---
title: "How long a via stub can a fast link take?"
topic: "Via stubs"
answer: "About 23.6 mm divided by the lane’s rate in GBd: 2.3 mm for 10GBASE-KR, 0.7 mm for PCIe 5. Past that, the stub costs more than about 0.5 dB at the link’s Nyquist frequency. A via to layer 3 of a 1.6 mm board leaves 0.94 mm, fine to about 25 Gb/s."
url: "https://nearfieldnotes.com/notes/via-stubs/"
site: "Near Field Notes"
author: "Michael Walker"
note: 17
family: "Signal integrity"
published: "2026-11-05"
revised: "2026-11-05"
revision: 1
video: "https://nearfieldnotes.com/videos/via-stubs/"
---

Note 17 · [Signal integrity](https://nearfieldnotes.com/families/si/) · Rev 1 · [Watch the video · 0:09](https://nearfieldnotes.com/videos/via-stubs/)

Via stubs

# How long a via stub can a fast link take?

**Short answer:** About 23.6 mm divided by the lane’s rate in GBd: 2.3 mm for 10GBASE-KR, 0.7 mm for PCIe 5. Past that, the stub costs more than about 0.5 dB at the link’s Nyquist frequency. A via to layer 3 of a 1.6 mm board leaves 0.94 mm, fine to about 25 Gb/s.

The figure sends one fast pulse into two vias that drop from the top layer to layer 3. In the first, the plated barrel runs on to the bottom of the board. That leftover length, the stub, rings and sends 13% of the pulse back.

In the second, a drill has cut the stub away from below. Only 0.9% comes back.

**Note 17’s opening figure: A pulse meets the via.** 20 ps wide, slowed 50 billion times

![Note 17’s opening figure: A pulse meets the via](https://nearfieldnotes.com/notes/via-stubs/fig-opening.png)

**Key**

- Pulse going in
- Echo coming back

*What the figure shows:* A 20 picosecond pulse runs along the top trace into two vias that drop to layer 3 of the MNT Reform's 6-layer board. In the first, plated through all six layers, the 0.94 mm of barrel below layer 3 rings and sends 13% of the pulse's energy back up the top trace as an echo. The pulse that goes on along layer 3 is smaller and trails ringing. In the second, back-drilled to 0.2 mm, 0.9% comes back.

Above each via, the voltage along 9 mm of top trace into it and 9 mm of layer 3 out of it. Below, the via itself in section through the MNT Reform motherboard 3.0, to scale, lighting up where the wave passes. Sent back counts the share of the pulse’s energy that returns up the top trace.

**The numbers**

| Via | Energy sent back | Energy through | Stub | Down the stub and back | Peak on layer 3 |
|---|---|---|---|---|---|
| Through all six layers | 13% | 87% | 0.94 mm | 13.4 ps | 0.82 V |
| Back-drilled | 0.9% | 99% | 0.2 mm | 2.8 ps | 0.99 V |

## What a via stub is

*One via on the MNT Reform motherboard 3.0*

A via carries a signal from one layer to another through a plated hole. Drilled straight through the board, its barrel runs from the top layer to the bottom, whichever layers the signal uses.

When the signal leaves on an inner layer, the barrel below it carries nothing on. That leftover length is the stub. It hangs off the line like a short wire with an open end. A [TDR](https://nearfieldnotes.com/glossary/#g-tdr) sees it as a dip, as [Note 16](https://nearfieldnotes.com/notes/tdr/) shows.

On a 1.6 mm, 6-layer board, a via from the top layer to layer 3 leaves 0.94 mm of stub.

**Fig. 17: The via, cut in half.**

![Fig. 17: The via, cut in half](https://nearfieldnotes.com/notes/via-stubs/fig-17.png)

*What the figure shows:* Figure 17: a section through the via, to scale. Six copper layers in 1.58 mm of FR-4: top, ground, layer 3, power, ground, bottom. The signal comes in on the top trace, runs down the plated barrel and leaves on layer 3. The barrel below layer 3, 0.94 mm of it to the bottom, carries no signal: that is the stub, open at its end.

A 0.4 mm drill with 0.8 mm pads on every layer, to scale. The planes are cleared 0.3 mm around the pads. The line shows the signal’s way through.

