Crossing over a diff pair - how?

Layout
1

I have a 2 sided board that has multiple diff pairs but no matter how I reroute it I always seem to end up crossing over at least one of them. One option I know is to but one device on the underside of the PCB, but in my case I am unable to put any device on the back.s I need to use VIAs to do a cross over.

Is there a recomended technique/layout to do this ??
and can I use microvias ??

Dave

2

HI,

First thought 4 layer board with internal ground planes? Depending on the size of the from the usual overseas fab houses are comparable to 2 layer these days.

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3

I agree with @Docara, if impedance is important, a relatively unbroken ground plane is almost essential. Tracks can cross pairs, but should do so at right angles.

4

I was refering to the need to swap over the tracks that form a diff pair - not other tracks crossing the diff pair. What is the recomended method/layout and should I use vias or mirco vias ??

I am stuck with a two layer board and with no devices on the backside.

5

If not tracks crossing the diff pair then what needs to swap over them?

I have never used micro vias, but according to what I have read micro-via is the very short via from top (or bottom) layer to the nearest internal layer. So the subject of micro vias starts in theory from 4 layer boards, but practically probably from 6 layer boards.

6

I’m confused by the word/phrase “Swap Over”…

Swap means ‘Exchange’ thus, ‘Exchange Over’ means What?

Perhaps a Sketch of what you want will help clarify… (I suspect you want one Track to Cross over the other (paired) Track?

Thus, you can use a Tie (it’s a Net-Tie having a Via part)… Could also place THT-Terminals and use a Jumper-Wire…

7

Sounds like the OP thinks they can form some sort of BUSS out of it somehow. Actually some kind of interchange between the busses. So yeah. Very confusing.

8

Maybe crossing +/-? Like this

+ _______ _____ -
\/
- ______/\_____ +

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9

If this is simply flipping +/-, then using vias works at speeds up to around 10 MHz. More timing and skew critical pairs like USB3 and HDMI are not compatible with 2 layer boards

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10

They are actually, I’ve routed USB3 on 2 layers. You still have almost 4mm of allowed intrapair skew.

11

Just add a zero-Ohm resistor in series with one signal and route the trace under the resistor. You could add a second resistor to balance out the effect of the resistor.

I have seen a dedicated smd part to do the cross over. Don’t remember the details.

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