back drill commonly applied
If you’re a PCB designer looking to maintain signal integrity in high-speed signal circuits, then you’ll want to implement back drill. Back drilling removes via stubs to reduce reflections, scattering and delay which can degrade signal transmission. Back-drilled vias are typically used in specialized applications and for the highest signal speed.
Technically speaking, back drilling is a process that’s performed to remove copper plating from the inside of a via on a printed circuit board (PCB). The stub length created by this process can affect signal transmission due to resonance and high-frequency repetitions which attenuate the signals. To minimize this, the via stub is drilled out from the backside of the via, hence the name.
You’ll find back drill in PCBs with highly compact designs and dense layers. It’s also frequently utilized in high-speed signal circuits to ensure minimal signal transmission reflections. Back drill can be difficult to manage during the fabrication process, as there are many factors which can influence the effectiveness of the technique. These include the chosen drilling method, the board’s contraction and expansion characteristics, and the precision of the drilling equipment.
Where is back drill commonly applied?
Another challenge is that the process can leave behind a hole on the surface of the board which may affect conductive paths and RF performance. The solution is to use alternate construction techniques that can eliminate the need for back-drilling, such as buried and blind vias, laser-drilled vias (microvias), or alternative stack-up arrangements.
In addition to these alternate methods, back drilling is still the most effective tool for managing stub length in high-speed signal circuits. By applying the right rules and ensuring proper documentation, PCB manufacturers can accurately create and back-drill these vias to maintain signal integrity.
Using Altium’s powerful tools, it’s easy to set up and apply the proper back drill rules for your PCB design. The Layer Stack Manager allows you to select the first and last layers for your back-drilled vias, and the Backdrill Figure will automatically update to reflect this selection. You can then open the Object Class Explorer and click on Net Classes to add the nets that need to be back-drilled. You’ll need to specify the original hole size as well as a back-drill oversize, which is usually a few sizes bigger to eliminate unwanted copper plating.
Finally, you can save the rule and close the Layer Stack Manager. The resulting back-drilled vias will appear in the Drill Table and will be identified by a color-coded label based on the original hole size and the oversize specification, as shown below. You can also enable the display of back-drilled vias in the Layer Stack Legend by checking the appropriate checkbox. This will allow you to visually establish the presence of these holes and verify their functionality in your design. You can also configure the Rule Summary dialog to display each back-drilled via pair, and you can use the Backdrill Report to generate a drill file listing for each of the pairs.