Pcb layout

<CRITICAL-RULE> NEVER use the Read, Write, or Edit tools on KiCad files (.kicad_sch, .kicad_pcb, .kicad_sym, .kicad_mod, .kicad_pro, .kicad_prl). ALL KiCad file manipulation MUST go through the kicad MCP tools. NEVER run kicad-cli commands via Bash. If an MCP tool returns an error, try different parameters — do NOT fall back to manual file editing.

EVERY KiCad operation has a corresponding MCP tool. Do NOT claim a tool does not exist without first listing all available tools. Key tools that MUST be used instead of file writes:

• add_symbol — create custom symbol definitions in .kicad_sym files
• create_symbol_library — create new .kicad_sym library files
• create_schematic — create new .kicad_sch files
• create_project — create new .kicad_pro project files If you find yourself thinking "there's no MCP tool for this," you are wrong. Check the tool list again. </CRITICAL-RULE>

KiCad PCB Layout

Place footprints and route traces to produce a manufacturable board. This skill assumes you have a completed schematic with a netlist.

Response Format

When this skill activates, print exactly:

Using pcb-layout to place footprints and route traces.

Then proceed directly to the Pre-flight Checks. This is a mechanical execution phase — do not ask the user what to do unless board size or layer count is ambiguous and not specified in specs/bom.md.

Report progress at natural milestones: after footprint placement, after critical trace routing, after autoroute, after post-route refinement. Keep status updates to one line each.

MCP Tools for This Skill

These are the kicad MCP tools you should be using during PCB layout:

Reading / inspection:

• list_pcb_footprints — list footprints on the board
• list_pcb_traces — list traces on the board
• list_pcb_nets — list nets on the board
• list_pcb_zones — list zones on the board
• list_pcb_layers — list layers on the board
• list_pcb_graphic_items — list graphic items on the board
• get_board_info — get board outline, layer count, design rules
• get_footprint_pads — get pad positions and net assignments
• get_footprint_bounds — get board-coordinate bounding box of a placed footprint
• check_placement — check if placing a footprint at (x, y) violates keepout or board edge

Netlist import:

• update_pcb_from_schematic — pull the schematic's netlist onto the board: loads the assigned footprints, binds every pad to its net, creates the .kicad_pcb if missing (requires pcbnew). Re-run after schematic edits; delete_stale=True removes parts dropped from the schematic.

Placing and moving footprints:

• place_footprint — place a footprint on the board
• move_footprint — reposition a placed footprint (warns on keepout/edge violations)
• remove_footprint — delete a placed footprint

Routing:

• add_trace — add a trace segment with layer, width, coordinates
• add_via — add a via at a coordinate
• autoroute_pcb — run Freerouting autorouter (requires Java 17+, auto-downloads JAR)

Post-routing refinement:

• add_copper_zone — create a copper zone (ground plane, power fill) with polygon outline
• add_keepout_zone — create a keep-out zone restricting tracks, vias, pads, copper pour, or footprints
• fill_zones — compute copper fills for all zones (requires pcbnew)
• set_trace_width — change width of existing traces by net, layer, or region
• add_thermal_vias — add a via array under a footprint pad (QFN thermal pads)
• remove_traces — delete traces by net, layer, or region
• set_net_class — create/update net classes with design rules (requires pcbnew)
• remove_dangling_tracks — clean up unconnected trace stubs

Drawing and annotation:

• add_pcb_text — add silkscreen text or other layer text
• add_pcb_line — draw lines (board outline on Edge.Cuts, etc.)

Verification and export:

• run_drc — run design rules check
• export_pcb — export PCB as PDF/SVG
• export_gerbers — export Gerber manufacturing files
• export_3d — export 3D model (STEP/VRML)
• export_positions — export pick-and-place file
• export_ipc2581 — export IPC-2581 data
• export_hierarchical_netlist — use instead of standard netlist export for hierarchical designs with sub-sheets

Footprint libraries:

• list_lib_footprints — browse .pretty library directories
• get_footprint_info — check pad dimensions and pin mapping

Pre-flight Checks

Before starting PCB layout, verify these prerequisites:

1. ERC gate cleared: Run run_erc and confirm violation_count = 0. If ERC has not been run or has violations, STOP and invoke the verification skill first.
2. Annotation check: Run validate_hierarchy on the root schematic. Netlist import fails if references contain ? or have duplicates across sheets. Fix with annotate_schematic (pass project_path for hierarchical designs) before proceeding.
3. Footprint verification: For each schematic component, call list_lib_footprints to verify the assigned footprint exists.
4. BOM cross-reference: If specs/bom.md exists, cross-reference footprints against the BOM's footprint column.
5. Board size constraints: Check board size from BOM artifact (if present) and configure board outline accordingly.
6. IPC design class: If BOM specifies IPC design class, configure design rules (trace width, spacing, annular ring) to match class requirements:
   • Class 1 (consumer): relaxed tolerances
   • Class 2 (industrial): standard tolerances (default)
   • Class 3 (high-rel): tightest tolerances

