KEMI Docs · Circuit Practice

Decoupling Capacitor Placement

Last updated: 2026-08-30

If '0.1µF on every IC' is just a convention to you, the layout ends up with capacitors herded into a corner of the board. Decoupling is half placement. Here is why it exists, where the parts go, and the mistakes that actually show up in design reviews.

Digital ICs draw current in bursts on every clock edge. The supply and its wiring are too inductive to respond in time. A decoupling capacitor is a local charge reservoir right at the IC that serves the burst on the spot and absorbs the resulting dips and ripple — and, as a bypass, it keeps the IC's high-frequency noise out of the wider power network.

100nF ceramic (X7R/X5R) sits at the sweet spot of capacitance, self-resonant frequency and price — hence decades of convention. The rule is one per power pin, not one per IC. An MCU with eight power pins gets eight. When the datasheet specifies a combination (100nF×N + 10µF), the datasheet wins.

1. Minimise pin-to-pad distance — a few millimetres. Every extra length is inductance.
2. Best case: power flows through the capacitor pad on its way to the pin.
3. Ground through a via straight into the plane, right at the pad — a long trace to ground gives half the benefit away.
4. For BGAs: opposite side of the board, directly under the power pins. That is the standard solution.

Bulk capacitors (10-100µF at the power entry and regulator outputs) hold up the low-frequency end; the 100nF at each pin handles the high end. They are a hierarchy, not substitutes: bulk at the entry, a few µF per block, 100nF at each pin.

• One capacitor 'shared' by several power pins — it is one per pin.
• Capacitor ground daisy-chained through another part's pad instead of a via to the plane.
• A single 10µF standing in for 100nF — large MLCCs resonate low and miss the high frequencies; parallel combinations win.
• Ignoring DC bias derating — small X5R parts lose much of their capacitance near rated voltage. Use 2x voltage margin.

This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.