Introduction
Printed circuit boards (PCBs) use plated through holes and vias to create interconnections between different layers in multi-layer board designs. 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.
This article provides a detailed overview of back drilling, examining the need for back drilling, how it is implemented, its effects, key design considerations and applications that utilize back drilling.
What is Back Drilling?
Back drilling is a controlled depth drilling process performed on already plated through holes in a PCB to remove a part of the plated hole barrel from the back side of the board. This eliminates a portion of the via stub that would otherwise extend all the way from the component layer to the bottom layer of the PCB.
As seen above, back drilling reduces the length of the remaining copper stub by removing those barrel segments that are not essential for connectivity, thereby minimizing via stub effects.
The name “back drilling” comes from the fact that this precision drilling step is performed from the back side (non-component side) of the finished PCB substrate. It is a highly controlled process using special CNC drill bits optimized for high accuracy and cylindricality.
Why is Back Drilling Used?
Back drilling provides several benefits for high speed PCBs which justify the additional fabrication complexity and cost:
Reduce Stub Effects
- The remaining via stub acts as a resonant antenna that creates impedance discontinuities and radiates EMI at higher frequencies.
- Back drilling the non-critical portions reduces the stub length, minimizing unwanted resonance and radiation.
Improve Signal Integrity
- Shorter stubs allow signals to propagate to adjacent layers with reduced reflections and attenuation.
- This improves signal quality and timing across layers, especially for multi-gigabit data rates.
Allow Tighter Routing Channels
- Stub reduction enables narrower channel-to-channel spacing for routing traces.
- Allows higher route density by preventing coupling between adjacent traces.
Permit Tighter Layer Stacking
- Shorter vias need thinner annular rings and anti-pad clearances.
- Permits tighter core stacking distances not possible with full-length vias.
Reduce EMI Emissions
- Shorter stubs radiate less EMI due to their reduced antenna effect.
- Helps meet EMI and EMC compliance requirements.
Enable High-Density BGAs
- Minimizes stub effects in densely spaced BGA packages.
- Allows escaping the large number of signals from BGA balls.
So in summary, back drilling provides tangible improvements in signal quality, EMI control and routing density for high speed digital and RF PCBs.
How is Back Drilling Performed?
Back drilling is carried out as one of the final fabrication steps after the main PCB manufacturing process is complete.
PCB Fabrication
- The PCB substrate is first fabricated using regular methods with all layers pressed and plated through holes formed.
Controlled Depth Drilling
- A specialized CNC back drill machine uses carbide or diamond coated drill bits to selectively drill into the barrel of chosen plated through holes from the back side.
- The drilling depth is precisely controlled to remove only the targeted portion of the barrel.
- Drill bit widths range from 0.2 mm to 1 mm for different hole sizes.
- Multiple hole sizes can be back drilled in one pass.
Deburring
- The holes are then deburred to remove any rough edges or resin smear.
Copper Plugging (Optional)
- For mechanical stability or conductive shielding, the back drilled holes can be plugged using copper.
This sequence allows surgically reducing via stub lengths with great precision on finished boards.
How Much is Drilled?
The amount of back drilling done depends on these considerations:
Via Technology Used
- Blind or buried vias already provide stub control by avoiding through holeconnections.
- Back drilling gives incremental benefit for boards with 100% plated through hole vias.
Routing Layers Affected
- Back drilling only helps layers with escape routing above the drilled segment of the hole.
- Other layers are unaffected by back drilling that hole.
Signal Data Rate
- Higher data rate signals benefit more from stub reduction. Low speed signals see little effect.
- Typically back drilling is applied for signals 5 Gbps and above.
Available Routing Channels
- If routing space is tight, back drilling allows pushing channels closer.
- In designs with adequate spacing, back drilling may not be required.
EMI Requirements
- Back drilling helps meet stringent emissions requirements.
- For moderate EMI levels, it may not provide significant benefit.
- Only holes carrying high speed signals need to be back drilled. Other via stubs with power or low speed signals are usually not back drilled.
So the amount of back drilling is chosen based on electrical performance needs and routing constraints for that specific design.
Design Guidelines
Here are some guidelines to follow when designing PCBs with planned back drilling:
- Clearly identify back drill locations on inner layers based on high speed routing needs.
- Maintain annular ring width equal to or greater than anti-pad clearance for structural reliability.
- Do not back drill holes under BGA packages or connectors.
- Separate back drilled and non-back drilled holes in different drill tool numbers.
- Use fiberglass reinforced dielectric material like FR4 to prevent fracturing.
- Confirm board shop has back drilling capability and quality standards.
- Request 0.25 to 0.5 mm smaller finished hole size than drill diameter to account for plating.
- Verify back drill registration capability and tolerances.
Following these design practices will result in a more manufacturable, reliable back drilled board.
Applications that Use Back Drilling
Some examples of PCB technologies and products that often utilize back drilling include:
- High speed digital boards with multi-gigabit data channels
- High frequency RF/microwave circuit boards
- Complex multilayer boards with 20+ layers
- HDI boards with high density interconnects
- High pin count BGA packages
- DDR3/DDR4 memory routing in computing
- Servers and network switching equipment
- Optical transceivers for data communications
- Telecommunication infrastructure systems
- Test and measurement equipment
So back drilling provides tangible benefits in many leading-edge electronic products demanding signal integrity and EMI control.
Impact of Back Drilling on Fabrication
Implementing back drilling does impact certain aspects of PCB fabrication:
- Additional drilling step increases fabrication time and cost.
- Tighter clearances may require thinner cores or prepregs.
- More advanced drilling equipment needed for precision back drilling.
- Panel yield can be affected if back drill registration is off.
- Needs additional data prep (drill drawings, NC drill files).
- Requires close planning between designer and board shop.
- Challenging on boards with thick cores or uneven layer stacks.
So designers should evaluate if back drilling gains offset the increased complexity, especially for cost-sensitive boards.
Conclusion
Back drilling is an extremely useful PCB fabrication technique for high density, high frequency boards used in applications like networking, telecom, aerospace, automotive and medical electronics. By selectively removing non-critical portions of plated through hole barrels, back drilling minimizes stub effects that can degrade signal integrity and emanate EMI at multi-gigabit data rates. With careful planning, back drilling allows meeting electrical performance and densification needs. Overall, back drilling is an important tool in the PCB designer’s toolkit for boards pushing the limits of speed and density.
Frequently Asked Questions
Does back drilling weaken the structural integrity of the PCB?
Back drilling can slightly reduce rigidity if many large holes are drilled deeply. But in most cases, restricting back drill depth to just a few inner layers prevents significant weakening while still reducing stubs.
Does back drill registration need to be very precise?
Moderately precise registration is needed – around 0.1mm to 0.15mm tolerance is typical. This prevents accidentally drilling into laminate or traces.
Can components be placed over back drilled holes?
It is not recommended to place surface mount components or connectors over back drilled locations as it risks damaging their connections during the back drilling process.
How do I indicate back drill locations on PCB layout?
Back drill holes are typically marked on inner layer plots using a distinct symbol or annotation to identify them for fabrication.
How much does back drilling increase PCB fabrication cost?
As an additional manufacturing step, back drilling can increase cost 20% or more depending on the number of holes drilled. But the cost may be offset by the performance and density benefits.
