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Document biscuit board laser ablation workflow
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‎docs/elements/board.mdx‎

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@@ -86,7 +86,8 @@ Usually you'll want to use an autorouter preset:
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- `autorouter="auto"` - Uses the [platform configuration](../guides/running-tscircuit/platform-configuration.md). For tscircuit.com this defaults to `sequential-trace`.
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- `autorouter="sequential-trace"` - Iterate over each trace and use tscircuit's fast built-in autorouter. This method is fast and deterministic but often fails with over 50 traces.
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- `autorouter="auto-local"` - Use the platform configuration, but only route locally (do not make API calls)
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- `autorouter="auto-cloud"` - Use the platform configuration for
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- `autorouter="auto-cloud"` - Use the platform configuration for
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- `autorouter="laser_prefab"` - Reserve [prefabricated vias](./via.mdx#prefabricated-vias-for-laser-routing) that can be reassigned during routing. Ideal for rapid-turn PCBs produced with laser ablation and mechanical drilling templates. See the [Biscuit Board Laser Ablation guide](../guides/tscircuit-essentials/biscuit-board-laser-ablation.mdx) for a complete walkthrough.
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For complex boards with over 50 traces, you should use `autorouter="auto-cloud"`
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to take advantage of tscircuit's cloud autorouters, which internally use the popular

‎docs/elements/via.mdx‎

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| outerDiameter | number \| string | "0.8mm" | Outer diameter of the copper annular ring |
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| pcbX | number | 0 | PCB X position of the via |
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| pcbY | number | 0 | PCB Y position of the via |
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| netIsAssignable | boolean | `false` | Marks the via as prefabricated so autorouters like `laser_prefab` can claim it for any compatible net |
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### Prefabricated vias for laser routing
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The [`laser_prefab` autorouter](./board.mdx#setting-the-autorouter) is designed
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for fabrication flows that combine laser ablation with pre-drilled or templated
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vias. These workflows start with a "biscuit" carrier board that already contains
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a precise matrix of holes. Copper is laminated onto the carrier and a laser
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ablates the top copper to define traces before the template's vias are filled or
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re-plated.
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When you mark a via with `netIsAssignable`, the autorouter exports an
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**assignable obstacle** so the router can reuse the via for whichever net needs
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to cross layers during routing. This lets you place a library of vias ahead of
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time while still giving the autorouter flexibility to finish the layout. You can
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still pre-assign a net to the via in your design; the autorouter only replaces
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that assignment if a different trace claims the via.
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:::tip
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Group all of the prefabricated vias together—usually in a separate subcircuit or
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component library—so you can reuse the same biscuit template across multiple
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projects. The `netIsAssignable` flag prevents unused vias from shorting random
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nets while remaining available to complete new connections.
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:::
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<CircuitPreview
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defaultView="pcb"
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code={`export default () => (
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<board width="10mm" height="10mm" autorouter="laser_prefab">
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<testpoint name="TP_TOP" footprintVariant="pad" pcbX={0} pcbY={4} layer="top" />
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<testpoint name="TP_BOTTOM" footprintVariant="pad" pcbX={0} pcbY={-4} layer="bottom" />
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<via
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name="V_ASSIGNABLE"
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pcbX={0}
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pcbY={0}
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fromLayer="top"
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toLayer="bottom"
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holeDiameter="0.3mm"
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outerDiameter="0.6mm"
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netIsAssignable
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/>
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<trace from="TP_TOP.pin1" to="V_ASSIGNABLE.top" />
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<trace from="V_ASSIGNABLE.bottom" to="TP_BOTTOM.pin1" />
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</board>
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)`}
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/>
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When the board renders, any assignable via that remains unused keeps its
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original net assignment. Otherwise, the autorouter replaces its net with the one
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required for the completed connection. For an end-to-end workflow, read the
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[Biscuit Board Laser Ablation guide](../guides/tscircuit-essentials/biscuit-board-laser-ablation.mdx).

