Component Placement and Floorplanning
Zones, connectors, decoupling and crystals, heat, mechanical limits, courtyards and assembly.
Floorplan before you place
Placement decides how short and clean the routing can be. It is where most of the layout quality is made, and it is much easier to move a part than a finished route. Do it in two passes: first a floorplan (zones on the board for each function), then the detailed placement of the parts inside each zone.
Worked example: zone areas for the example board
Using the bill of materials from Design Workflow: Schematic to Board (the 22 small passives are shared between zones), each zone needs:
| Zone | Parts (courtyards) | With routing space ×2.3 |
|---|---|---|
| Connector and protection | 109 mm² | 251 mm² with routing space |
| Regulator and bulk capacitors | 100 mm² | 230 mm² with routing space |
| MCU, crystal and decoupling | 159 mm² | 366 mm² with routing space |
| Sensor | 30 mm² | 69 mm² with routing space |
| LEDs, button and headers | 177 mm² | 406 mm² with routing space |
- Total courtyard area 575 mm². A board of 45 × 35 mm = 1 575 mm² gives a ratio of 2.74: enough room for the 2.3 factor, with some left for the mounting holes and edges.
- The MCU zone is the largest: all the decoupling capacitors and the crystal go inside it, so that the high-speed traces stay short.
Connectors and mechanical limits
Start with the parts that are fixed by the outside world: connectors, switches, LEDs, mounting holes, the board outline and any parts that must line up with the enclosure. Put them first and lock them in place.
- Connectors on the edge, with the mating direction clear of other parts; group by function (power, signals, programming) and put the noisy ones (motor, mains) away from the sensitive ones.
- Mounting holes in the corners, at least 3 mm from the edge, with a keep-out for the screw head and washer (about 6 mm diameter for M3).
- Height limits: tall parts (electrolytics, connectors, heat sinks) where the enclosure allows; check the 3D view against the case.
- Antennas: follow the module’s data sheet; a typical Wi-Fi or Bluetooth module needs a copper-free keep-out of about 15 mm beyond the antenna end (375 mm² here), at the board edge, away from metal and batteries.
- Mark orientation on the silkscreen (pin 1, polarity, connector names), so that a person can assemble it without the drawing.
Worked example: tolerance stack for a connector cut-out
- The USB connector must fit the opening in the case. The independent position errors are: connector placement (pick-and-place) ±0.10 mm; board outline (router) ±0.20 mm; case cut-out position ±0.15 mm; case mounting holes versus board holes ±0.15 mm.
- Worst case: 0.60 mm. Statistically (root-sum-square): √(0.10² + 0.20² + 0.15² + 0.15²) = 0.31 mm.
- Make the cut-out larger than the connector by about 0.30 mm each side (a little more than the RSS value, less than the worst case) and check the 3D model. More about tolerance stack-ups: Fits and Tolerances.
Group by function
- Place the schematic blocks as blocks: power, MCU, analog, drivers, interfaces. The signals between blocks run in a short, simple way if the blocks are next to each other in the order of the signal flow.
- Keep analog and digital apart but connected through one ground plane (no splits, see Routing, Vias and Return Paths): sensitive analog circuits sit in their own region, away from switching regulators, clocks and motor drivers.
- Put the high-current paths first: the regulator, inductor and capacitors of a switching converter go as a tight group (see Power, Ground and Decoupling).
- Let the part with the most connections set the layout: usually the microcontroller. Rotate it so that the pins with the most traffic face the parts they talk to.
- Use the rats-nest (the thin lines that show unrouted connections): move parts until the lines are short and do not cross much.
Decoupling, crystals and sensitive parts
These are the parts where placement matters electrically, not just physically.
- Decoupling capacitors: one 100 nF per supply pin, within 2 to 3 mm, with the supply and ground connections short and a via to the plane right next to the pad (see Power, Ground and Decoupling).
- Crystals and oscillators: as close to the chip as possible (under 10 mm), on the same side, with the load capacitors next to the crystal and a ground guard around it; no other signals run under the crystal.
- Reset, boot and analog reference pins: keep the traces short, away from clocks and switching nodes.
- Switching nodes: the fast-changing copper of a buck converter is a small, compact shape: no longer than needed and away from sensitive signals.
- Sensors: keep temperature sensors away from heat, microphones and accelerometers away from vibration and noisy power, and magnetometers away from magnets and large currents.
Worked example: crystal load capacitors
- A crystal specified for a load capacitance CL = 12 pF works at its stated frequency when the capacitance it sees, CL = C1C2 ÷ (C1 + C2) + Cstray, equals 12 pF.
- With equal capacitors C1 = C2 = C this is C ÷ 2 + Cstray. For a stray capacitance of 3 pF (pads, short traces and the chip’s pins): C = 2 × (12 − 3) = 18 pF (check: 18 ÷ 2 + 3 = 12 pF).
- A longer trace adds stray capacitance (about 0.1 pF per millimeter on FR-4 with a plane below): 10 mm adds 1 pF and pulls the frequency by a few parts per million. A crystal 50 mm away can add 5 pF, the error is bigger than the crystal’s tolerance, and the oscillator may not start.