**The numbers**

| Layer | What it is | Depth of its center |
|---|---|---|
| F.Cu | Top | 0.018 mm |
| In1 | Ground | 0.327 mm |
| In2 | Layer 3 | 0.635 mm |
| In3 | Power | 0.945 mm |
| In4 | Ground | 1.254 mm |
| B.Cu | Bottom | 1.562 mm |
| Stub | Barrel below layer 3 | 0.94 mm |

The MNT Reform laptop’s motherboard has vias like this on its PCIe lines. The processor module reaches the M.2 slot over two pairs, one each way.

Each pair leaves the module’s socket on short top-layer traces, drops to layer 3 and comes back up at the slot. All eight vias are plated through all six layers, so each one carries a 0.94 mm stub.

**Fig. 17b: Where the pairs run.**

![Fig. 17b: Where the pairs run](https://nearfieldnotes.com/notes/via-stubs/fig-17b.png)

*What the figure shows:* Figure 17b: the MNT Reform motherboard 3.0 from the top, 276 by 94 mm, with a 26 by 32 mm window enlarged. In it, the top layer's copper, the M.2 slot at the top left and the processor module's socket at the right. The PCIe pairs to the M.2 slot leave the module on short top-layer traces, drop to layer 3 through vias, run under the copper and come back up through vias at the slot. The arrow points at one of those vias.

The whole board from the top, and the box around the pairs enlarged. The copper is the top layer’s, read from the board’s design files. The arrow points at the via in the sections above.

**The numbers**

| Pair, line | End | Drill | Pad | Plated |
|---|---|---|---|---|
| PCIe RX N | M.2 slot | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe RX P | M.2 slot | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe TX N | M.2 slot | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe TX P | M.2 slot | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe RX N | Processor module | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe RX P | Processor module | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe TX N | Processor module | 0.4 mm | 0.8 mm | F.Cu to B.Cu |
| PCIe TX P | Processor module | 0.4 mm | 0.8 mm | F.Cu to B.Cu |

## A quarter-wave notch

*Three stubs, swept to 60 GHz*

An edge that runs down the stub bounces off its open end and comes back up. At the frequency where the stub is a quarter wave long, the bounce arrives back exactly out of step. The open end then looks like a short at the junction, and almost nothing gets past.

`f_notch = c/(4 L √(ε_r))`

where f_notch: frequency of the notch (Hz); c: speed of light (m/s); L: length of the stub (m); ε_r: relative permittivity of the FR-4 (No unit).

`= (300 mm/ns)/(4 × 0.945 mm × 2.12) = 37.4 GHz`

So the notch falls as one over the stub’s length. In FR-4 it sits near 35 GHz divided by the stub in mm.

**Fig. 17c: How much gets through.**

![Fig. 17c: How much gets through](https://nearfieldnotes.com/notes/via-stubs/fig-17c.png)

*What the figure shows:* Figure 17c: how much of a sine gets through the via, S21 in dB, from 0 to 60 GHz. Back-drilled to 0.2 mm it stays within half a dB. With this board's 0.94 mm stub it falls away to a notch at 37.4 GHz and recovers past it. With the 3.2 mm board's 2.6 mm stub the notch drops to 14 GHz, with a second one near 41 GHz.

S21 of the via alone, from 12 mm of top trace to 12 mm of layer 3. The model has no loss, so its notches have no floor. They are cut off at −30 dB.

**The numbers**

| S21 at | Back-drilled | This board | Thick board |
|---|---|---|---|
| Stub | 0.2 mm | 0.94 mm | 2.6 mm |
| 5 GHz | −0.01 dB | −0.08 dB | −0.56 dB |
| 10 GHz | −0.03 dB | −0.32 dB | −4.00 dB |
| 12.5 GHz | −0.04 dB | −0.51 dB | −11.85 dB |
| 16 GHz | −0.06 dB | −0.89 dB | −7.22 dB |
| 20 GHz | −0.09 dB | −1.54 dB | −1.27 dB |
| 28 GHz | −0.16 dB | −4.42 dB | −0.04 dB |
| 37.4 GHz | −0.24 dB | −56.73 dB | −3.55 dB |
| 50 GHz | −0.33 dB | −2.32 dB | −0.43 dB |
| Notch | 176.7 GHz | 37.4 GHz | 13.8 GHz |

Quarter-wave notch

### This board’s stub notches at 37.4 GHz

**Field marks.** [S21](https://nearfieldnotes.com/glossary/#g-s21) falls away to a deep notch at 37.4 GHz and comes back above it. Back-drilled to 0.2 mm, the same via loses under 0.3 dB all the way to 40 GHz, and its notch moves out to 177 GHz.

