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One of the challenges in PCB design and manufacturing is maintaining signal integrity. Back drilling, also known as controlled-depth drilling, is used to remove the conductive via stubs from the copper sleeves within PCB through-holes, improving signal integrity.
This article introduces the principles and features of back drilling technology, offering design parameters and guidelines to help you improve signal integrity through PCB back drill.
What is Back Drilling in PCB?
Back drilling refers to the secondary drilling of plated-through holes during PCB manufacturing. Unwanted stubs are removed after plating through-holes are manufactured using a slightly larger drill bit, leaving only the active interconnect area. Unlike traditional through-hole vias, which penetrate the entire PCB board, back drilling selectively removes excess material to transform the through-hole into a “partial through-hole”.
This process mitigates reflection and resonance issues in high-frequency signal transmission effectively, improves impedance matching and enhances signal integrity. Therefore, any high-speed PCB project is an application that can benefit from backdrilling, such as data center components and telecom and communication boards.
Why Need PCB Backdrilling
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Improving signal integrity: A via on multilayer PCBs is located between the top and bottom layers, extending beyond the last layer, and this creates a stub of copper that remains unused. Subsequently, when dealing with high frequency, these stubs begin to behave like antennas, receiving the signal sent to them and unintentionally reversing it back to the source, creating distortion, crosstalk, and other issues with signal integrity.
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- For example, a 400G Ethernet backplane design can be achieved without attention to via stubs. However, the PCB does not pass compliance testing for IEEE insertion loss or return loss. It found that it’s because of stubs.
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Enabling subsequent design upgrades and iterations: A design for 3–5 Gbps may find stub vias as survivable, but if this design, 8–10 years down the line, wants to take that same architecture and convert it to 25–56 Gbps, those stub vias have to go. Starting with an entirely clean board allows the architecture to compete later for speed increases.
How is Back Drilling Done?
Before diving into the specific process of back drilling, let’s first understand the characteristics of circuit boards that undergo backdrilling.
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Most back-drilled boards are rigid PCBs and the number of layers typically ranges from 8 to 50.
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The board thickness is usually above 2.5 mm and the size tends to be larger.
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The minimum drill diameter for the first drilling is generally ≥ 0.3 mm.
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The back-drilled hole is typically about 0.2 mm larger than the original drilled hole.
Process of Back Drilling
There are specific steps to manufacture back dilled vias as follows:
- Vias are drilled fully through the PCB, just like any other assembly, then the copper barrels of the vias are plated.
- Only vias related to high-speed signals that could create stubs are marked for backdrilling. A CNC drill returns from the opposite side and trims the unused copper barrel to the level just above the last connected layer.
- Finally, the PCB is washed to remove drilling residues or leftover copper debris inside the back-drilled holes, thus ensuring clean and reliable electrical performance.
Backdrilling Parameters
Back drill hole size: Typically, approximately 10 mil larger than the original via hole (i.e. the initial drilling size).
Back drill hole clearance:
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There should be 20 mil clearance from the inner layer perimeter circuit patterns. (The IPC standard requires a minimum of 10 mil.)
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The minimum spacing between backdirlled vias is 10 mil.
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The minimum spacing between a back drill via and an outer layer pad is 15 mil (the IPC-6012A standard requires a minimum of 20 mil).
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Ensure a dielectric thickness of 6–8 mil between plated-through and non-plated-through layers.
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Minimize stub length for optimal signal integrity; however, actual production tolerances (depth tolerance and board thickness tolerance) typically range from 4–8 mil.
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The manufacturers verify the stub depths (±2–3 mils tolerances are common) to make sure it is not cut into the signal layers.
Design Considerations
Several key factors require careful consideration when designing back drilling:
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The drilling depth can be calculated theoretically:
Drilling depth = Distance from the plate surface to the top of the target layer – Safety margin – Drill bit length compensation.
For example, for a PCB with a total thickness of 1.6 mm, the back drilling depth for holes extending from the surface layer (L1) to layer L5 should be “the thickness from L1 to L5 plus 0.05 mm compensation” (to avoid damaging layer L5).
This calculation must be extremely precise, taking into account all compensation factors, and represents a core control point in production. Reducing the back drill depth by 1 mil (i.e., the residual stub depth increases by 1 mil), results in a signal loss of approximately 0.25%.
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Limit back drilling to high-speed vias or differential pairs or signal nets over ~3 Gbps. Power and low-speed control via DO NOT require back drilling.
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Stubs should be 1/10 of the signal wavelength. For instance, 25 Gbps translates to a signal wavelength of ~24 mm in FR4, so anything longer than 2–3 mm should be removed.
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The accuracy for depth must be above the last linked layer. Too shallow keeps the stub in place, too deep breaks a connection. Also, the thicker the PCB, the more likely via stubs create problems, and need backdrilling.
Comparison to Other Via Types
Before deciding which via type to use, it’s helpful to compare back drill vs blind via and other common options. This comparison table shows how each via affects signal integrity, cost, and manufacturing complexity.
Table: Backdrilled Vias vs Blind Vias vs Buried Vias vs Through-Hole Vias
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Via Type
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Description & Characteristics
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Cost & Complexity
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Best Use Case
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Standard Through-Hole Vias
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Low
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Low-speed or low-cost designs
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Blind Vias
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Medium–High
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Higher-density designs where stub-free connections are required
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Buried Vias
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High
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High-density or high-performance designs
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Backdrilled Vias
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Medium
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High-speed designs needing low stub impact
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In short, back drilling in PCB manufacturing provides a cost-effective alternative to blind and buried vias for high-speed performance.
Future of Backdrilling in PCB
The backdrilling in PCB significantly improves signal integrity in high-speed PCBs by removing excess stubs from vias, thereby reducing reflections and interference. As 5G, high-speed computing and artificial intelligence technologies advance rapidly, the demand for back drilling will continue to grow.
Back drilling technology is advancing towards greater precision and automated, intelligent control.
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Higher precision: The accuracy of back drilling depth control has improved from ±5 mil to ±2 mil or even finer.
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Enhanced automation: EDA tools now offer smarter back drilling rule settings and automated recognition algorithms.
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Technology integration: Back drilling can be combined with techniques such as via filling and laser drilling.
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Simulation integration: The effects of back drilling are incorporated directly into signal integrity simulation models.
⚡Wanna enhance your PCB signal integrity with back drilling?
At ELEPCB, our experienced engineers provide professional PCB back drilling services and design support to ensure your high-speed PCBs perform reliably and efficiently.
FAQ
A1: No, only high-speed vias that have the critical signal need to be backdrilled. For instance, power vias or low-speed control signals are unlikely to generate stubs when they are transmitting.
A2: No. Blind vias do not create stubs because they only go to the layer in which you would be connecting them. Backdrilling goes all the way through, and the excess is removed. Backdrilling is less expensive than making a blind via because it’s just an additional step in multilayer builds.
A3: Accurate enough that it only drills up to just before the last connected layer. If it doesn’t go deep enough, the stub remains. If it goes too deep, the connection will be missing.
A4: No. Most designers only offer back drilling for their critical high-speed differential pairs and leave other vias alone to avoid a manufacturing upcharge.
A5: Most CAD systems allow you to denote what drilling is needed within drill tables or fabrication notes. You just have to effectively communicate with your PCB designer so they know.
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