Circuit board glossary

Item: Impedance Control

Description:

For printed circuit boards with high frequency signals, the signal integrity has to be provided by good impedance characteristics of the conductors. To ensure the desired impedance, an appropriate calculation model is chosen by the PCB manufacturer according to the layer-buildup and printed circuit board layout.

Popular impedance calculation models are

  • Single Stripline
  • Dual Stripline
  • Embedded Microstrip
  • Surface Coated Microstrip
  • Surface Microstrip

see also Design and Manufacture of Controlled Impedance PCBs
from Polar Instruments GmbH.

To check the finished printed circuit board for the desired impedance, an additional test coupon is placed in the production panel`s data, which is measured after production by impedance measuring equipment: impedance control.

For the multilayer boards popular today, small tolerances of 5-10% can be guaranteed through modification of layer-build-up, conductor geometry and layout.

Influence on impedance of printed circuit boards:

  • Distance of signal layer and and potential layer
  • Geometry of the conductors (layout)
  • Conductor heighth (= copper thickness)
  • layer-buildup
  • Dielectric constant (εr) of the base material

Additional information: Impedance Control at Multi-CB.