Practical Notes on Backdrilling

A plated through hole is convenient, but the unused portion of the barrel below the last layer it serves is not electrically neutral. That leftover copper acts as a stub, and at high frequencies it behaves as a resonant element that degrades the channel. Backdrilling removes the stub mechanically, and understanding its limits is essential before specifying it on a drawing.

What a Via Stub Does to a High-Speed Channel

When a signal enters a through via at an inner layer and the barrel continues to the far side of the board, the excess length is a short transmission line with an open circuit at one end. Energy coupled into that stub travels down, reflects, and returns out of phase. The result is a notch in the insertion loss curve and a bump in the return loss at the frequency where the stub length equals a quarter wavelength.

For a thick backplane the stub can be several millimetres long, which places the first resonance in the low gigahertz range and directly inside the band of interest. Even a shorter stub narrows the effective bandwidth of the channel, raising jitter and closing the eye. Via stub control is therefore not an exotic refinement; it is a normal part of designing a high-speed link.

How Backdrilling Removes the Stub

Backdrilling, also called controlled depth drilling or counterboring, uses a drill slightly larger than the finished via to remove the unused plated barrel. The tool enters from the side opposite the signal layers and stops at a controlled depth, cutting away the stub while leaving the barrel that carries the connection intact. The hole is then re-plated or left as bare laminate depending on the process.

The operation is performed after plating, so the drill removes both copper and resin. Depth control is the whole difficulty: too shallow and a long stub remains, too deep and the drill damages the barrel or the laminate that the signal layers depend on. The target is normally expressed as a maximum remaining stub length rather than as a nominal depth.

Drill Depth, Tolerance and the Remaining Stub

Every process has uncertainty, and backdrilling carries three sources at once: the thickness of the finished panel, the accuracy of the drilling machine, and the registration between the drilled hole and the layer being protected. A common specification allows 0.2 mm to 0.3 mm of remaining stub, which is short enough to push the first resonance well above the channel bandwidth.

Panel thickness variation is often the largest contributor, because the drill depth is measured from the surface and any extra thickness becomes extra stub. Fabricators therefore measure the panel before the operation and adjust the program per panel. Stating the requirement as a remaining stub length, and letting the fabricator choose the drill depth, gives the best result.

Impedance Control and Return Path Effects

Removing the stub also changes the local geometry of the via. The barrel that remains is short, so its capacitance to the surrounding planes is small and the impedance discontinuity at the transition is reduced. Consistent depth across all the vias in a bus matters as much as the absolute depth, because a bus with varying stub lengths has varying impedance along its length and skews the signals that pass through it.

The return path has to be considered at the same time. Backdrilling a via that sits inside a plane keeps the antipad clearance intact, but widening the hole can encroach on adjacent copper and change the clearance to neighbouring nets. The high-speed design rules cover the clearance and antipad choices that keep impedance control intact through the transition.

Resonance and Insertion Loss

The classic stub resonance occurs where the stub length is a quarter of the wavelength in the dielectric. At that frequency the stub looks like a short circuit and the channel suffers a deep loss notch. Below resonance the effect grows gradually, appearing as increased insertion loss and a rising group delay variation that shows up as jitter in the time domain.

Backdrilling shifts that resonance upward, typically by a factor of five or more when the stub is cut from several millimetres to a fraction of one. Above 10 Gbps the channel can also be limited by the via barrel itself, by the pad capacitance and by laminate loss. Backdrilling removes one dominant impairment but does not make a poor stack-up good.

When Backdrilling Is Worth the Cost

The operation adds a drilling step, extra handling, and inspection, and it consumes panel area around each hole. It is worthwhile when the channel bandwidth approaches the stub resonance, which in practice means signals above roughly 5 to 10 Gbps, thick backplanes, or connectors routed through many layers. Below that, the cost rarely returns a measurable benefit.

It is not worthwhile to backdrill every via on a board. Only vias on high-speed nets need the treatment, and only on the side where the stub exceeds the target length. A layer pair near the surface has almost no stub to remove, so the specification should be applied selectively and marked clearly on the fabrication drawing.

Design Rules That Make Backdrilling Practical

Plan the via stack so that high-speed signals change layers near the surface or in the middle of the board, whichever side has the shorter unused barrel. Keeping high-speed nets on adjacent layer pairs limits the stub length without any drilling at all, and it is free. Backdrilling is the remedy of last resort, not the first tool to reach for.

Give every backdrilled via enough clearance to its neighbours, because the finished hole is larger than the original. Antipads on the planes that the hole passes through have to accommodate the enlarged diameter, and the drill must not cut into the pads of the layers that remain connected. Spacing rules in the via types guide are a useful reference for clearance planning.

Working with the Fabricator on Tolerances

Ask for the achievable remaining stub and the depth tolerance before finalising the design, since these vary with drill size, panel thickness and equipment. Provide the via list with the required maximum stub for each net class, and note whether the removed hole should be plugged or left open, because an open backdrilled hole can trap flux during assembly.

Plating thickness on the remaining barrel is also worth confirming. The plating guide explains how barrel copper thickness relates to current capacity and to the thermal stress of assembly, and backdrilling removes some of the material that was deposited, so the remaining wall should still meet the requirement.

Alternatives When Backdrilling Is Not Possible

Blind and buried vias remove the stub by construction, because a via that ends inside the board has no unused barrel at all. They solve the same problem more elegantly but at the cost of sequential lamination, so the choice between the two is usually a cost and lead time question rather than a technical one for moderate layer counts.

Where neither is available, a thinner board reduces the stub length directly, and moving the signal to a layer pair that is closer together helps. Routing choices on the package and the connector can also reduce the number of layer transitions. The high-frequency laminate guide covers the material side of the same trade-off.

FAQ

What is an acceptable remaining stub length? Most fabricators work to 0.2 mm to 0.3 mm, which pushes the first resonance well above the channel bandwidth. Tighter values are possible but cost more and may not improve a real link.

Does backdrilling damage the via? Not when the depth is controlled. The risk is drilling too deep and cutting into the barrel that carries the signal, which is why the requirement is specified as a maximum remaining stub rather than a fixed depth.

Can backdrilled holes be plugged? Yes, and it is often advisable. Plugging keeps flux and solder out of the open hole during assembly and protects the remaining barrel from contamination. gopcb notes the requirement on the fabrication drawing so the step is not missed.