**Look-alike.** A 3.2 mm board leaves 2.6 mm of stub under the same via. It notches at 13.8 GHz, and again at three times that, 41 GHz.

**Range.** Any via whose barrel runs on past the layer the signal leaves on. Half the stub, twice the frequency.

## Long before the notch

*The longest stub for six links*

A link never runs at the notch. What matters is its Nyquist frequency, half its baud rate: the tone of its fastest pattern, 1010. That is where a channel’s loss is usually judged.

The stub already costs about 0.5 dB at a third of the notch and 1 dB at 45% of it. That held for every stub from 0.5 to 4 mm. Keep the notch three times above Nyquist, and the stub must be shorter than

`L_max = c/(6 √(ε_r) R)`

where L_max: longest stub the lane can take (m); c: speed of light (m/s); ε_r: relative permittivity of the FR-4 (No unit); R: baud rate of the lane (Bd).

`= (300 mm/ns)/(6 × 2.12 × 10.3 GBd) = 2.3 mm for 10GBASE-KR`

That is 23.6 mm divided by the rate in GBd.

**Fig. 17d: The longest stub each link takes.**

![Fig. 17d: The longest stub each link takes](https://nearfieldnotes.com/notes/via-stubs/fig-17d.png)

*What the figure shows:* Figure 17d: the longest stub each link can take before it costs more than 0.5 dB at the link's Nyquist frequency. 10GBASE-KR, 10.3 GBd: 2.3 mm. PCIe 4, 16 GBd: 1.5 mm. USB4, DP UHBR20, 20 GBd: 1.2 mm. 25G Ethernet, 25.8 GBd: 0.9 mm. PCIe 5, PCIe 6, 32 GBd: 0.7 mm. 112G PAM4, 53.1 GBd: 0.4 mm. Lines mark this board's 0.94 mm stub and the 3.2 mm board's 2.6 mm.

Before the stub costs about 0.5 dB at the link’s Nyquist frequency. GBd is per lane. PCIe 6 and 112G send four levels a symbol, PAM4. The rest send two, NRZ.

**The numbers**

| Link | Rate a lane | Longest stub | This board fits | Thick board fits |
|---|---|---|---|---|
| 10GBASE-KR | 10.3 GBd | 2.28 mm | Yes | No |
| PCIe 4 | 16 GBd | 1.47 mm | Yes | No |
| USB4, DP UHBR20 | 20 GBd | 1.18 mm | Yes | No |
| 25G Ethernet | 25.8 GBd | 0.91 mm | No | No |
| PCIe 5, PCIe 6 | 32 GBd | 0.74 mm | No | No |
| 112G PAM4 | 53.1 GBd | 0.44 mm | No | No |

Stub limits

### This board’s stub stops just short of 25G Ethernet

**Field marks.** The 0.94 mm stub fits every link up to USB4’s 1.2 mm. 25G Ethernet takes 0.91 mm, so it misses by 0.03 mm.

**Look-alike.** The same via on a 3.2 mm board leaves 2.6 mm. That is past even 10GBASE-KR’s 2.3 mm.

**Range.** PCIe 6 runs at PCIe 5’s 32 GBd, so it takes the same 0.7 mm. 112G PAM4 at 53 GBd takes 0.4 mm, which only back-drilling or a blind via gets under.

## Thicken the board yourself

*1.6 to 3.2 mm, plated through or back-drilled*

Backplanes and big server boards often run 3 mm thick or more. The via to layer 3 stays the same, but the barrel below it grows with the board.

Slide the board thicker and watch the fastest lane the stub allows fall. Then back-drill it.

