Calculators / 05
Electrical Length and Critical Length Calculator
A trace becomes a transmission line when a reflection from its far end can no longer hide inside the edge that launched it. This calculator applies the rule the topic page derives — the round trip against a third of the rise time — so the two can never disagree.
Reading the numbers
- In air, light covers an inch in 84.7 ps; a dielectric slows it by √Dk.
- Below the critical length the reflection returns while the edge is still rising and merges into it. Above it the reflection arrives as a separate step, and the line needs a controlled impedance and a termination.
- It is the rise time, not the clock rate, that decides: a 10 MHz clock with a 100 ps edge is already electrically long on any trace longer than about 0.10 in at Dk 3.8.
Every value is in the address bar as you change it, so a link reproduces the calculation exactly.
Type values with SI prefixes — 100n, 2.2p, 16G — or
drag the sliders; a bare number keeps the prefix already shown.
The formulas
Go deeper — assumptions, how it is checked, and sources
Assumptions. One driver, one receiver, a uniform line. Microstrip has an effective Dk below the laminate's because part of its field is in air; stripline's equals the laminate's. Use the line impedance calculator to get εeff.
How it is checked. At Dk = 1 the delay is the vacuum value; delay scales linearly with length and with √Dk; and the critical length is exactly where the round trip equals tr/3.
Source. E. Bogatin, Signal and Power Integrity — Simplified, 3rd ed., Pearson, 2018, ch. 7.
Laminate presets. Each preset is copied from its manufacturer’s own document for one named construction, and every value is typical, not guaranteed: Shengyi S1141 technical data sheet (2021), a standard-grade FR-4 measured on a 1.6 mm 8×7628 board; Isola 370HR Dk/Df tables, Rev. C (2020), core 1×2116 at 51 % resin; Panasonic MEGTRON 4 R-5725, data sheet 22040132 (2022), 2116 at 56 %; MEGTRON 6 R-5775(N), 22040130 (2022), 2116 at 56 %; MEGTRON 7 R-5785(GE) and (GN), 22040128 (2022), 2116 at 53 % and 55 %; and Rogers RO4350B at its design Dk of 3.66, not the 3.48 process Dk. Dk and Df change with frequency, construction and test method, so the preset reads them at a 10 GHz reference, as this calculator has no frequency input. It interpolates only between points measured by one method, and outside them it uses the nearest tabulated point and says so. Two cores of the same product can differ by more than two products do, so sign off with your fabricator’s stackup values. The sources are rows C-27 to C-33 of the claim ledger.
Related
Sources
- Isola — 370HR Dk/Df tables C-27
- Panasonic — MEGTRON 4 data sheet C-28
- Panasonic — MEGTRON 6 R-5775(N) data sheet C-29
- Panasonic — MEGTRON 7 data sheet C-30
- Rogers — RO4350B laminates C-32
- Shengyi — S1141 technical data sheet C-33
Rows marked with a claim id are tracked in the claim ledger, which records what each source can and cannot establish.