October: Impedance

What does impedance mean?

Technical definition (source: German wikipedia)
Impedance, also referred to as alternating current resistance, is the ratio of electrical voltage at a consumer (component, cable, etc.) to current consumption. In general, this physical variable is advantageously expressed as a complex-valued function of the frequency.

From the point of view of the PCB manufacturer, impedance adjustment is a complex requirement. A large number of PCB parameters have a direct effect on impedance, which is why they must be manufactured to very low tolerances.

Which requirements must an impedance PCB fulfil?

  • Base materials and prepregs have been defined
  • Layer assembly is impedance-adjusted
  • Signal routing complies with impedance models
  • Signal tracks have been marked
  • Reference planes have been clearly assigned
  • Nominal values and tolerances of the impedance have been determined

What effect does the selection of the PCB manufacturer have on impedance?

The selection of the manufacturer can have far-reaching effects on impedance, as different manufacturers tend to prefer different base materials and constructions.

Impedance PCBs must be coordinated with the manufacturers beforehand. Changing them without prior coordination involves great risks for the impedances obtained.

Which PCB processes have an effect on impedance?

Table 1

Table 2

Which impedance models can be used?

Table 3

How and where is impedance measured?

Table 4

An impedance is measured using the TDR (Time Domain Reflectometry) measurement method.
There are four ways of measuring impedance. The traditional variant is the test coupon adjusted by the manufacturer to the PCB conditions and placed in the production panel. Customers can implement test structures in the panel edge or directly on the PCB.
In addition, differential impedances can also be measured directly.

What tolerances are commonly used?

Common tolerances are ±10% of the rated value of the impedance.
In principle, lower tolerances are possible, but require precise evaluation.