Backdrilling

As time goes by, higher and higher speed signals are hosted in electronic circuits. A transmitted signal will be subjected to noise and crosstalk. Therefore, incorporating design and manufacturing processes to mitigate these problems becomes a necessity.

Let’s take a look at the image given below. It represents the cross-section of a via:

You can see how the signal is traveling from layer two to layer four. It is easy to see that there are no connections from layer five downwards. The portion of the via from layer five to the bottom layer is called the via stub. The main objective of backdrilling is to remove this via stub. In other words, it removes the unused section of the plated through-holes. It can be performed from either side of the PCB at various depths varying from via to via. A backdrill size is usually 6 to 10 mils larger than the original tooling. In manufacturing, fabricators must be careful not to drill beyond (over drill) the calculated depths. Under drilling is also imperative to avoid since it might leave unacceptable stubs.

Backdrilling is available in the standard Allegro PCB Designer license. It is integrated into the Constraint Manager. The first step to backdrill is identifying the nets to be potentially targeted. This is done by defining a property that serves as a threshold for the maximum stub length.

Once the nets are identified and the stub lengths calculated, PCB Editor considers the defined parameters to place the necessary backdrill.

From the point of view of the Padstack Editor, backdrill parameters can also be defined:

You may refer to the Backdrill section of PCB Editor Product Manual for detailed information and a step-by-step guide on how to set up this feature in your design.

Team PCBTech

Cadence Design Systems