**Note 17’s explorer: Via stubs.**

![Note 17’s explorer: Via stubs](https://nearfieldnotes.com/notes/via-stubs/fig-explore.png)

*What the figure shows:* The via to layer 3 as the board under it thickens from 1.6 to 3.2 mm, plated through or back-drilled, and the fastest NRZ lane its stub costs under 0.5 dB at Nyquist. Plated through, that falls from 24.9 Gb/s on the 1.6 mm board to 9.2 Gb/s on the 3.2 mm one. Back-drilled to 0.2 mm, it is 118 Gb/s on every board.

The bar is the fastest NRZ lane whose Nyquist frequency the stub costs under about 0.5 dB. As the board thickens, the layers below layer 3 spread out evenly. Back-drilled, 0.2 mm is left on every board, good to 118 Gb/s, past the end of the scale.

**The numbers**

| Board | Via | Stub | Notch | Fastest lane |
|---|---|---|---|---|
| 1.6 mm | Plated through | 0.94 mm | 37.4 GHz | 24.9 Gb/s |
| 1.6 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 1.8 mm | Plated through | 1.2 mm | 30.3 GHz | 20.2 Gb/s |
| 1.8 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 2 mm | Plated through | 1.4 mm | 25.9 GHz | 17.3 Gb/s |
| 2 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 2.2 mm | Plated through | 1.6 mm | 22.6 GHz | 15.1 Gb/s |
| 2.2 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 2.4 mm | Plated through | 1.8 mm | 20.0 GHz | 13.3 Gb/s |
| 2.4 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 2.6 mm | Plated through | 2.0 mm | 18.0 GHz | 12.0 Gb/s |
| 2.6 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 2.8 mm | Plated through | 2.2 mm | 16.3 GHz | 10.9 Gb/s |
| 2.8 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 3 mm | Plated through | 2.4 mm | 14.9 GHz | 10.0 Gb/s |
| 3 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |
| 3.2 mm | Plated through | 2.6 mm | 13.8 GHz | 9.2 Gb/s |
| 3.2 mm | Back-drilled | 0.2 mm | 176.7 GHz | 117.8 Gb/s |

## The eye on a thick board

*25 Gb/s, three vias compared*

The decibels show up as [closed eyes](https://nearfieldnotes.com/glossary/#g-eye-diagram). These are 25 Gb/s of random bits after 5 cm of FR-4 and the via, with no equalization. [Note 8](https://nearfieldnotes.com/notes/eye-diagram/) shows how an eye is built.

**Fig. 17e: The eye after the via.**

![Fig. 17e: The eye after the via](https://nearfieldnotes.com/notes/via-stubs/fig-17e.png)

*What the figure shows:* Figure 17e: three eyes at 25 Gb/s after 5 cm of FR-4 and the via. 1, the 1.6 mm board with its 0.94 mm stub: 58% open. 2, a 3.2 mm board with a 2.6 mm stub: its rails split into many lines and the eye is 4.7% open. 3, the same 3.2 mm board back-drilled to 0.2 mm: 62% open.

Every bit laid over the next, two bits wide. The darker the shade, the more bits passed there. The bar is the opening at the best moment to sample, as a share of the full swing.

**The numbers**

| Eye open at | 1.6 mm board | 3.2 mm board | 3.2 mm, back-drilled |
|---|---|---|---|
| Stub | 0.94 mm stub | 2.6 mm stub | 0.2 mm left |
| 10 Gb/s | 79% | 78% | 79% |
| 16 Gb/s | 71% | 61% | 72% |
| 25 Gb/s | 58% | 4.7% | 62% |

Eye after the via

### A 2.6 mm stub closes the eye to 4.7%

**Field marks.** On the 3.2 mm board the stub’s echo comes back 36 ps later, about one bit at 25 Gb/s. It lands on the next bit, each rail splits in two, and the eye is 4.7% open.

**Look-alike.** The 1.6 mm board’s 0.94 mm stub leaves it 58% open. Back-drilled, the 3.2 mm board opens to 62%, a little better than the thin board, since only 0.2 mm is left.

**Range.** At 10 Gb/s all three are 78 to 79% open. At 16 Gb/s the thick board falls to 61% and the others stay above 70%.

## What each fix buys

*The same via, four ways*

There are four ways to deal with a stub, cheapest first. Leaving it costs nothing. Routing the pair to the far layer costs nothing either, if the layer is free. Back-drilling adds a drilling step from the far side. A blind via needs a board built in stages, which is worth it only if the board needs that anyway.