‎docs/guides/tscircuit-essentials/biscuit-board-laser-ablation.mdx‎

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---
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title: Biscuit Board Laser Ablation
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description: Learn how to route PCBs against a prefabricated via template using the laser_prefab autorouter and the netIsAssignable via property.
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---
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import CircuitPreview from "@site/src/components/CircuitPreview"
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# Biscuit Board Laser Ablation
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The **laser_prefab** autorouter pairs tscircuit with a fabrication process that
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combines laser ablation and a "biscuit" carrier board. Fabricators pre-drill a
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matrix of vias into an inexpensive substrate—the biscuit—then laminate thin
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copper to the top and bottom. A UV or IR laser ablates the copper, defining the
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traces while leaving the via barrels intact. Because the vias are prefabricated,
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you route against a known template instead of drilling new holes for every
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board.
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This guide walks through designing for that workflow. You'll learn how to:
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1. Prepare a via template that can be reused across designs.
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2. Configure your board to route with `autorouter="laser_prefab"`.
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3. Mark vias as assignable so the autorouter can claim them dynamically.
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4. Validate that the generated routing honors the biscuit's geometry.
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## 1. Build a reusable via template
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Create a library subcircuit that contains every via location available on the
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biscuit board. Mark each via with `netIsAssignable` so it becomes an assignable
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obstacle when the router runs. The vias can optionally include default net names
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to serve as documentation for repeated layouts.
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```tsx title="src/biscuit-template.tsx"
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import { Fragment } from "react"
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export const BiscuitTemplate = () => (
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<Fragment>
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<via name="B1" pcbX={-4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B3" pcbX={4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B4" pcbX={-4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B6" pcbX={4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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</Fragment>
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)
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```
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Keep the template focused on the vias themselves. Copper features such as fiducials or
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alignment marks should live in their own subcircuits so you can swap templates
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without affecting the mechanical stackup.
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## 2. Place the biscuit template on your board
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Include the template inside your `<board />` before adding components. Because
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the vias are already drilled in the physical biscuit, avoid translating or
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rotating the template in a way that misaligns the coordinates.
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<CircuitPreview
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defaultView="pcb"
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code={`export default () => (
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<board width="20mm" height="20mm" autorouter="laser_prefab">
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<group name="biscuit" pcbX={0} pcbY={0}>
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<via name="B1" pcbX={-4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B3" pcbX={4} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B4" pcbX={-4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B6" pcbX={4} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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</group>
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<testpoint name="TP_TOP" footprintVariant="pad" pcbX={0} pcbY={9} layer="top" />
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<testpoint name="TP_BOTTOM" footprintVariant="pad" pcbX={0} pcbY={-9} layer="bottom" />
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</board>
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)`}
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/>
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If your biscuit provides fiducials or tooling holes, align them with your CAD
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origin so the panelization step can easily merge your routed copper with the
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prefabricated template. Keep the board outline slightly inside the biscuit's
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usable area so the laser can clear debris without hitting the rails.
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## 3. Route with `autorouter="laser_prefab"`
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Once the template is in place, add traces exactly as you would for a conventional
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board. The router treats assignable vias as neutral territory: any net can claim
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them provided both layers are available and no design rules are violated.
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```tsx title="src/laser-prefab-example.tsx"
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<board width="20mm" height="20mm" autorouter="laser_prefab">
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<BiscuitTemplate />
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<chip
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name="U1"
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footprint="soic8"
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pcbX={-6}
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pcbY={0}
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connections={{ pin1: "R1.pin1", pin8: "B6.top" }}
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/>
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<resistor name="R1" resistance="1k" footprint="0402" pcbX={6} pcbY={0} />
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<trace from="TP_TOP.pin1" to="B2.top" />
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<trace from="B2.bottom" to="TP_BOTTOM.pin1" />
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</board>
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```
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During routing, tscircuit emits a **simple route JSON** that marks each
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`netIsAssignable` via as an obstacle with the `netIsAssignable` flag. The
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`laser_prefab` preset looks for that flag, allowing it to reserve a via for any
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trace that needs to change layers. The [integration test in the core
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library](https://github.com/tscircuit/core/blob/main/tests/autorouting/laser-prefab.test.tsx)
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verifies the behavior end to end.
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### Reserving specific vias
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Sometimes you know that certain vias must stay tied to a particular net—perhaps
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they connect to ground pour or stitching features. In that case, omit the
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`netIsAssignable` flag on those vias and route them manually. The router will
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leave them untouched while still consuming the rest of the biscuit template as
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needed.
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## 4. Validate the routed output
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After calling `circuit.renderUntilSettled()`, inspect the PCB view or export the
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Gerber/ODB++ data to confirm that:
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- Every claimed via matches a real location on the biscuit template.
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- Unused vias remain isolated and keep their original net names.
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- Trace clearances respect your fabrication limits.
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<CircuitPreview
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defaultView="pcb"
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code={`export default () => (
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<board width="20mm" height="20mm" autorouter="laser_prefab">
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<group name="biscuit">
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<via name="B2" pcbX={0} pcbY={6} fromLayer="top" toLayer="bottom" netIsAssignable />
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<via name="B5" pcbX={0} pcbY={0} fromLayer="top" toLayer="bottom" netIsAssignable />
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</group>
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<testpoint name="TP_TOP" footprintVariant="pad" pcbX={0} pcbY={9} layer="top" />
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<testpoint name="TP_BOTTOM" footprintVariant="pad" pcbX={0} pcbY={-9} layer="bottom" />
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<trace from="TP_TOP.pin1" to="B2.top" />
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<trace from="B2.bottom" to="B5.top" />
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<trace from="B5.bottom" to="TP_BOTTOM.pin1" />
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</board>
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)`}
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/>
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If you need to adjust routing priorities, you can provide a custom autorouter
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object instead of the preset. Just make sure the implementation understands the
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`netIsAssignable` obstacles that tscircuit produces.
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## Additional tips
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- **Version your templates.** Include a `templateVersion` prop or layer marker so
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your fabrication team can confirm they are loading the correct biscuit.
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- **Simulate thermal load.** Prefabricated vias sometimes have smaller annular
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rings. Run a design-rule check (DRC) to ensure high-current nets can handle the
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reduced copper area.
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- **Document drill tolerances.** Share the biscuit's drill chart with your team so
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component footprints can account for any off-center holes.
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With these practices, you can create a repeatable laser ablation workflow that
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leverages tscircuit's autorouting while taking full advantage of prefabricated
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via templates.

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