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I am designing an I/O board for an NVIDIA Jetson Nano SOM module. The board contains the HDMI, USB2.0, CSI, SD card and power system and GPIOs.

As I am beginner with high-speed designs, I have a problem, please help me to understand.

So for USB 2.0 90 ohm impedance is required, so I calculated the following:

PCB transmission line calculation screenshot

There I have selected 1 oz copper thickness and FR4 material. My PCB manufacturer has a capability of minimum 0.15 mm spacing and 0.15 mm track width. So after entering all the details, I got 0.77 mm as track width and 0.15 mm as track spacing.

For my reference I have downloaded the official Rpi CM4 IO board Kicad files, which are using 0.15 mm as width and 0.25 mm as gap. I have verified the copper core thickness is 1 oz.

So could you please help me to understand, what's the parameter which is causing the difference in width and gap?

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    \$\begingroup\$ You're using a two layer board so the traces have to be very wide. On a 4 layer they'd be much smaller due to the extra ground plane. \$\endgroup\$ May 5, 2021 at 14:17

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The difference is the H value. 60 mil is 1.524 mm, which suggests you are using a standard 2 layer board. For high speed interfaces like the ones you mention, you should use a 4 layer board. That way the reference plane is much closer, so your H value will be smaller. Here's JLC's standard 4 layer dimensions:

enter image description here

And here's their 2 layer:

enter image description here

In the 4 layer stack, the Prepreg value corresponds to H in your software. In the 2 layer stack, the Core value corresponds to H. Try experimenting with the values there, and you'll come closer to the values you've observed on the CM4 carrier board.

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  • \$\begingroup\$ So how can I use this tool to calculate the W and S for 4 layer board ?? \$\endgroup\$ May 5, 2021 at 14:21
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    \$\begingroup\$ Just copy the relevant values your PCB manufacturer provides about their stackup. For example, for the 4 layer one I've linked, you'd set H = 0.18 mm, T = 0.035 mm. I can't see what H_t is on your software's graphic, but cross reference it like the ones I've shown. \$\endgroup\$
    – awjlogan
    May 5, 2021 at 14:25
  • \$\begingroup\$ I received the following stackup details from the manufacture. link. And after putting the prepreg value in "H" get impedance different. Please help what I am doing wrong \$\endgroup\$ May 6, 2021 at 9:40
  • \$\begingroup\$ @Abhishekbhatia Note in your link that they are quoting the Single impedance, not differential. Your \$Z_O\$ looks wrong, Saturn PCB Design (and some others online) give a value close to the manufacturer's value. \$\endgroup\$
    – awjlogan
    May 6, 2021 at 10:04
  • \$\begingroup\$ Hope by using this stackup details,I can calculate S and W itself from tool and W=17.5 mil and S=5 mils ryt ? \$\endgroup\$ May 6, 2021 at 10:21

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