Impedance Controlled PCBS

For the optimum transmission of signals between semiconductors

Background

High data rates and the increasing importance of electromagnetic compatibility (EMC) demand a precisely defined, reproducible impedance behaviour.

Impedance, which is also called alternating current resistance, specifies the relationship between the electrical voltage and the current draw on a consumer (component, conductor etc.). From the point of view of a PCB manufacturer, impedance adjustment is a complex requirement. Many PCB parameters have a direct effect on impedance, which means that really small tolerances are required.

Applications

Controlled impedance PCBs are often used in the field of digital technology.

Benefits

  • Optimum product function
  • Checking and calculation of impedances and stack-ups
  • Impedance measurements on special coupons

Special features

We're happy to help you with the design of a controlled impedance PCB.

Important parameters:

  • Stack-ups and the associated insulation gaps between the layers
  • Track widths
  • Dielectric constants (epsilon r of base materials and solder mask)
  • Conductor geometry

The impedances in the PCB are distributed into different groups; ±10% from the nominal impedance value is usual. Lower tolerances are possible in principle but require a closer check:

  • Microstrip (typical requirement of 50 Ω ±10%)
  • Differential (typical requirement of 100 Ω ±10%)
  • Coplanar

The following information is important to the PCB manufacturer in order to ensure the reliable fabrication of impedance PCBs:

  • The base materials and prepregs are defined.
  • The Stack up is impedance-matched.
  • Signal routing corresponds to the impedance models.
  • The signal tracks are identified.
  • Reference planes are clearly assigned.
  • Nominal values and impedance tolerances are defined.

Contact info

Rainer Jäckle