**Fig. 17f: Four ways through.**

![Fig. 17f: Four ways through](https://nearfieldnotes.com/notes/via-stubs/fig-17f.png)

*What the figure shows:* Figure 17f: the same via four ways, each a section through the board. 1, left as it is: the barrel runs to the bottom, 0.94 mm of stub, notch at 37 GHz. 2, the pair leaves on the bottom layer instead: the whole barrel carries the signal and nothing is left over. 3, back-drilled from below: 0.2 mm left, notch at 177 GHz. 4, a blind via that stops at layer 3: no stub.

Each section is the via drawn to scale, with the signal’s way through it. Routed to the far layer, the whole barrel carries the signal.

**The numbers**

|  | Fix | Stub | Notch |
|---|---|---|---|
| 1 | Leave it | 0.94 mm stub | 37.4 GHz |
| 2 | Far layer | No stub | None |
| 3 | Back-drill | 0.2 mm left | 177 GHz |
| 4 | Blind via | No stub | None |

Key to via stubs

## Does this via need a fix?

1. 1a Stub shorter than 23.6 mm ÷ the lane’s GBd Leave it
2. 1b Stub longer than that Go to 2
3. 2a The pair can leave on the far layer Route it there
4. 2b It has to leave on an inner layer Go to 3
5. 3a The board is built in stages anyway A blind via
6. 3b A standard board Back-drill, 0.2 mm left

This board’s 0.94 mm stub passes 1a up to 25 GBd.

#### Conditions

- **Board:** MNT Reform motherboard 3.0, 6 layers, 1.58 mm. Its design files are open hardware under CERN-OHL-S v2.
- **Stack-up:** Six 35 µm copper layers and five 0.274 mm FR-4 layers, εr 4.5. The top trace sees 3.4 with the air above it.
- **Via:** 0.4 mm drill, 0.8 mm pads on every layer, planes cleared 0.3 mm around the pads. Top layer to layer 3: 0.62 mm of barrel used, 0.94 mm left below.
- **Where:** The PCIe pairs from the processor module’s socket, U1, to the M.2 slot, J10
- **Model:** Every piece is a transmission line: 12 mm of 50 Ω top trace in, the used barrel as a 45 Ω line, the stub as an open 45 Ω line, 12 mm of 50 Ω layer-3 trace out. Each pad on a layer the signal doesn’t use adds 0.05 pF to ground. Both ends matched. No loss in the via.
- **Solved:** In time, by finite differences, for the pulse. In frequency, with chained ABCD matrices, for S21, the limits and the eyes. The two agree within 0.1 dB up to 34 GHz.
- **Pulse:** 1 V, 20 ps wide at half height
- **Eyes:** NRZ, 1500 random bits, edges 0.35 of a bit long, 5 cm of FR-4 with a loss tangent of 0.02 before the via, no equalization
- **Links:** Each standard’s published rate per lane, in GBd
- **Thicker boards:** The layers below layer 3 spread out evenly. Everything above stays as built.

Simulated only. Nothing on this page has been measured on a board yet.

#### Limits

- A line model, not a 3D field solve. The barrel’s 45 Ω and the pads’ 0.05 pF are set by hand, not solved, so the notch could sit a few GHz away.
- One line on its own. The pair’s other line, the ground vias beside it and the space between the planes are left out. The RX vias have no ground via within 1.6 mm.
- With no loss, the notches go all the way down. Real ones are shallower and a little wider.
- The eyes are this via’s alone. A real link adds its connector, its package and its equalizer.

#### Files

- viasim/line1d.py The pulse in time
- viasim/freq.py S21, the notch, the limits and the eyes
- layout/site_board.py Where the pairs run, read from the board file

#### Sources

- [MNT Reform](https://mntre.com)

What should the next note be? [Ask for one](https://nearfieldnotes.com/#ask-for-a-note)

---

Source: https://nearfieldnotes.com/notes/via-stubs/

Cite as: Michael Walker, “Via stubs”, Near Field Notes, Note 17, Rev 1, 5 Nov 2026. https://nearfieldnotes.com/notes/via-stubs/

Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/via-stubs/#fig-17.