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Backdrilling Vias for improved Signal integrity

Backdrilling Vias for improved Signal integrity

 

Backdrilling, also known as Controlled Depth Drilling (CDD) technique that removes the unused portion or stub of copper barrel from a through-hole in a PCB with conventional numerically controlled (NC) drill equipment. It can be applied to boards where stubs cause SI degradation, with minimal design and
layout considerations. When a high-speed signal travels between PCB layers through a copper barrel, it can be distorted. If the signal layer usage results in a stub being present, and the stub is long, then that distortion can become significant.

These stubs can be removed by re-drilling those holes after the fabrication is complete, with a slightly larger drill. The holes are back drilled to a controlled depth, close to, but not touching, the last layer used by the via. Allowing for fabrication and material variations, a good fabricator can back drill holes to leave a 7mil stub, ideally the remaining stub will be less than 10mil.

When the drill bit is drilled, drill tip touches the copper foil of substrate board generates micro-current to sense the height of board surface, and then drill down according to the set drilling depth. It will stop when reach drilling depth.

 

Benefits of Backdrill

  •  Reduced deterministic jitter
  •  Lower bit error rate (BER)
  •  Less signal attenuation with improved impedance matching
  •  Increased channel bandwidth
  •  Increased data rates
  •  Reduced EMI radiation from the stubs
  •  Reduced excitation of resonance modes
  •  Reduced via-to-via crosstalk
  •  Aspect ratio can be neglected (in contrast to blind vias)

 

Back drilling eliminates detrimental PTH via stub effects that distort signals passing through them.

 

Back Drilling Advantages and Disadvantages

 

Signal integrity and high-speed data transmission rates are critical aspects of PCB design. It is a common issue in high-frequency PCB designs where a transmitted signal is distorted owing to several layers.

This is when a PCB for back drills comes in handy.

 

Here are the advantages and disadvantages of using back drilling.

AdvantagesDisadvantages
  • It reduces the number of bit mistakes per unit time or Bit Error Rate (BER).
  • It increases the channel bandwidth and data rates by removing stubs that cause distortion.
  • There is a decrease in signal layer distortion problems called deterministic jitters. These include timing errors caused by EMI, noise-related propagation, and via-to-via crosstalk in the PCB.
  • Resonance frequency mode excitation is reduced.
  • Better impedance matching and reduces stubs EMI/EMC radiation due to reduced signal attenuation.
  • Easy to produce and cost-effective.
  • It is only suitable for high-frequency boards between 1GHz and 3GHz with no viable blind vias.
  • A unique technique must be employed to avoid damaging traces and planes lateral to the backboard hole.

 

Conclusion:

A back drill is a useful drilling process for removing problematic stubs from solder masks on a PCB. Problematic stubs often cause deterrent jitters like signal crosstalks. It is essential in improving signal integrity in hole technology boards.

Sample specification:

 

 

If you’d like to know more about the process and capability of back-drill, please contact us.

 

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