The Brief Summary About PCB Impedance Control

PCB board with controlled impedance trace is very important for signal transmission quality. Especially for quick switching parts with signal rise times of 1 to 2ns, the signal integrity must be guaranteed through good impedance characteristics in the conductor trace connections.


What is the PCB Impedance Test Coupons

The PCB impedance test coupons is a small PCS of bare printed circuit board on which only have impedance track line and the holes where connected to impedance trace on it.

Typically it is designed by PCB manufacturer to put it on the PCB production working panel to be produced by the same process of PCBs. So the coupons has the same features as the exact impedance traces on the PCB board. The holes on the coupons is conductive to it’s internal layers which have impedance trace.


The impedance value of the track in the internal layers can be tested by the probe of the impedance test machine via touching these holes.


Single Ended PCB Impedance Control and it’s Coupons

Single ended PCB impedance also called the single side PCB impedance, which is a type of impedance generated from a single track line not coupled to an adjacent line. The typical single end-ed impedance value is 50+/-5ohm.

It’s test coupons designed as a small PCB with only single track plus the holes for the test point purpose.

Single Ended PCB Impedance Test Coupons
Single Ended PCB Impedance Test Coupons Design

Differential PCB Impedance Control and it’s Coupons

It is the impedance generated from a pair of transmission lines on the circuit board. The typical differential impedance value is 80-100 ohms with a tolerance +/-10%.

It’s coupons designed as a small PCB with single track plus the holes for the test point purpose

Differential Impedance Control Test Coupons
Differential Impedance Control Test Coupons Design

How to Use the Test Coupons to Verify the Impedance Value on the PCB

As the impedance coupons has the same layer stack up & the same track as it’s PCBs, and the holes on the coupons conducted it’s impedance trace accordingly. Thus,it is very easy and simple to use a impedance test machine (for example Polar CITS880s) to plug the test probe into the holes on the coupons to verify the impedance value.

PCB Impedance Control Test Machine

Matched impedance value with it’s track line is crucial to electronics functional after PCB assembly. Normally, a processional PCB manufacturer always offer the impedance test coupons and it’s test report to the PCB buyer along with the shipment without any extra added cost.