# Microstrip Characteristic Impedance

There are two IPC documents which are frequently referenced for approximate microstrip and stripline impedance formulas:

The IPC-D-317A has been superseded by IPC-2251 which can be purchased from www.ansi.org for about $110.

The IPC-D-317A formulas seem to be good for RF4 to about 5% but fall apart for other dielectric constants.

FR4 has an wildly varying dielectric constant from 4.5 to over 5.0.

For calculators using IPC-D-317A see http://www.emclab.umr.edu/pcbtlc/.

The default impedance calculations for Protel 2004 are based on IPC-D-317A.

Microstrip impedance formulas from IPC-D-317A:

Embedded microstrip impedance formulas from IPC-D-317A:

Stripline impedance formulas from IPC-D-317A:

Asymmetrical stripline impedance formulas from IPC-D-317A: