Free Engineering Tool
Via Stub Length Calculator
Find the quarter-wave resonant frequency of your via stub and check if back-drilling is needed. Enter stub length and Dk to get the resonance — or specify your max frequency to find the longest allowable stub.
Via Stub Calculator
Two modes: enter a stub length to check its resonance, or enter a frequency limit to find the max stub.
Design Inputs
Length of unused via barrel below the signal layer
NRZ bit rate — used to check if stub is acceptable (5th harmonic rule)
Results
Quarter-Wave Resonance
✓ OK — resonance is 44% above signal bandwidth (25 GHz)
Formula Used
f_res = c / (4 × L × √εr) = 2.998×10⁸ / (4 × 1.016 mm × √4.2)
Quarter-wave resonance model assumes a shorted transmission-line stub. Real via stubs have additional parasitic capacitance from pads and antipads that shift the resonance slightly lower. For critical channels (25 Gbps+), validate with 3D EM simulation and use back-drilling or blind/buried vias.
Need back-drilling? AtlasPCB offers controlled-depth back-drill with ±5 mil accuracy on every order.
Get Instant QuoteThe Physics
Why via stubs kill high-speed channels
A via stub is a short, open-ended (or shorted) transmission line connected to the signal path. At its quarter-wave resonant frequency, it presents a very low impedance to the signal — effectively a short circuit — creating a deep notch in the channel's S21 (insertion loss). Energy at that frequency is reflected back to the source and never reaches the receiver.
Quarter-wave resonance
f = c / (4 × L × √εr)The stub resonates when its electrical length = λ/4
5th-harmonic rule
For NRZ signaling, the 5th harmonic of the fundamental frequency must pass cleanly. This means f_res must exceed 2.5 × bit_rate. At 25 Gbps, the stub must resonate above 62.5 GHz.
The fix: back-drill
Back-drilling removes the unused barrel from the opposite side, shortening the stub to a controlled residual (typically 5–10 mil). This pushes the resonance well above the signal bandwidth.
FAQ
Via stubs & back-drilling, answered
What is a via stub?
The unused portion of a through-hole via barrel below the signal layer. It acts as a resonant stub that creates a notch in the channel. Critical above 5–10 Gbps.
How is resonant frequency calculated?
f_res = c / (4 × L × √εr). Speed of light divided by four times the stub length times the square root of dielectric constant. Standard quarter-wave formula.
When is back-drilling needed?
When stub resonance falls in your signal bandwidth. Rule of thumb: at 10 Gbps on FR-4, max stub ≈ 145 mil; at 25 Gbps ≈ 58 mil; at 56 Gbps ≈ 26 mil.
What are alternatives to back-drilling?
Blind/buried vias (no stub), via-in-pad with microvias, sequential lamination, or routing on layers near drill entry. Back-drilling is the most cost-effective for standard PTH construction.
Need back-drilling on your board?
Controlled-depth back-drill with ±5 mil accuracy, TDR-verified impedance, high-speed signal integrity — 1-piece MOQ.