In short: An unused via barrel left below your signal transition acts as a resonant transmission-line stub that steals energy at a quarter-wave null frequency, and back-drilling mechanically removes that stub so multi-gigabit links stay clean.
If you have ever stared at an insertion-loss plot that dives into a sharp notch somewhere between 8 and 20 GHz and wondered what killed your 25 Gbps SerDes margin, the answer is very often a via stub. This article explains the physics of the stub, why back-drilling removes it, and how to decide when the extra fabrication cost is worth it. For a refresher on the different via geometries involved, see our guide to via types in PCB.
What a via stub actually is
A plated through-hole (PTH) via is drilled straight through the entire board stack, even when the signal only needs to travel between, say, layer 1 (top) and layer 4. The portion of the barrel below layer 4 — the part that carries no useful signal — is the stub. Electrically it is not inert copper; it is a short, unterminated transmission line hanging off your signal path.
Because the far end of that barrel is open-circuited, it behaves exactly like a quarter-wave open stub. At the frequency where the stub's electrical length equals a quarter wavelength (λ/4), the open end reflects a signal back that arrives 180° out of phase, creating a resonant short at the via junction. The result is a deep suck-out in the frequency response.
Rule of thumb: the first resonant null appears near f ≈ c / (4 · L · √εr), where L is the stub length and εr the dielectric constant. A 1.5 mm stub in FR-4 (εr ≈ 4.2) resonates around 9–10 GHz — squarely inside a 25 Gbps channel.
Why resonance matters at gigabit speeds
At low speeds a 1 mm stub is harmless — its resonant frequency sits far above the signal content. But modern interfaces push fundamental and harmonic energy into the exact band where stubs resonate:
- PCIe 5.0 (32 GT/s) — Nyquist at 16 GHz
- 112 Gbps PAM-4 SerDes — Nyquist near 28 GHz
- DDR5 at 6400 MT/s — meaningful energy past 5 GHz
When a resonant null lands on or near Nyquist, you lose insertion loss precisely where the equalizer is already working hardest, and you add group-delay distortion that closes the eye. Stubs also degrade return loss and worsen crosstalk because the resonant structure radiates.
Back-drilling: removing the dead copper
Back-drilling (also called controlled-depth drilling or counter-boring) is a secondary drill pass that uses a slightly larger bit to bore out the plating of the stub portion from the back side of the board, stopping short of the last active layer. What remains is a small residual stub — the distance from the last used layer down to where the drill stopped.
Typical practice keeps the back-drill stub under ~10 mils (0.25 mm). A high-performance fab can hold a drill-stop tolerance of about ±4–5 mils, which is why you must leave a keep-out clearance between the last signal layer and the back-drill depth.
Modeling and verifying the fix
You do not back-drill on intuition — you simulate. A 3D full-wave field solver (Ansys HFSS, Cadence Clarity, or Siemens HyperLynx) extracts an S-parameter model of the via with and without the stub. Engineers then run the channel through an IBIS-AMI statistical simulation (Keysight ADS, Cadence Sigrity SystemSI) to confirm the eye opens. The tell-tale signature of a stub in the extracted S21 is a narrow, deep notch; after back-drilling that notch moves far higher in frequency and shallows dramatically.
| Scenario | Stub length | First null | Verdict at 28 GHz |
|---|---|---|---|
| No back-drill | 1.6 mm | ~9 GHz | Fails — null inside band |
| Back-drilled | 0.2 mm | ~70 GHz | Passes — null well clear |
Alternatives exist when back-drilling is impractical: blind/buried vias and micro-vias remove the stub by construction, and skip/through-via optimization can help. But those raise board cost and layer-count constraints, so for most HDI backplanes a back-drill pass remains the cheapest effective fix. Teams designing such boards often lean on specialist high-speed PCB design services to close signal-integrity margins. For the companion topic of controlling impedance across these transitions, read our note on controlled-impedance routing, and browse the full CourseTron course catalog for structured training.
Practical design guidelines
- Route the fastest signals on outer-to-upper layers so the natural stub is short.
- Specify back-drill depth and residual-stub tolerance explicitly on the fab drawing — do not leave it to the fabricator.
- Add non-functional pad removal (back-drill and NFP together) for best results.
- Keep a ground-return via near every signal via so the back-drill does not orphan the return path.
Want to master signal integrity, via modeling, and channel simulation hands-on? The CourseTron High-Speed PCB & Signal Integrity course walks you through HFSS extraction and eye-diagram analysis on real backplane designs.
Frequently asked questions
How long can a via stub be before it matters?
There is no fixed number — it depends on your Nyquist frequency. A good heuristic is to keep the stub's λ/4 resonance at least 2× above your signal's Nyquist. For 28 GHz Nyquist that means the stub resonance should sit above ~56 GHz, which typically means a residual stub under ~0.25 mm.
Is back-drilling the same as a blind via?
No. A blind via is formed during the original lamination/drill sequence and never has a stub. Back-drilling starts with a normal through-hole and mechanically removes the unused copper afterward, leaving a small residual stub and a counter-bored hole.
Does back-drilling weaken the board or the via?
Slightly — it removes plating and leaves a resin-filled counter-bore, so you lose a little mechanical and current-carrying capacity in that barrel. This is why power and high-current vias are generally not back-drilled, and why fabs cap how close the drill may approach the last active layer.
Can simulation alone decide the back-drill depth?
Simulation sets the target, but manufacturing tolerance sets the floor. Because drill-stop depth varies by several mils, you design the keep-out so that even the worst-case residual stub still keeps the resonance clear of your band.