PCB Board Stray Inductance

All PCB Board conductors are inductive, and at high frequencies, the PCB Board Stray Inductance of even quite short pieces of wire or printed circuit traces may be important. The inductance of a straight wire of length L mm and circular cross-section with radius R mm in free space is given by the first equation shown in below Figure:

The inductance of a strip conductor (an approximation to a PC track) of width W mm and thickness H mm in free space is also given by the second equation in above Figure.

In real systems, these formulas both turn out to be approximate, but they do give some idea of the order of magnitude of inductance involved. They tell us that 1 cm of 0.5-mm of wire has an inductance of 7.26 nH, and 1 cm of 0.25-mm PC track has an inductance of 9.59 nH—these figures are reasonably close to measured results. At 10 MHz, an inductance of 7.26 nH has an impedance of 0.46 Ù, and so can give rise to 1% error in a 50-Ù system.