Heat: where the warm parts go
Every part that dissipates power heats the board around it. Put the warm parts (regulators, drivers, power resistors) near an edge or in an area with copper to spread the heat, and keep temperature-sensitive parts (electrolytic capacitors, sensors, crystals, batteries) away from them (see Thermal Design on the Board).
Worked example: an electrolytic capacitor next to a regulator
- A regulator dissipating 1.2 W with a thermal resistance to ambient of 62 K/W runs about 74 K above the air.
- The life of an electrolytic capacitor roughly doubles for each 10 K it is cooler (the Arrhenius rule of thumb). A 2 000 h part rated at 105 °C lasts:
| Capacitor temperature | Life | In years |
|---|---|---|
| 105 °C | 2 000 h | 0.2 years |
| 85 °C | 8 000 h | 0.9 years |
| 65 °C | 32 000 h | 3.7 years |
| 45 °C | 128 000 h | 14.6 years |
A capacitor 5 mm from the regulator could run at 85 °C instead of 45 °C: 16 times shorter life. Move it 15 to 20 mm away, or use a 105 °C part and put it on the cold side of the board.
Assembly: spacing, orientation and sides
| Case | Minimum courtyard spacing |
|---|---|
| Pick-and-place assembly | 0.25 mm |
| Hand assembly, 0603 and larger | 0.5 mm |
| Hand rework of ICs (access for the iron) | 1.0 to 2.0 mm |
| Tall parts next to small ones (shadowing in reflow) | at least the height of the tall part |
- Orientation: place polarized and multi-pin parts with the same orientation (all pin 1s to the left or top) and put the resistors and capacitors on a 0° or 90° grid: it speeds assembly and inspection.
- One side if you can: placing all parts on the top side lets you reflow once and keeps the bottom plane clean. Put parts on both sides only if you must, with the heavy ones on the first-reflowed side (see Soldering).
- Fiducials: two or three round copper marks (1 mm) in opposite corners help a machine find the board position, and three are needed for fine-pitch.
- Panel edges: keep parts 3 mm or more from V-cut edges and 5 mm from tab positions (see Manufacturing Rules, Files and Design Review).
- Through-hole parts: keep them together on one side to make wave or hand soldering easier.
- Silkscreen: reference designators visible and not under the part, a polarity mark for each polarized part, and nothing on pads.
Test points and probe access
- Add test points for every power rail, ground (several), the main clocks, reset and key interface signals; a 1.0 to 1.5 mm pad or a through-hole loop for a scope hook.
- Place them on the top side, on a 2.54 mm grid if you want a header or a probe fixture, and label them.
- Leave room for the programming connector and keep it clear of other parts so that the cable fits.
- Provide a ground point near the scope probe test points, so that a short ground spring can be used.
A placement checklist
- Connectors, holes, LEDs and switches match the enclosure drawing.
- Parts are grouped by function and in signal-flow order; the rats-nest is short.
- Decoupling capacitors are beside their pins; the crystal is under 10 mm from the chip.
- Warm parts have copper and are away from the sensitive parts.
- No part is closer than its courtyard allows; the tall parts will not shade the small ones.
- Orientation is consistent; pin 1 and polarity marks are visible.
- Test points, fiducials and the programming header are present.
- The board outline, keep-outs and antenna areas are clear of copper.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Cannot route: traces cross everywhere | Parts in the wrong order or orientation | Rotate and swap parts; move the high-pin-count part to the center |
| Connector does not fit the case | No tolerance stack; wrong reference | Check the 3D model; add cut-out margin |
| Microcontroller will not start (crystal) | Crystal too far away, wrong load capacitors | Move it close; recalculate the load capacitors |
| Noisy sensor reading | Sensor near the switching regulator or clock | Move the sensor away; separate the power path |
| Capacitors fail early | Placed next to a hot part | Move away; use a 105 °C type |
| Parts tombstone or misplace | Parts too close, uneven pad sizes | Increase spacing; symmetrical pads (see Surface-Mount Soldering) |
Related
- PCB Design and Layout: the overview of this guide.
- Design Workflow: Schematic to Board: sizing the board.
- Routing, Vias and Return Paths: the next step.
- Thermal Design on the Board: heat in more detail.
FAQ
Where do I start placing parts?
With the connectors, mounting holes and anything fixed by the enclosure; then the largest part (usually the microcontroller), then the rest in order of connection.
How close should decoupling capacitors be?
Within 2 to 3 mm of the supply pin, with a short, wide connection and a via to the ground plane at the pad.
How far should the crystal be from the MCU?
As close as you can, under 10 mm, with the load capacitors next to the crystal and a ground guard.
Should I put parts on both sides?
Only when the board is crowded: one side is cheaper and easier to assemble and inspect.
What spacing do SMD parts need?
0.25 mm between courtyards for machine assembly, 0.5 mm or more for hand work, and more where you need to rework.
References
- IPC-7351B (courtyards and land patterns); IPC-7525 (stencils); IPC-A-610 (acceptability).
- Montrose, M. I., Printed Circuit Board Design Techniques for EMC Compliance, IEEE Press.
- Component data sheets (crystal load capacitance, antenna keep-outs, regulator thermal data); Nichicon and Panasonic aluminum electrolytic capacitor technical notes (life).
Last reviewed: October 2026. Spacings, keep-outs and life figures are typical: follow the part data sheets and your assembler’s rules.