1

I am attempting to determine the trace width needed for routing a differential signal of 100 Ω on a PCB I am designing.

This is a hobby project and the differential traces are for gigabit ethernet.

I'm very tempted to just forget about it, and just go with that manufacturer's numbers, but I am curious why different calculators spit out such wildly different numbers.

I found some EE exchange posts which provided unsatisfactory answers:

Link1 Link2

The second link seems to hint that some calculators take some extra parameters into consideration.

Having never routed differential signals the size difference these calculators are coming up with is a little alarming but perhaps this is normal?

Parameters:

  1. Trace Separation: 4mil
  2. Trace thickness: 1.4mil
  3. Dielectric Thickness: 3.5mil
  4. Er: 4.05

The manufacturer provides a stack up as well as a 2D calculator for determining trace width: here The manufacture's calculator computes 3.49mil trace width.

When I attempted to validate this against other online calculators I find that some agree and some differ substantially.

EEWeb: Says the same numbers from my manufacture would be 160ohms!

Everythingrf: Agrees with the manufacture.

Can someone explain why they are different and what I need to consider when deciding trace width using these tools?

ocrdu
  • 9,195
  • 22
  • 32
  • 42
Erik Kinstler
  • 13
  • 3

1 Answers1

0

JLCPCB and EverythingRF use the same formula which only applies to circumstances where the trace-width-to-dielectric-thickness ratio is less than 1 (W/H < 1).

(EEWeb has a good explanation on their webpage but I didn't check if they use the same formula).

In your case the ratio is quite close 1: Trace width (W) is 3.49 mil, and dielectric thickness (H) is 3.5 mil. The ratio goes beyond 1 as the separation gets higher. So if you find that the dielectric thickness is less than or equal to the required trace width then you should review the stack-up.

JLCPCB's first stack-up recommendation for 1.6mm PCB seems correct to me but the 2nd one doesn't.

Rohat Kılıç
  • 33,940
  • 3
  • 29
  • 85