How Back Drilling Can Be Used to Salvage PCBs
Back drilling is a technique that removes unneeded conductive via stubs. Stubs are a significant source of signal reflection and impedance discontinuities that cause signal distortion and data errors, particularly with high-speed signals. The shorter stubs resulting from back drilling also help to reduce crosstalk between adjacent signal traces. As such, back drilling is commonly used in PCBs with high-speed signal designs like Ethernet, USB, and HDMI.
To perform backdrill, select the first and last layers of your PCB from the Layer Stack Manager. Then, click the Back Drills tab within the Layer Stack Properties panel to set the layer-spans that will be back drilled. The default value is all layers, but you can also choose to limit the back drills to specific nets such as high-speed nets.
The next step is to specify the size of the back drilled holes. The width of the back drill is typically 8 mils larger than the diameter of the primary hole. This is a common design rule that can be found in most manufacturing documentation, but the exact value can vary by manufacturer. Finally, it’s important to note that a back drill must be positioned accurately with respect to the original hole and must coincide with its edge and bottom edges. This is important to ensure accurate alignment and prevent stub overlap.
How Back Drilling Can Be Used to Salvage PCBs With Signal Integrity Issues
Another method to minimize signal interference is by using buried or blind vias. However, these methods are not always viable from a cost and manufacturing standpoint. Moreover, they can introduce additional signal loss and other undesirable effects. Back drilling can provide a cost-effective and reliable solution to manage signal quality for high-speed signals in dense PCBs.
To use the back drilling feature, open the PCB Rules and Constraints Editor and create a new Maximum Via Stub Length (Back Drilling) design rule. The scope of this rule determines which traces or pads will be back drilled. A typical scope would be InNet(‘HighSpeedNets’). Once the back drilling rule is in place, select the Ethernet RX nets and click the > icon to add them to the list of targets for back drilling. You can also right-click on the Unique Holes panel in Hole Size Editor mode and select Drill Table to view a report of all back drilling events.
The report will also provide a summary of the back drilling settings. Then, you can click the Export button to save the report to a file. The file name can be customized in the export dialog box. Once the file is saved, it will be available for use within EDGE Capture Pro and other CAD tools. The report is also automatically emailed to your fabricator as well as the manufacturer’s engineering team. This can be an effective way to communicate the desired back drill specifications with your manufacturer. This will help to avoid any confusion or misunderstandings during the manufacturing process.