USB Trace impedance conundrum

[dope40]

Hi ,
I have recently been making a board with a usb connector and FTDI FT232RL.
The problem is that i am using a 2 layer board. I used Saturn PCB Calculator , but the traces have
to be too big .

So , the conundrum is that when i saw the SparkFun USB to Serial Breakout - FT232RL https://www.sparkfun.com/products/12731, in the board file the traces are 10 mil ( 0,254 mm ) and are not side by side .



Does someone know how is this possible ?? Its my first time using usb , so my knowedge on the subject is really poor . During my reading i found a post ( Sorry cant find the link ) where i read that you can use a 18 mil ( 0.457 mm ) and a 6 mil (0.152 mm) gap .



I am also using a STF202 USB Filter with ESD Protection .

My questions are :
- Will my design work ?
- Why / How does SparkFuns works ?
- Am i missing something ?

All help well be very appreciated .

 

[KlausST]

Hi,

It seems you try to generate traces with controlled impedance...

FT232RL are USB1.1 standard devices.
The USB data rate is 10Mbaud.
This frequency isn't very critical. And the trace length is very short. Also not critical.
With 10Mbaud you need some meters of trace length to come in the critical area.

So you can use almost any trace width. Relax.

Klaus

 

[FvM]

I agree with KlausST. Trace impedance is almost irrelevant for a few cm of 12 MHz FS( Full Speed) USB connection. It matters for HS or SS.

The substrate height and Er must be known for trace impedance calculation. The given trace parameters would achieve 90 Ohm differential impedance for a thin FR4 substrate of about 0.5 mm FR4 respectively a multilayer with ground plane. Standard 1.55 mm substrate requires considerably wider traces. You can use free Saturn PCB tool to calculate trace impedances.