Layout Process

1. Import netlist — update_pcb_from_schematic creates the board file if missing; footprints appear in a cluster with all nets bound. Net names match the schematic's netlist (local labels sheet-prefixed, e.g. "/SIG") — read them with list_pcb_nets.
2. Board setup — define board outline, layer count, design rules.
3. Place footprints — group by function, then optimize.
4. Route critical traces — power, high-speed, sensitive analog.
5. Route remaining traces — signal interconnects (or use autoroute_pcb).
6. Post-route refinement — review and improve routing quality: a. Set net classes for power nets: set_net_class (e.g., "Power" class with 0.5mm+ width) b. Widen power traces: set_trace_width on power nets (VIN, VOUT, SW, GND paths) c. Add thermal vias: add_thermal_vias under QFN/exposed pads d. Clean up: remove_dangling_tracks to remove autorouter stubs e. Remove and re-route if needed: remove_traces + add_trace for problem areas
7. Add copper zones — add_copper_zone for ground planes on both layers, then fill_zones.
8. Run DRC — fix violations, re-run until clean.
9. Export — Gerbers, drill files, pick-and-place, BOM.

Footprint Placement Strategy

Group by functional stage — keep components from each schematic stage physically together. The schematic's block diagram maps directly to PCB placement regions.

Placement priority order:

1. Connectors — fixed by mechanical constraints (edge of board)
2. ICs / active components — central to their stage
3. Decoupling caps — as close as possible to IC power pins (< 3mm)
4. Input/output passives — near their IC, following signal flow
5. Bulk caps and inductors — near but not blocking routing channels
6. Test points, mounting holes — last, fill remaining space

Orientation conventions:

• Align ICs with pin 1 in a consistent corner (top-left preferred)
• Polarized caps: positive toward the supply side
• Diodes: cathode band facing consistent direction per stage
• Connectors: mating face toward board edge

Spacing minimums:

• IC to decoupling cap: < 3mm center-to-center
• Between 0805 passives: 1.0mm edge-to-edge minimum (1.5mm comfort)
• Between ICs: 3–5mm edge-to-edge for routing channels
• Components to board edge: 1.0mm minimum (2.0mm for wave solder)

Use place_footprint with coordinates in mm. Use move_footprint to adjust after initial placement. Use get_footprint_pads to check pad locations before routing.

Trace Routing

Trace Width by Current

Current (A)	Min width (mm)	Recommended (mm)	Notes
< 0.3	0.15	0.25	Signal traces
0.3–1.0	0.25	0.5	Low power
1.0–2.0	0.5	0.75	Moderate power
2.0–3.0	0.75	1.0	Power traces
3.0–5.0	1.0	1.5	High current
> 5.0	Calculate	Calculate	Use IPC-2152

These assume 1oz (35um) copper, 10C rise, outer layer. Inner layers need ~2x width for the same current.

Routing Rules

Power traces first:

• Route VIN, VOUT, and GND connections before signal traces.
• Use the widest trace the space allows, not the minimum.
• Keep power loops tight — short, wide traces from regulator input cap through the IC to the output cap.

Signal traces:

• Default 0.25mm width for logic signals.
• Keep parallel runs short to minimize crosstalk.
• Route differential pairs together with matched length.
• Avoid 90-degree corners — use 45-degree bends or arcs.

Return paths:

• Every signal needs a return path. On a 2-layer board, route GND traces alongside signal traces.
• On 4+ layer boards, use a continuous ground plane — do not split it under signal traces.

Sensitive traces:

• Analog inputs: guard ring or ground trace on both sides.
• High-impedance nodes: minimize trace length, keep away from switching signals.
• Clock/PWM lines: route away from analog sections.

Use add_trace with layer, width, and coordinate list. Use add_via to transition between layers.

Via Usage

• Signal vias: 0.3mm drill / 0.6mm annular ring (default)
• Power vias: 0.4mm drill / 0.8mm annular ring, use multiple vias in parallel for high current (one via ≈ 1A for 1oz copper)
• Thermal vias: under thermal pads, 0.3mm drill, array of 4–9 vias connecting to ground plane
• Place vias close to pads, not in the middle of trace runs

Post-Autoroute Refinement

After using autoroute_pcb, the board has connectivity but needs refinement before it's manufacturing-ready. The autorouter uses minimum-width traces for everything and has no concept of power integrity or thermal design.

Step 1: Define net classes

Use set_net_class to establish design rules for power nets:

• Create a "Power" class with track_width=0.5 (or wider per current table) for VIN, VOUT, and other power rails.
• Create a "HighCurrent" class with track_width=1.0 for switch nodes (SW) and high-current ground paths.

Step 2: Widen power traces

Use set_trace_width to bring power traces up to their net class width:

• set_trace_width(width=0.5, net_name="VIN")
• set_trace_width(width=1.0, net_name="SW")
• For ground nets routed as traces (before zone fill): widen to 0.5mm+

Step 3: Add thermal vias

Use add_thermal_vias for any QFN or exposed-pad IC:

• add_thermal_vias(reference="U1", rows=3, cols=3, spacing=1.0, via_drill=0.3)
• Thermal vias connect the exposed pad to the ground plane on the opposite layer for heat dissipation.

