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.
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%)
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.