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Back Drill PCB: What It Is and How to Use It

PCB manufacturing and design come with various challenges that may affect operations and efficiency. And one problem you need to watch out for is retaining signal integrity. Fortunately, a back drill PCB is effective at handling this issue.

So, when transmitting signals on a PCB, there’s a high chance of distortion from signal noise, crosstalk, and other unwanted effects. Interestingly, Via stubs may also affect signal integrity, but you can avoid it with back drilling.

But the question here is: “What is a back drill PCB?” This article will explore back drilling and how to use the technique for your PCB designs.

What is a Back Drill PCB?

Multiple printed circuit boards

Multiple printed circuit boards

The sad truth about high-speed PCB designs is the sheer amount of problems they have. No doubt, maintaining signal integrity tops the list of concerns. But it’s easy to solve with proper design and manufacturing practices.

Interestingly, Vias have a non-functional section called the Via stub. Sadly, this part causes annoying signal integrity problems in high-speed PCBs. And the stub end can create signal reflections that interfere with the board. Thankfully, you can counter this with back drilling.

In truth, the process involves re-drilling a slightly larger hole than the PTH to remove the via stub. So, back-drilled holes reduce the stub’s length to about ten mils.

Note: Stubs more than ten mils will produce signal reflections.

Why is Back Drilling Required?

PCB drilling

PCB drilling

Back drilling is necessary for shortening the via stub. Usually, this unused portion of the via’s copper barrel won’t contribute to the PCB’s design and will only interfere with high-speed signals.

Typically, back drilling results in a Via stub lower than ten mils, crippling the copper barrel’s ability to create signal reflections.

For example, you have a through-hole Via drilled from layers one to ten in a 10-layer stack-up. And your designs require signals in layers one to three; the via will have a stub after the third layer. Unfortunately, this unused portion creates high-frequency reflections and resonance.

Therefore, you’ll need back-drilling to remove the extra copper plating after the third layer. Also, your back drill must be larger than the PTH’s original size. Otherwise, you won’t remove all the unwanted copper.

Advantages of Back Drilling

PCB drill bits

PCB drill bits

Here are the benefits of using back drilling on your PCBs:

  • Back drilling dampens signal attenuation. And the process reduces the stub’s enhanced impedance matching while mitigating its EMI/EMC radiation.
  • You can also avoid signal distortion issues with back drilling. Via stubs are notorious for causing deterministic jitter and are the results of signal crosstalk, EMI, and noise.
  • Back drilling reduces via-to-via crosstalk.
  • Reducing deterministic jitter with back drilling lessens your signal’s overall BER (Bit Error Rate).
  • Diminished excitation of resonance modes.

Complexities of the Back-Drilling Process

Machine drilling PCB

Machine drilling PCB

Here are some areas you may find tricky while using the back-drilling process:

Back Drilling Depth Control

Many things could go wrong with back drilling. For instance, if the back drilling machine is inaccurate, you may have low via tolerance. Hence, it’s crucial to control your back drill depth.

Further, several ‘outside’ factors may affect your back drilling depth. For example, you may have drill resistance, drill tip angle, effective contact between the cover board & the measuring unit, and board warpage. 

Hence, you must choose suitable drilling methods and materials during production. So, you should get higher depth control for more accurate results.

Back Drilling Accuracy Control

As mentioned earlier, accuracy is critical for back drilling. So, your preferred drilling method, board contraction & expansion, and equipment precision can affect the overall back-drilling accuracy.

How much Leftover Stub Length Can You Have?

Drill working on a PCB

Drill working on a PCB

When back drilling, you must figure out how much residual stub length won’t affect your PCB. However, deciding this factor depends on several considerations, including your preferred signal integrity and practical manufacturing processes.

Typically, decreasing the maximum leftover stub length and increasing the Vias for back-drilling will fuel manufacturing costs.

Here is a table showing the leftover stub length and corresponding signal loss.

FAQs

Why is back drilling also known as control depth drilling?

You can also call this process control depth drilling because manufacturers back-drill these holes to set and control depth.

What are the best alternatives to back drilling?

Laser-drilled vias, buried & blind vias, and alternative stack-up arrangements are other techniques that can control stub length.

Why is it hard to get smaller stub lengths (less than 10 mils)?

Back drilling below ten mils requires more drill machine accuracy. And it’s incredibly challenging to attempt. Also, going below ten mils increases manufacturing costs.

Final Words

Designing high-speed PCBs is tough. You must solve several challenges to ensure your board performs as expected. However, maintaining signal integrity requires extra measures. And that’s where you need back drilling.

Back drilling solves a huge problem in most PCBs. Via stubs are notorious for creating reflections that can influence the original signal. Luckily, back drilling can cut off any unused stub portion, which retains signal integrity. 

Interestingly, back drilling reduces stub length to 10 mils or less–halting any chance of generating annoying reflections.

Do you have any questions? Please contact us, and we’ll be happy to help.

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