PCB propagation delay calculator
Delay per length for stripline and microstrip traces from the dielectric constant, the delay of a given trace, and the length that gives a target delay.
- Delay per length
- 6.836 ps/mm
- Effective Er
- 4.2
- Velocity, share of c
- 48.8 %
- Length for the target delay
- 14.63 mm
How this is calculated
tpd = √Er,eff / c c = 299 792 458 m/s stripline: Er,eff = Er microstrip: Er,eff from Hammerstad and Jensen (1980), u = w/h widened for copper thickness: εe(u) = (Er + 1)/2 + (Er − 1)/2 · (1 + 10/u)^(−a(u) b(Er)) (the same model as the impedance calculator)
- Stripline
- The field is wholly inside the dielectric, so delay depends only on Er and not on width or height (TI SCAA082A).
- Microstrip
- Part of the field is in air, so the trace is faster than stripline in the same laminate. The model includes copper thickness; solder mask raises Er,eff further and is not included. With 1 oz copper it reproduces the TI SCAA082A table (0.5 mm and 1 mm on 1.5 mm, Er 4.6: 3.045 and 3.164 against the TI values of 3.046 and 3.165).
- Er
- Use the laminate value at your signal frequency from the fab stackup. A field solver Er,eff can be entered directly with Known Er,eff.
- Texas Instruments SCAA082A, High-Speed Layout GuidelinesNovember 2006, revised August 2017: section 1.3.1 and Table 2
- E. Hammerstad, O. Jensen, Accurate Models for Microstrip Computer-Aided Design (effective permittivity with copper thickness)IEEE MTT-S International Microwave Symposium Digest, 1980, pp. 407 to 409
- NIST, CODATA value: speed of light in vacuum, 299 792 458 m/s, exactCODATA 2022
Questions
For stripline in Er 4.2, 6.84 ps/mm or 173.6 ps/in. TI SCAA082A gives 715 ps per 100 mm for stripline in Er 4.6. Microstrip on the same laminate is faster, about 5.8 to 6.0 ps/mm for thin copper on a trace one to two times as wide as its height above the plane, because part of the field is in air. Thicker copper puts more field in air: 0.1 mm over 0.1 mm in 1 oz copper is about 5.6 ps/mm.
The wave sees an effective Er between air (1) and the laminate. In TI SCAA082A, a microstrip on Er 4.6 has an Er,eff of 3.05 to 3.17 and runs 582 to 593 ps per 100 mm, against 715 ps for stripline.
Not for stripline. For microstrip a wider trace puts more of its field in the laminate and is slightly slower: in the TI table, going from 0.5 mm to 1 mm raised the delay from 582 to 593 ps per 100 mm.
Enter the budget as the target delay. At 6.84 ps/mm, a 10 ps budget is 1.46 mm of trace. The DDR length matching calculator does this for a whole byte lane against its strobe.