back drilling differ from conventional drilling
Back drilling is a method used in PCB manufacturing to remove via stubs or portions of plated through-holes that extend beyond the intended signal layer. This method reduces via-to-via crosstalk and deterministic jitter, allowing higher data transmission rates and improving signal integrity on high-speed circuit boards. Back drilling requires careful planning and understanding of the fabrication process to ensure it is implemented correctly.
Via stubs are conductive stubs of copper that extend from a via in a printed circuit board (PCB). This stub increases impedance matching between adjacent traces, which can cause distortion and degrade signal quality. Via stubs can also contribute to other undesirable effects, including crosstalk between different signal paths on the board.
Stub lengths can be minimized using alternative construction techniques, such as blind and buried vias or stack-up arrangements where traces are moved closer to the end of the stub. However, these methods are not always cost-effective or feasible, and may require complex routing and redesign. Back drilling is the most cost-effective solution to reducing stub length and improving signal integrity.
How does back drilling differ from conventional drilling
To perform back drilling, the board manufacturer drills holes slightly larger than the original plated-through-hole diameter. This re-drilling method removes the extra copper plating on the hole, and allows the back drill bit to reach closer to the final signal layer, eliminating the stub and minimizing the remaining stub length. This method is typically done using mechanical drilling technology, but can be performed with laser or plasma etching as well.
In order to properly back drill a circuit board, the PCB designer must define the layers that can be back drilled and the maximum stub length allowed. This is accomplished by selecting the Back Drills tab in the Layer Manager, then clicking Add a Back Drill. Once the back drilling rules are set up, the PCB is drilled to the appropriate layer. The stubs are marked with two colors: red and blue, so the engineer can easily identify which stubs were back drilled.
Once the stubs are trimmed, it is essential that the PCB be washed to remove any conductive debris from the back drilling process. This step is essential to prevent signal integrity problems and improve the performance of the circuit board.
The last thing you want is to send a PCB to the assembly house that has unreliable signal quality due to improper back drilling. To avoid this, it is important to have a good communication flow with the board fabricator during the design phase. This includes defining the maximum stub length, establishing the proper rules for back drilling and ensuring that the design is fabricated to these specifications.
Ideally, the original through-hole and pad dimensions are used for the back drilled holes so that there is no change in signal transmission or impedance matching. By carefully defining the back drilling rules in the Design Rule Editor and clearly communicating the design intent to the board manufacturer, back drilling can be successfully implemented on every board.
