Component Placement Strategy: Laying Out a PCB Before You Route It
Experienced layout engineers often say that placement is eighty percent of the work. Good placement makes routing short and obvious; poor placement forces long tracks, extra vias and compromises in grounding that no amount of careful routing can undo. Beginners tend to rush placement because the ratsnest looks overwhelming, then spend hours untangling tracks.
This article gives a repeatable order for placing parts, using a typical microcontroller board as the example: a USB connector, a 3.3 V regulator, an STM32 or ATmega microcontroller with a crystal, a few sensors on I2C, and some headers.
Before You Place Anything
- Know the enclosure. Get the board outline, mounting hole positions and any height limits from the box or mechanical drawing. Draw the outline on
Edge.Cutsfirst. - Set the grid. Placing on a 0.5 mm or 1.27 mm grid keeps parts aligned and makes routing neater. Use View > Grid Properties or the grid drop-down in the toolbar.
- Set design rules. Enter your fab's clearances in File > Board Setup so that DRC courtyard and clearance checks work while you place.
- Spread the parts out. After Update PCB from Schematic, all footprints arrive in one cluster. Select groups and drag them roughly beside the board.
A Proven Placement Order
- Mechanically fixed parts: mounting holes, connectors, switches, LEDs, displays and anything that must line up with a hole in the enclosure. Lock them with the lock command once placed so they cannot move by accident.
- The main IC: put the microcontroller roughly central, oriented so its pins face the parts they connect to. Rotating an IC by 90 degrees can remove half the crossings in the ratsnest.
- Decoupling capacitors: place each 100 nF capacitor right next to the power pin it serves, on the same side, before anything else claims that space.
- Crystal and its load capacitors: as close as possible to the oscillator pins, with nothing else routed between them.
- Power section: regulator, input and output capacitors grouped tightly near where power enters, with a short path to the load.
- Interface parts: ESD protection right behind the USB connector, series resistors near the driving pin, pull-ups anywhere on the net.
- Everything else: sensors, headers and indicator LEDs, following the ratsnest to keep lines short and uncrossed.
Think in Functional Zones
Divide the board into zones that match the blocks on your schematic. Keep noisy circuits away from sensitive ones, and keep high current paths short and away from signal areas.
| Zone | Placement guidance |
|---|---|
| Power input and regulation | Near the power connector; keep switching regulators away from analog inputs |
| Digital core | Microcontroller, memory and crystal together in the middle |
| Analog front end | Close to the sensor connector, away from switching nodes and clocks |
| High current loads | Motor drivers and relays near their output connectors, on the edge |
| User interface | Buttons, LEDs and displays where the enclosure allows access |
Zones do not need separate ground planes. A single solid ground plane with sensible zoning works better for almost every student project than a split plane, as explained in the ground plane article in this series.
Using the Ratsnest as a Guide
The ratsnest lines show the shortest unrouted connection for each net. While placing, aim for short lines that do not cross. Two tricks help a lot.
- Hide the ratsnest for GND and power nets while placing signal parts, because these nets will be handled by planes and pours. In the Appearance panel, open the Nets tab and toggle ratsnest visibility per net or net class.
- Rotate and mirror parts to untangle crossings before routing. Swapping a resistor's orientation is free now and costs a via later.
- Watch for nets that must be short: the crystal, the regulator feedback divider, the switch node of a buck converter, and USB data lines.
Placement Rules for Easy Assembly
- Keep parts at least 0.5 mm from the board edge, and more if the board will be panelised with V-grooves; check your fab's guidance.
- Put all SMD parts on one side if possible. Two-sided assembly costs more and needs a second reflow pass.
- Orient polarised parts such as diodes and electrolytic capacitors the same way, so inspection is quick.
- Leave room for a soldering iron tip around through-hole parts and test points.
- Do not place tall parts where they block a connector or a programming header.
- Make sure no courtyards overlap; DRC reports this as a courtyard overlap error.
Finally, press Alt+3 to check the board in the 3D Viewer. Tall electrolytic capacitors next to a USB plug, or a header under a display, are much easier to see in 3D than in 2D.
Summary
Place fixed mechanical parts first, then the main IC, then decoupling capacitors and crystals that must sit close to it, followed by power, interface and general parts. Group the board into functional zones, use the ratsnest to minimise crossings, and check courtyards and the 3D view before you route a single track. Time spent here pays back many times over during routing and debugging.