TDR View

TDR View in the TRC Plus is provided to give an indication of how DC trace resistance on fine line traces causes a rise in the TDR test trace of an impedance controlled trace.

Designers will specify a characteristic impedance for impedance controlled traces – and where the DC resistance is low the TDR reflection remains a flat line. However, as line width and trace thickness reduce below 100microns and 4oz copper the DC resistance becomes significant and manifests itself as a rising trace. When measuring the characteristic impedance the DC resistance must be removed from the TDR trace in order to measure an accurate figure of characteristic impedance.

TRC Plus allows you to predict the DC resistance and the TDR view provides a simplified impression of a test result for a given trace length to allow the specifying authority to decide whether the resistance is significant enough to require removing from the TDR measurement.

CITS880s offers two alternate methods of removing DC resistance from the characteristic impedance measurement – Launch Point Extrapolation or DC resistance compensation. Both are valid and the method used is down to the choice of the board specifier.

Consider the traces below:

50 Ohm single ended trace on a fine line rigid flex design

3 mil line width 50 Ohm single ended trace with 3 Ohm series resistance

The single ended trace above on a fine line rigid flex design exhibits significant DC trace resistance. This manifests itself as an upward slope over the length of the measured coupon.

It is important that the DC resistance is removed from the TDR measurement before correlating with a field solver.

100 Ohm differential pair with 3 mil line widths

3 mil line width 100 Ohm differential pair with 5 Ohms DC resistance

As with the single ended case above, the differential trace, also on a fine line rigid flex design, exhibits significant DC trace resistance, shown as an upward slope over the length of the measured coupon.

100 Ohm differential pair with 8 mil line widths

8 mil line width 100 Ohm differential pair with low DC resistance

This differential pair with 8 mil line widths and heavier copper exhibits only a small amount of DC resistance over the line length. With broader geometries such as this it is safe to dismiss the effects of trace resistance on the impedance measurement.

What happens if I don't remove the DC resistance component from the measurement?

If you don't remove the DC resistance it will appear “as if” the TDR is measuring high. Some fabricators may assume this high measurement has resulted because the datasheet value for Er is too high and use a field solver to goal seek the “correct” Er – this is an incorrect assumption, and any field solver / TDR correlation should only be studied a) if the DC resistance is small enough to be ignored, or b) the DC resistance has been de-embedded