How to use TDR to test the line impedance of PCB board

1. Industry standards for impedance testing

I have posted many pictures of impedance testing before, and I will post another one for you to see. Impedance is not a stable straight line as imagined, but fluctuates. It will be affected by the probe or open circuit at the front and back ends, and there will also be fluctuations in the middle due to the production process.

So, how do we judge the test results? How do we determine whether the impedance of the produced PCB meets the requirements? First, let’s look at the IPC specification. The measurement interval recommended by IPC2557A is 30%~70% of the DUT.

Let’s take a look at the testing habits of Intel and current mainstream board manufacturers. In order to avoid the influence of the Launch area and the reflection area, the recommended test interval is 50% to 70% of the DUT.

 
To use TDR to test line impedance, we must first understand the requirements of the test range in order to accurately understand the measurement results.

2. The influence of the probe on the impedance test results

Generally speaking, the following probes are used during TDR testing:

Among them, board factories usually use handheld probes for point testing, and the influence of the probe is inductive. SI laboratories often use SMA to connect test cables, and SMA may be capacitive in impedance testing. The influence of the two probes on the test results is shown in the following figure:
 

Due to the inductive or capacitive influence of the Probe, the final test result of the DUT will have a slight deviation.