Backdrill is the process of removing stubs from plated-through-holes (vias) in multilayered PCBs by the second drilling and allowing to transfer signals from one layer to another layer of the board. Backdrill can be applied to any type of circuit board, but usually used on boards with eight layers or more. When high-speed signals pass through PCB layers, they may be distorted. This distortion will be significantly increased if the stubs in the signal layer are long. Proper back drill length is a crucial process for PCB manufacturers to minimize signal integrity issues in PCB fabrications.
The harmfulness of the stubs
Via stub is the main reason which introduces the undesired distortion. Stubs are the unnecessary but conductive part of the through-hole that extends farther than the inner layer of the last connection. And the stub may cause reflections and interference with capacity, inductance, and impedance. Especially in the backplanes and thick circuit boards, stubs can withstand significant signal integrity interference.
The benefits of the backdrill
As a finishing step in the PCB manufacturing process, backdrill uses the technology of controlled depth drilling compatible with traditional CNC drilling equipment. Key benefits of the backdrilling PTH vias include:
l Increase channel bandwidth
l Increase data rates
l Reduce the application of blind vias and buried vias, and reduce the difficulty of the boards
l Reduce the thickness of the boards
l Less signal attenuation and improve signal integrity
l Reduce deterministic jitter
l Reduce EMI radiation from the stubs end and via-to-via crosstalk
The difficulties of the backdrilling process
Generally, the backdrill diameter should be slightly larger than the primary drill diameter to remove the copper from the vias, and normally the backdrill is 0.2mm larger than the vias. And the space between trace and plane should larger enough to ensure the backdrill process will not accidentally drill through traces and planes located near backdrilling. Besides, there are some difficulties during the backdrilling process.
1. The control of the back-drill depth
Since the backdrill is a process of blind holes through the depth control function of the drilling machine. The tolerance is affected by drilling equipment accuracy and dielectric thickness tolerances. In addition, the accuracy is also easily affected by other factors, such as the angle of the drill tip, the resistance of the drill, the bow and twist of the PCB, and the contact effect between the measurement unit and the cover plate, etc. Therefore, manufacturers should pay more attention to selecting more suitable drilling materials and drilling methods to best control the accuracy.
2. Precise control of backdrill technology
The accuracy of the backdrill is very important because it is the second drill. Many factors, such as the drill size, drilling, equipment, or PCB materials, will affect the accuracy of the backdrill, which is of great significance to the quality of the terminal process of the PCB manufacturers.
Backdrill is a cost-effective strategy that significantly reduces the unwanted signal distortion caused by PTH via stub structures. For this technology, NextPCB has years of experience and superb technical ability. If you have a PCB project of PCB prototype assembly or PCB mass production, please feel free to contact us for more details or place orders directly on the website.