PCB Back Drilling

Printed circuit board (PCB) use plated through holes and vias to create interconnections between different layers in multi-layer board design. In high speed PCBs, these plated through holes can cause unwanted stub effects that deteriorate signal integrity at higher frequencies. Back drilling is a specialized PCB fabrication technique that removes a portion of the plated through hole barrel to eliminate this stub effect.

1.  What is Back Drilling.

Back drilling, also known as controlled depth drilling, is used to remove conductive via stubs from PCB vias. As part of the vias, stubs can severely impact signal integrity in high-speed PCB design.

In addition, stubs can cause the signal to reflect back from the stub end, then disturbing the original signal. In other words, if the stub is very long, the distortion will be severe. To solve this problem, back drilling is performed, in which most of the stubs can be removed by re-drilling the holes using a slightly larger drill bit.

2. How PCB back drilling overcomes signal integrity issues

After making the plated through holes, use a slightly larger drill bit to re-drill the holes from the backside to remove these stubs. Drill the hole from backside to a predetermined depth that is close to, but does not touch, the last layer used by the hole.

Ideally, the remaining stub should be less than 10 mils. the diameter of the back-drilled hole is a little bit larger than the diameter of the plated through-hole. Typically, the diameter of the drill bit for the back-drilling hole is 8 mils to 10 mils larger than the diameter of the original drill bit because the clearance between the circuits and the wall of the back-drilling hole must be large enough to avoid accidentally drilling into the circuits adjacent to the back-drilling hole during the back-drilling process.

3.  The process of back drilling

     3.1Through-hole with stubs extending out of the signal path

    3.2 Back drilling with a slightly larger drill bit

    3.3 Reduced the stub by back drilling

4. Situations requiring back drilling

Generally, it is recommended to consider using back-drilling when the circuit track on the PCB has signals with a rate of ≥1Gbps. However, designing high-speed interconnect circuits is a complex systematic project, which also needs to consider factors such as the driving capability of the chip and the length of the interconnect circuit. Therefore, circuit simulation is the most reliable way to judge whether back drilling is needed.

5. General Features of Back Drilling PCB

  - Back drilling is for rigid PCB

   - Generally used for multilayer PCB than 8 layers 

  - PCB Thickness more than 2.5mm

  - Back drilling hole is 0.2mm larger than the original through-hole 

  - The depth tolerance of Back drilling +/-0.05mm

6. Advantages and disadvantages of PCB back drilling

   6.1 Advantages 

- Back drilling helps to reduce signal attenuation, ensuring a stronger and more reliable signal. In addition, this technique helps to minimize the effect of stubs on impedance matching, thereby reducing EMI/EMC emissions.  

- Back drilling is also an effective way to prevent signal distortion issues. Via stubs are well-known for causing deterministic jitter, which can result from signal crosstalk, EMI, and noise. By removing these stubs, back drilling can help to eliminate the sources of deterministic jitter, improving signal quality and preventing signal distortion issues.  

- Back drilling helps to minimize crosstalk between vias.  

- By implementing back drilling, the deterministic jitter in your signal can be reduced, which can result in a decrease in the overall bit error rate (BER) of the signal.  

- Reduced excitation of resonance modes.  

- Minimize the usage of buried and blind vias to simplify PCB production.  

- Minimal impact on design and layout.  

- Expanded channel bandwidth.  

   6.2 Disadvantages 

One disadvantage of back drilling is that it is only appropriate for high-frequency boards with a frequency range between 1GHz and 3GHz and that do not have feasible blind vias. In addition, special techniques must be used to prevent any damage to lateral circuits and planes caused by back-drilling in the PCB.