PCB Back Drilling

By Dhirajprinted Circuit Board
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Back Drilling in PCB

PCB Back drilling, also known as controlled depth drilling, is a technique for removing stubs from plated through-holes or vias on high-speed multi-layered PCBs. The stubs are the parts of the vias that aren’t needed. Back drilling can be used on any type of PCB. Drilling is a time-consuming and costly procedure in PCB fabrication, as we all know. It’s also a technique that presents certain technological obstacles, and the back drilling process is no exception.

PCB Back Drilling, Back Drilling in PCB, Back drill,

Da: Primary Drill
Db: Back Drill
Dc: Anti-pad Size
Dd: Back drill – signal distance
MCL: Must Cut Layer
MNCL: Must Not Cut Layer
T: Target Layer Depth
S: Stub Length

Why Back drilling?

 

Signal reflections and impedance discontinuities are caused by the stubs. The stubs will also have an impact on signal integrity. If the PCBs have a high bit error rate (BER), deterministic jitter, signal attenuation, or other EMI issues, back drilling may be the best solution. If the stub is too long, the signal distortion will be significant.
Why do we use back drilling to regulate signal quality in high-speed signal applications rather than blind vias or buried vias? Because back drilling is less expensive than blind and buried vias technologies. Back drilling can also significantly lessen the difficulties of PCB production.

How does via stubs cause jitter in high signal circuits?

 

A signal divides at the intersection between the internal trace and the stub as it travels down the length of a PTH. The signal that travels the length of the stub is reflected before returning to the trace junction, where it splits again, with some of the signal going back to the source and the other following the trace.
In this way, the timing of a reflected wave can be so incorrect that its troughs cancel out the peaks (or vice versa) of an incoming signal. While this may not be a concern at lower bit rates, the noise generated by the split path provided by the via stub rises as the signal speed increases. To put it another way, jitter causes attenuation, greater BERs, and EMI emission in the presence of other electrical fields.

Advantages of Back Drilling

  • Reduce noise interference & deterministic jitter.
  • Reduce the use of buried & blind vias and reduce the difficulty of PCB production.
  • Improve signal integrity.
  • Lower bit error rate (BER).
  • Local thickness reduction.
  • Minimal design and layout impact.
  • Increased channel bandwidth.
  • Increased data rates.
  • Less signal attenuation with improved impedance matching.
  • Reduced excitation of resonance modes.
  • Reduced via-to-via crosstalk.
  • Reduced EMI/EMC radiation from the stub end.
  • Lower costs than sequential laminations.

Typical Process Flow:

Back drill Typical Process Flow, Typical Process Flow in Back drill.

Back Drill Test:

Back Drill Test, PCB Back Drill Test, Test in Back drill.
  • Probe TP1 and Back drill : Open = Accept
  • Probe TP2 and Back drill : Short = Accept
  • If both Accept, coupon is PASSED

Supplying Data for Back drill :

  • Supply back drilling information via NC file (Gerber, Excellon II, etc.)
  • Depth definition :

A) Separate NC file for every depth per side of the board. A legend or Text file (fab drawing) explains to which layer to drill and from which side.

Separate NC file, PCB back drill, Back drill in PCB.

B) Single NC file per side including all depths by tool number. A separate file (fab drawing) explains to which layer to drill and from which side.

Single NC file, PCB back drill, Back drill in PCB.

Defining Back drill:

  • Provide a unique drill Aperture for back drilled holes.

Back drilled holes of different depths required to have unique apertures

Back drilled holes from different sides required to have unique apertures

  • Provide Updated Drill Table that has separated drill apertures for each unique back drill requirement
  • Fabrication note identified in drill table, and details the following:

Back drill from which side of the board

Back drill to what depth, identifying what layer in the back drill is to remain electrically intact.

Cerra Systems is one of the leading PCB manufacturing solutions providers with PCB manufacturing plants located in North America, Asia with certifications for MIL, AS, TS, ISO, & NADCAP. Our products are Rigid, Flex, Rigid-flex, HDI, RF, and hybrid PCBs addressing up to 64 layers and 2 mil technology. Cerra Systems  Major  PCB Market is Aerospace, defense, Medical, computing, cellular & Industrial.

We supply a wide range of Printed Circuit Board in India and backplane products from very small, double-sided rigid PCB to high layer count, high-density multi-layer boards to large tabletop size backplanes. We have dedicated and quick-turnaround operations capable of delivering PCB prototypes with a lead time as short as 24 hours. We also offer quick-turn prototyping with a lead time as short as 5 days, depending on the technology and layer counts.

For more details, visit us- http://cerrasystems.com/

Manufacturing Partners Profile includes: 
Argus Sytems (AESPL) –  PCBA, Cable Assembly, Box Build. 
Emsxchange – PCBA, Cable Assembly, Box Build.

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