A decoupling (bypass) capacitor is a small local reservoir of charge next to an IC. When the chip switches and suddenly needs current, the capacitor supplies it immediately, instead of the current having to travel through the inductance of long supply traces.

Why 100 nF?

There is nothing magical about the value. A 100 nF ceramic capacitor is cheap, available in every package, and has low enough impedance over the frequency range where most digital logic needs help. It became the standard starting point, and most IC datasheets recommend it.

What matters more than the exact value is having one capacitor per supply pin, placed close to it.

Add bulk capacitance too

Small capacitors handle fast edges but run out of charge quickly. Larger "bulk" capacitors cover slower load changes.

  • Per IC supply pin: 100 nF ceramic (X7R).
  • Per IC or per group of ICs: 1 µF to 10 µF ceramic.
  • Where power enters the board, and at regulator outputs: 10 µF or more, as the regulator datasheet specifies.

Always follow the IC's datasheet first — microcontrollers, radio modules and regulators often state exact values and positions.

Placement matters more than value

Trace and via inductance is what a decoupling capacitor is fighting. A perfect capacitor placed far away is a poor decoupler.

  1. Put the capacitor as close to the supply pin as possible.
  2. Keep the loop small: supply pin → capacitor → ground, with short, wide traces.
  3. Connect the ground side to the ground plane with its own via, right at the pad.
  4. Route power so that it reaches the capacitor before the pin where you can, not the other way around.
  5. Do not share one via between several capacitors.

Choosing the part

  • Dielectric: X7R is the usual choice. Avoid Y5V. See C0G vs X7R vs X5R vs Y5V.
  • Voltage rating: at least double the rail voltage, because ceramic capacitors lose capacitance under DC bias.
  • Package: 0603 or 0402 keeps inductance low; 0805 is fine for hand-built boards.

Common mistakes

  • One capacitor shared between several supply pins.
  • Capacitors grouped neatly in a corner of the board instead of beside each IC.
  • Forgetting analog supply pins (AVCC, VREF), which often need their own filtering.
  • No bulk capacitor at the power input, so the board misbehaves when a motor or radio switches on.

Find 100 nF and bulk values in Capacitors — try searching 100nF 0603.