default net names to serve as documentation for repeated layouts.

src/biscuit-template.tsx

import { Fragment } from "react"

export const BiscuitTemplate = () => (
  <Fragment>
    <via name="B1" pcbX={-4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B3" pcbX={4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B4" pcbX={-4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B6" pcbX={4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
  </Fragment>
)

Keep the template focused on the vias themselves. Copper features such as fiducials or alignment marks should live in their own components so you can swap templates without affecting the mechanical stackup.

2. Place the biscuit template on your board

Include the template inside your <board /> before adding components. Because the vias are already drilled in the physical biscuit, avoid translating or rotating the template in a way that misaligns the coordinates.

export default () => (
<board width="20mm" height="20mm" autorouter="laser_prefab">
  <group name="biscuit" pcbX={0} pcbY={0}>
    <via name="B1" pcbX={-4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B3" pcbX={4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B4" pcbX={-4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B6" pcbX={4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
  </group>

  <testpoint name="TP_TOP" footprintVariant="pad" pcbX={0} pcbY={9} layer="top" />
  <testpoint name="TP_BOTTOM" footprintVariant="pad" pcbX={0} pcbY={-9} layer="bottom" />
</board>
)

Keep the board outline slightly inside the biscuit's usable area so the laser can clear debris without hitting the rails.

3. Route with autorouter="laser_prefab"

Once the template is in place, add traces exactly as you would for a conventional board. The router treats assignable vias as neutral territory: any net can claim them provided both layers are available and no design rules are violated.

src/laser-prefab-example.tsx

<board width="20mm" height="20mm" autorouter="laser_prefab">
  <BiscuitTemplate />

  <chip
    name="U1"
    footprint="soic8"
    pcbX={-6}
    pcbY={0}
    connections={{ pin1: "R1.pin1", pin8: "B6.top" }}
  />
  <resistor name="R1" resistance="1k" footprint="0402" pcbX={6} pcbY={0} />

  <trace from="TP_TOP.pin1" to="B2.top" />
  <trace from="B2.bottom" to="TP_BOTTOM.pin1" />
</board>

During routing, tscircuit emits a simple route JSON that marks each netIsAssignable via with the netIsAssignable flag. The laser_prefab preset looks for that flag, allowing it to reserve a via for any trace that needs to change layers. The integration test in the core library verifies the behavior end to end.

Reserving specific vias

Sometimes you know that certain vias must stay tied to a particular net—perhaps they connect to ground pour or stitching features. In that case, omit the netIsAssignable flag on those vias and route them manually. The router will leave them untouched while still consuming the rest of the biscuit template as needed.

4. Validate the routed output

After calling circuit.renderUntilSettled(), inspect the PCB view or export the Gerber/ODB++ data to confirm that:

• Every claimed via matches a real location on the biscuit template.
• Unused vias remain isolated and keep their original net names.
• Trace clearances respect your fabrication limits.

export default () => (
<board width="20mm" height="20mm" autorouter="laser_prefab">
  <group name="biscuit">
    <via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
    <via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
  </group>

  <testpoint name="TP_TOP" footprintVariant="pad" pcbX={0} pcbY={9} layer="top" />
  <testpoint name="TP_BOTTOM" footprintVariant="pad" pcbX={0} pcbY={-9} layer="bottom" />

  <trace from="TP_TOP.pin1" to="B2.top" />
  <trace from="B2.bottom" to="B5.top" />
  <trace from="B5.bottom" to="TP_BOTTOM.pin1" />
</board>
)

If you need to adjust routing priorities, you can provide a custom autorouter object instead of the preset. Just make sure the implementation understands the netIsAssignable vias that tscircuit produces.

Additional tips

• Version your templates. Include a templateVersion prop or layer marker so your fabrication team can confirm they are loading the correct biscuit.
• Simulate thermal load. Prefabricated vias sometimes have smaller annular rings. Run a design-rule check (DRC) to ensure high-current nets can handle the reduced copper area.
• Document drill tolerances. Share the biscuit's drill chart with your team so component footprints can account for any off-center holes.

With these practices, you can create a repeatable laser ablation workflow that leverages tscircuit's autorouting while taking full advantage of prefabricated via templates.