100-ohm diff on 2-layer PTH board

[Andrew Holme]

I'm designing a board with a 100-ohm diff. pair carrying LVDS. For economy,
I want to make a 2-layer PTH board with a solid copper ground plane on the
bottom. The PCB fab said this was impossible because of board thickness.
After playing with a trace impedance calculator, I see what they mean, at
least as far as normal track widths are concerned. To get 100-ohms
differential on a 62 mil thick 2-layer board, I need 60 mil wide tracks
spaced 40 mil apart. So my question is: what's wrong with that?

TIA

 

[Helmut Sennewald]

I'm designing a board with a 100-ohm diff. pair carrying LVDS. For
economy, I want to make a 2-layer PTH board with a solid copper ground
plane on the bottom. The PCB fab said this was impossible because of
board thickness. After playing with a trace impedance calculator, I see
what they mean, at least as far as normal track widths are concerned. To
get 100-ohms differential on a 62 mil thick 2-layer board, I need 60 mil
wide tracks spaced 40 mil apart. So my question is: what's wrong with
that?

TIA

Hello,

In principle it will work.
Your other traces should have a distance of 3 times the board thickness
if they run in parallel over some length. That's a clearance of 0.2inch.

If you have a small board, you could use a thinner board, e.g. 20mil?

Best regards,
Helmut

 

[Joerg]

TXline claims 55 ohms odd mode Z for your dims on FR4, which is 110
ohms diff. You could also go 30 mil traces with a 10 mil gap for 100
ohm differential, which is 50 ohms odd mode per trace.

Nothing wrong with either one.

Agree. Controlled impedance lines anywhere between 50 and 300 ohms have
even been done on double-sided phenolic. Still got some of those in the
garage.

 

[David L. Jones]

I'm designing a board with a 100-ohm diff. pair carrying LVDS. For
economy, I want to make a 2-layer PTH board with a solid copper ground
plane on the bottom. The PCB fab said this was impossible because of
board thickness. After playing with a trace impedance calculator, I see
what they mean, at least as far as normal track widths are concerned. To
get 100-ohms differential on a 62 mil thick 2-layer board, I need 60 mil
wide tracks spaced 40 mil apart. So my question is: what's wrong with
that?

Nothing is inherently wrong with that.

BTW, don't get fixated on differential impedance, you don't actually need it
in your case:
http://www.speedingedge.com/PDF-Files/diffsig.pdf

Dave.