Has Anyone Routed USB2.0 on Standard 2-LayerBoard?

[pmtwiss]

Hey Guys,

Making a wuick test board using Advanced Circuit's $33, 2-Layer Board special.

here are some of the specs:
- 6 mil trace / space minimum
- 15 mil drill minimum hole size

I need to route USB on this board, but it is a simple 1oz copper 2-layer board. Thus from bottom layer to top layer is ~59 mils. By my calculations, in order to meet 90-ohms differential and adhere to their rules for this cheap board, I have to route a differential pair of 6 mil spacing and 40mils wide each trace.

I checked and checked again(used Saturn, and another internet calculator), and it seems right due to the fact that the plane under is 59 mils away.

Has anyone done this before? Just checking before I do it...

Thanks,
-Peter

 

[loosemoose]

that sounds about right. you can lessen the impact of impedance discontinuities by using shortest traces possible and no vias.

 

[pmtwiss]

Hey Loosemoose(awesome name!)

Yeah thanks for the confirmation, I have done tons of high-speed stuff but when I did the calculation for 90-ohm on a 2-layer 62 mils board, the trace came to ~40 mils...which are gigantic....I was shocked.

Basically, I never knew the plane distance effect a differential impedance that much....single-ended of course, but differential... I guess just never thought about it much.

-Peter

 

[loosemoose]

thanks.
for single ended 50 ohm traces the rule of thumb is 1:1 ratio, meaning the trace width is about the same as the height above the plane.
so a 5 mil trace that is 5 mils above the plane is ~ 50 ohms (-+10%).

here is a link to an article you might be interested: http://analoghome.com/articles/an004.pdf.

LM

 

[pmtwiss]

Hey LM,

Read that article. Nice thanks! I knew of the coplanar waveguide technique, by using Saturn PCB tool, however they do not do it for differential pairs.

Do you know of a CPWG-differential pair calculator?

Thanks

 

[loosemoose]

try the rogers corp calculator.

https://www.rogerscorp.com/acm/technology/index.aspx

you will need to sign up, but it is free.

 

[pmtwiss]

hey LM, I checked out their site and signed up but I didn't see anything like that...all I saw was a copper thinness, temp calculators...and a microwave calulator app.

I found this:
https://obrazki.elektroda.pl/79_1309087178.gif

What program is this?

THanks,
-Peter

 

[loosemoose]

look for the Microwave Impedance Calculator. the file is called mwi2010.exe.

 

[pmtwiss]

OK so I got it thanks,

works ok, give me ballpark area...but from the pic of that other calculator...it is off.

The rogers says 90-ohm differential is ~25 mils width, this pic says 14 mils...kind of a big difference.

I guess I can choose right in them middle and be fine.

-Peter

- - - Updated - - -

OK so I got it thanks,

works ok, give me ballpark area...but from the pic of that other calculator...it is off.

The rogers says 90-ohm differential is ~25 mils width, this pic says 14 mils...kind of a big difference.

I guess I can choose right in them middle and be fine.

-Peter

 

[loosemoose]

maybe it is better if you ask the board shop to give you the geometries. They most likely will have a better tool to verify.