What really causes series inductance of capacitors?
When current flows, there is by definition a magnetic field around it. This leads to self-inductance for any conductor with a varying current.

As a capacitor is a low impedance at AC (the precise amount depends on frequency of course) then a real capacitor looks like this:

C1 is the nominal capacitor, R1 is the equivalent series resistance, L1 is the equivalent series inductance and R2 is the leakage resistance.







simulate this circuit &ndash; Schematic created using CircuitLab

You will note we now have a damped series resonant circuit; below self resonance it is capacitive, at resonance it is resistive and above it is inductive.

The value of ESL depends on both the materials and the size of the device; for a reverse geometry device in a 0204 surface mount package it might be as low as 300pH; a typical 0402 surface mount ceramic is about 680pH.

For decoupling and coupling devices this matters in a high speed world.

Let's do a quick calculation. If I am decoupling a device that has internal switching rates of 200 picoseconds (not at all uncommon and has frequency artifacts at 2.5GHz) and I use an 0402 0.1uF device, then the actual impedance is about 4.3 ohms and it is inductive.

You did read that correctly; the capacitor is now acting as an inductor.

Typical surface mount ESLs:

0402 680pH: 0603 about 900pH: 0805 about 1.2nH

A 1 inch track at 4 thou (quite common) has about 5nH of inductance, for reference. This is the reason decoupling devices need to be so close to the actual power pin being decoupled. A device that is even as little as 1/2 inch away at these frequencies may as well not exist.

The inductance for a PCB trace assumes it is over a plane; the precise value will vary based on distance to the plane (because it impacts total return path and round trip time). I have found the value above to be a good (conservative) starting point for PCB designs. The actual inductance is specifically dependent on total current path distance for the loop.

So the reason for ESL? Physics.