Step 4: Clean up

Use remove_dangling_tracks to remove autorouter stubs — trace ends that connect to nothing. These are cosmetic but will show as DRC warnings.

If any trace routing is unsatisfactory, use remove_traces to delete traces on a specific net, then re-route manually with add_trace.

Step 5: Add copper zones

Use add_copper_zone on both layers:

• Ground fill on F.Cu and B.Cu (2-layer) or dedicated ground plane on In1.Cu (4-layer)
• Set clearance to match design rules (typically 0.3–0.5mm)
• Then call fill_zones to compute the actual copper fill

Step 6: Final DRC

Run run_drc and fix any remaining violations. Common post-refinement issues:

• Clearance violations from widened traces — may need to re-route nearby signals
• Zone fill islands — delete or connect them

Checklist

IMPORTANT: Use TodoWrite to create todos for EACH checklist item below.

• Pre-flight: verify ERC = 0 violations
• Pre-flight: run validate_hierarchy — no unannotated/duplicate refs
• Pre-flight: verify all footprints exist
• Import netlist: update_pcb_from_schematic (check skipped is empty)
• Define board outline and design rules
• Place footprints by functional group
• Route critical traces (power, high-speed)
• Route remaining traces (or autoroute)
• Post-route: define net classes for power nets
• Post-route: widen power traces
• Post-route: add thermal vias for QFN/exposed pads
• Post-route: clean up dangling tracks
• Add copper zones (ground planes)
• Run DRC — must show zero violations

Layer Stack

2-layer board (default for simple designs):

• F.Cu: signal and power traces
• B.Cu: ground fill + remaining traces
• Route most signals on top, use bottom for crossings

4-layer board (for complex or noise-sensitive designs):

• F.Cu: signal + power
• In1.Cu: ground plane (continuous, no splits)
• In2.Cu: power plane
• B.Cu: signal + power

Rule of thumb: if the design has > 1 switching regulator, > 20 ICs, or any signal > 10MHz, use 4 layers.

Copper Zones (Ground Planes)

• Add a ground zone on B.Cu for 2-layer boards, on In1.Cu for 4-layer.
• Zone clearance: 0.3mm (match design rules).
• Thermal relief on pads: 0.5mm spoke width, 4 spokes.
• Remove isolated copper islands (DRC will flag these).
• On 2-layer boards, add ground zone on F.Cu as well in unused areas to improve shielding and reduce etching.

Silkscreen and Fabrication

• Reference designators: 1.0mm height, 0.15mm line width minimum.
• Move refdes out of pad areas — readability over density.
• Add board name, version, date on silkscreen.
• Add polarity marks near polarized components if not in footprint.
• Add pin-1 indicators near IC footprints if ambiguous.

Use add_pcb_text for silkscreen annotations. Use add_pcb_line for board outline on Edge.Cuts layer.

Design Rule Defaults

Standard 2-layer PCB fab capabilities (most manufacturers):

Parameter	Minimum	Recommended
Trace width	0.15mm	0.25mm
Trace spacing	0.15mm	0.2mm
Via drill	0.3mm	0.3mm
Via annular ring	0.15mm	0.2mm
Pad to pad	0.2mm	0.25mm
Copper to edge	0.3mm	0.5mm

DRC and Export

After routing is complete:

1. Run run_drc — fix all violations before exporting.
2. Common DRC errors:
   • Clearance violation — move trace or via, increase spacing.
   • Unconnected net — route the missing connection.
   • Copper zone island — delete the isolated island or connect it.
3. When DRC is clean, export:
   • export_gerbers — full Gerber set for manufacturing
   • export_positions — pick-and-place file for assembly
   • export_3d — 3D model for mechanical fit check

Full export deliverables (post-DRC):

• Gerber set (copper layers, mask, silkscreen, drill, board outline)
• Drill files
• BOM export
• Pick-and-place file
• 3D model (STEP) for mechanical review
• Fabrication notes: layer stack, copper weight, finish, IPC class, solder mask color
• Assembly drawing

Post-export verification: Confirm Gerber layer count matches design, BOM export matches schematic BOM.

Verifying Footprints in KiCad Libraries

Use list_lib_footprints on .pretty directories to browse available footprints. Common libraries:

• Resistor_SMD.pretty — 0402, 0603, 0805, 1206, etc.
• Capacitor_SMD.pretty — ceramic SMD caps
• Capacitor_THT.pretty — electrolytic, film through-hole
• Package_TO_SOT_SMD.pretty — SOT-23, SOT-223, TO-263, etc.
• Package_SO.pretty — SOIC, SSOP, TSSOP
• Package_QFP.pretty — QFP, LQFP, TQFP
• Connector_PinHeader_2.54mm.pretty — standard pin headers
• Diode_SMD.pretty — SMD diodes
• Inductor_SMD.pretty — SMD inductors

Use get_footprint_info to check pad dimensions and spacing before committing to a footprint.

Extensibility

For complex boards (>30 components, 4+ layers, high-speed signals), consider creating a pcb-plan artifact following the same pattern as schematic-plan. For simpler boards, pre-flight checks and existing placement strategy are sufficient.