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100 ohms differential pairs impedance matching + length matching



I want to route high speed digital signals of 600 mhz and 6 ghz. datasheets and design guides strictly says it should be 100 ohms differential impedance. I also want to length matching them. I am using KiCAD, can't\won't use any other package.

I used 0.1 trace width and 0.2 spacing (i though it is 0.3 but it is actually 0.2). my PCB vendor is AllPCB and their 4-layer stackup shows pre-prep is 0.122mm of height https://www.allpcb.com/4_layer_pcb.html

I plugged these numbers in Kicad calculator and it showed like in attached picture.

I already routed these signals, was very tiresome to length match them!

please inform me what to do to achieve the requirements... and what is the best approach professional designers use in order to do it.

I mean, you don't design the board multiple times to be able to fit all PCB manufacturers with their different stackups right? or do you specify a custom stackup all the time if you are doing it for professional job.. and is it really much expensive?

does length matching has to do with this?

I had to put vias (0.3mm size) for nearly half of these diff. pairs... will this affect anything?

for your kind help!

NOTE: attached picture was for 0.3 gap which is not correct, correct one is 0.2mm. for 0.2mm kicad says Zodd= 68 ohms which is not going to be 100 ohms diff... i need it to be 50 ohms.

EDIT: I achieved 50 ohms Zodd by making 0.15mm width and 0.1mm spacing, but i feel these are too close and too small... is this correct to choose?


reference board has this:

width of trace: 0.1 mm
spacing: 0.2 mm (= actual space between the 2 tracks, not from track center to track center).

which is like mine.

but their stackup specs are the following:

prepreg height: 60 um.
copper height: 18 um. (mine is standard 35).

I plugged them on kicad calculator and it turns out to have around 51.75 ohms Zodd. seems about correct right?


I might be wrong about reference stackup, therefore I attached the sheet showing its details. please correct me

I looked around and found that I used 0.1mm trace with 0.2mm gap + 0.122mm prepreg thickness and 35um copper thickness... both kicad and saturn pcb tool showed around 100-103 ohms differential which is nice! also, all signals are length matched with help of 0.3mm vias. is there anything else i need to worry about? please inform me.

Now for single signals (not differential), there are 60 of them which needs to be 50 ohms. kicad shows 0.2mm to be suitable (=51.1 ohms) while saturn shows around 0.19mm (50.1 ohms). gonna go with saturn calculations and modify the tracks.

what do you think or recommend?


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