PCB design basics

Why it matters

Almost every electronic design ends up on a printed circuit board (PCB). Electrical engineers draw the schematic, often lay out the board, and review layouts for current capacity, noise and safety. A circuit that works in simulation can fail on a board with traces that are too thin, a ground return that loops across the board, or a decoupling capacitor placed too far away. KiCad is a free, professional-grade, open-source tool used by hobbyists and companies alike (https://www.kicad.org).

The workflow

1. Schematic capture: place symbols (resistors, LEDs, connectors, ICs) and wire them. Give every part a reference designator (R1, D1, J1) and a value.
2. Electrical rules check (ERC): catches unconnected pins, outputs driving outputs and missing power flags.
3. Footprint assignment: link each symbol to its physical land pattern, such as a 0805 surface-mount resistor, a 5 mm through-hole LED or a 2-pin 2.54 mm header. The footprint must match the part you will actually buy; check the datasheet's dimensions.
4. Board layout: draw the board outline, place parts, then route copper traces between pads. Unrouted connections show as thin "ratsnest" lines.
5. Design rules check (DRC): checks clearances, trace widths, drill sizes and unrouted nets against the manufacturer's capabilities.
6. Fabrication outputs: Gerber files (one per copper, mask and silkscreen layer), a drill file, and for assembly a bill of materials (BOM) and pick-and-place file.

Layout basics

• Layers: a two-layer board has copper on top and bottom, connected by vias (plated holes). Four-layer boards add inner planes, typically ground and power.
• Ground: use a solid ground pour or plane where you can. Return current flows directly under its signal trace at high frequencies, so a continuous ground keeps loops small and noise low.
• Decoupling: put a 100 nF capacitor right next to each IC power pin with short connections to ground.
• Trace width and current: wider traces carry more current with less temperature rise. IPC-2152 gives charts; on common 1 oz copper (35 µm) an outer trace about 0.25 mm wide handles roughly half an amp for a modest temperature rise, and about 1 mm handles a couple of amps. Use a calculator such as the one built into KiCad, and get high-current designs reviewed.
• Clearance: higher voltages need larger spacing between conductors (creepage and clearance). Anything connected to mains must follow the applicable safety standards (such as IEC 62368-1) and be designed and reviewed by qualified engineers; it is out of scope for this course.
• Placement first: good placement (connectors on the edge, decoupling close, short high-current loops) makes routing easy.

Worked example: an LED indicator board

Schematic: a 2-pin header J1 for 3 to 9 V from a battery, a 1 kΩ resistor R1, a red LED D1, and back to J1. At 9 V with a 2 V LED, I = (9 - 2) / 1000 = 7 mA and the resistor dissipates P = I^2 R = (0.007 A)^2 * 1,000 Ω = 49 mW, fine for an 0805 resistor rated 125 mW. Layout: a board about 20 mm by 15 mm, J1 on the edge, 0.3 mm traces, a ground pour on the bottom, mounting holes, and silkscreen labels for + and -. That small board teaches the whole workflow.

Ordering boards

Many fabricators make small two-layer boards cheaply in a few days. You do not need to order one for this lesson. Any board you do build later should stay on battery power of 9 V or less, and anything beyond that needs proper training and supervision.

Common mistakes

• Footprint mismatch: a SOT-23 pinout that doesn't match the transistor you bought, or a connector drawn mirror-image.
• Skipping ERC and DRC, or ignoring their warnings.
• Thin traces for power, or long, looping high-current paths.
• Split or broken ground under fast signals.
• No silkscreen labels or polarity marks, which makes assembly and testing error-prone.

Recap

• Schematic, ERC, footprints, layout, DRC, Gerbers: the same steps in every tool.
• Solid grounds, close decoupling, adequate trace width and clearance.
• Keep your own boards low-voltage; anything touching mains needs certified design and review.

Try it

Open the Do tab to lay out the LED indicator board in KiCad and generate its fabrication files.