What is PCB Back Drilling?
The process of PCB back drilling, also referred to as controlled depth drilling, involves removing the stub in multilayer PCBs to create vias. The aim of back drilling is to facilitate the flow of signals between different layers of the board without interference from unwanted stubs.
To provide a clearer explanation of the back drilling process, let’s consider an example.
Suppose there is a 12-layer PCB with a through-hole connecting the first and 12th layers. The aim is to connect only the first layer to the 9th layer, while keeping the 10th to the 12th layers unconnected. However, the unconnected layers create “stubs” that can interfere with the signal path, resulting in signal integrity problems. Back drilling involves drilling out these stubs from the reverse side of the board to improve signal transmission.
What is the purpose of PCB back drilling?
PCB back drilling is used to remove the unused portion of a plated through-hole (via) that extends beyond the last layer of the PCB. This process helps reduce signal integrity issues, such as stub resonance and signal reflections, which can occur in high-speed digital designs.
What is the “aspect ratio”?
FAQ about printed circuit boards › TERMINOLOGY
The relationship between the diameter of the hole and its length. When a manufacturer states that their production has an “aspect ratio” of 8:1 it means, for example, that the hole’s diameter is 0.20 mm in a 1.60 mm thick PCB.
For HDI structures, the aspect ratio for microvia is limited to 1:1, but 0.7-0.8:1 is preferable to easy the plating.
What are the benefits of PCB back drilling?
The benefits of PCB back drilling include improved signal integrity, reduced insertion loss, enhanced crosstalk control, and increased bandwidth. By removing the unused portion of the via barrel, back drilling minimizes stub resonance and signal reflections, resulting in cleaner and more accurate signal transmission.