Design guide

Designing for 0402: a practical guide

CopperPCB · October 1, 2026

0402 is the smallest part size we place. It's small enough for dense designs and large enough to build reliably on short runs and rework by hand. Here's how to lay out a board that goes through assembly cleanly.

Know your size codes

Case sizes come in two naming systems, and mixing them up is the most common 0402 mistake. In the imperial code the digits are hundredths of an inch; in the metric code they're tenths of a millimetre.

Footprints

Use the manufacturer's recommended land pattern or an IPC-7351 footprint at nominal density. The stock KiCad and Altium libraries are fine. Avoid hand-drawn pads that are much larger or smaller than the part; oversized pads let small parts float and skew during reflow.

Preventing tombstoning

Tombstoning is when one end of a small chip lifts off its pad and the part stands on end. It happens when one pad melts before the other, and it's most common at 0402. The fixes are all about keeping both pads the same:

• Symmetric pads. Both pads the same size and shape.
• Thermal relief on planes. A pad tied straight into a ground pour heats far more slowly than its partner. Use thermal-relief spokes or a short trace into the plane.
• Matched traces. Keep the traces into each pad similar in width.

Vias and solder mask

• Keep open vias out of pads. Solder wicks down an open via and starves the joint. Move the via off the pad, or have your fab fill and cap it.
• Tent vias near small parts so solder can't bridge to them.
• Keep silkscreen off pads. Most fabs clip it automatically, but it's better to lay it out clean.

Spacing and fiducials

Respect the courtyard outlines in your footprints and don't let them overlap. That spacing is what lets the machine place parts without disturbing their neighbours, and lets us inspect and rework every joint. Add two or three global fiducials, bare copper dots near opposite corners of the board, so the machine can align precisely to your copper.

Mind the physics of small parts

Capacitors lose capacitance under voltage

High-value ceramic capacitors in small cases (X5R and X7R types) can lose a large share of their capacitance once DC voltage is applied, and the effect gets worse as the case shrinks. A 10 µF part in 0402 running near its rated voltage may deliver only a fraction of that. Check the manufacturer's DC bias curves, which Murata, TDK and Taiyo Yuden publish for nearly every part, and step up to a larger case or higher voltage rating when it matters.

Resistors have a power limit

A standard 0402 resistor is rated for about 1/16 W. Check the dissipation in current-sense, pull-down and LED-limiting resistors, and move to 0603 or 0805 where you need more headroom.

Before you send your files

• Every part is 0402 (imperial) or larger.
• Footprints match the manufacturer's land pattern.
• Small passives tied to planes use thermal relief.
• No open vias in pads.
• Two or three global fiducials.
• Your pick-and-place file and parts list use the same reference designators.

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