PCB Design Basics

Everything so far — circuits, firmware, real-time behavior — eventually has to live on a physical board. Turning a schematic into a manufacturable PCB introduces its own set of decisions.

From schematic to layout

A schematic captures what connects to what, expressed as a netlist. Laying out the PCB is about deciding where each component physically sits and how copper traces realize each net — a problem with no single correct answer, only better and worse trade-offs for a given board's constraints.

Footprints

Every component on a schematic needs a matching footprint: the exact pad pattern, sized and spaced to the part's physical package, taken from its datasheet. Getting a footprint wrong — wrong pad pitch, wrong pad size for the package — is one of the most common reasons a board comes back from manufacturing with a part that simply won't solder down correctly.

Layer stack-up

A simple board might use two copper layers (top and bottom); anything denser usually needs four or more. A common 4-layer stack-up:

Layer 1: Signal (top)
Layer 2: Ground plane
Layer 3: Power plane
Layer 4: Signal (bottom)

Dedicating whole layers to ground and power gives every signal a clean, low-impedance return path directly above or below it — which matters enormously for the signal integrity covered in the next sub-lesson — at the cost of a more expensive board.

Design rules

Manufacturers publish minimum design rules: trace width and spacing, via diameter, and clearance to board edges or other copper. Two practical ones to internalize:

• Trace width vs. current: wider traces carry more current without excessive heating; a quick rule of thumb for 1 oz copper is roughly 20 mA per mil of trace width for external traces — but for anything power-critical, use the manufacturer's actual current-capacity charts.
• Via sizing: smaller vias save space but cost more to drill reliably; most fabs have a comfortable default via size that's worth using unless you have a specific reason not to.

Component placement strategy

Before routing a single trace, placement decisions set most of the board's eventual quality:

• Group components by function, and keep noisy digital sections physically separated from sensitive analog ones.
• Place decoupling capacitors as close as possible to the IC power pins they protect — placement here matters more than almost any other layout decision on the board.
• Keep high-speed or high-current paths short and direct before worrying about aesthetics.

Why this matters in practice

A schematic that's electrically perfect can still produce a board that doesn't work reliably if the layout ignores these constraints — undersized traces overheat, missing decoupling causes resets under load, and bad footprints mean a respin before the board even gets powered on. Layout isn't an afterthought to the schematic; it's where a meaningful fraction of a board's real-world reliability is decided.

The next sub-lesson, Signal Integrity Fundamentals, covers what changes once your traces are carrying high-speed signals.