Author Topic: Spec'ing differential pairs in CAD software  (Read 248 times)

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Offline Northy

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Spec'ing differential pairs in CAD software
« on: October 22, 2020, 03:06:18 pm »
Hi all,

Although I'm working in Eagle, I thought this was more of a generic question. If it's not in the correct place, then I'm sorry.

How do people convey their differential pairs and controlled impedance trace requirements to their board manufacturer?

I've gone with a specific width/spacing for the matched pairs and added text notes to the files at the moment, to allow the manufacturer to adjust them.

Is this a good way to do it?

The thing I'm worried about is the software adjusting the track widths when I create the gerbers. Can software do any sort of track width consolidation? Are there some settings I should be looking at?

Thanks,

G
 

Offline Northy

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Re: Spec'ing differential pairs in CAD software
« Reply #1 on: November 11, 2020, 01:22:03 pm »
Incase this helps anyone in the future I just added the following note to the gerber files and the PCB purchasing spec, and the PCB manufacturer will adjust them to suit:

Note per controlled impedance (differential signal traces):

- Top Layer, 9mil wide/5mil gap to be 100 Ohm
- Top Layer, 7mil wide/5mil gap to be 90 Ohm
 

Online Alex Eisenhut

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Re: Spec'ing differential pairs in CAD software
« Reply #2 on: November 11, 2020, 05:47:26 pm »
Typically it's done on the fab drawing. You call out the data like you did, and add any other special instructions.

I also add a readme.txt file to the fabrication package that describes the stackup, the function of each layer, and repeats the special instructions. But make sure they expect it, otherwise it may slow down and confuse the process rather than help.

It's weird that it's still done this way. There are other ways to send manufacturing info to a manufacturer, like ODB++, and some other formats I forget, but every time I asked I was told that plain Gerber/NC/text files are good enough!

And that was even for 24+ layers, mixed materials, blind and buried vias, etc

So to answer your question, yes, that's a good way to do it!
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Offline Northy

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Re: Spec'ing differential pairs in CAD software
« Reply #3 on: November 11, 2020, 06:10:13 pm »
Thanks for the reply.

I've added it to the documentation layer of Eagle, then I spit out a Notes.gbr from that layer.
I also create a PCB purchasing spec document for each PCB (for quality/safety) and I've added it to that too.

Thanks,

G
 

Online nctnico

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Re: Spec'ing differential pairs in CAD software
« Reply #4 on: November 12, 2020, 02:35:22 pm »
Hi all,

Although I'm working in Eagle, I thought this was more of a generic question. If it's not in the correct place, then I'm sorry.

How do people convey their differential pairs and controlled impedance trace requirements to their board manufacturer?
Well, you don't. You specify the board stack-up (copper thickness, dielectric material type & thickness) or select an existing stackup and use that to calculate the trace widths / distances. A board manufacturer doesn't care about your design / trace widths. Their only job is to fabricate a board within the given mechanical tolerances.

IOW: the trace widths and distances are to be calculated during the PCB design stage and stay fixed from there on. This does require planning.
« Last Edit: November 12, 2020, 02:37:15 pm by nctnico »
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