# The width of the traces and the size of the vias

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#### Budda

##### Newbie level 5
Traces and Via's

well up until recently i had only designed a couple of simple boards at uni, these were really just a matter of playing dot to dot joining connections together.

Now im looking at designing a "high speed board" and have been reading about trace impedance and a milliion other things.

So i understand that when you design high speed pcbs you use say a 50ohm impedance by a hieght width calculation, but does it matter what width your traces are compared to frequency? I have seen some boards with traces that are 4mil and others seem to have traces 16mil or more is this just a matter of preference and board size??

And my other question is what determines the size of the via you use? is it just whatever fits on the track and doesnt take up to much room?? there doesnt seem to be much information on this topic.

Kevin

#### Sukohi_Berkut

##### Junior Member level 2
Re: Traces and Via's

Hi Budda

It is true that the impedance of the trace depends on the width/height above the plane also the dielectric constant.

if you are designing baords upto 6 -10 layers, all you have to do is specify the board shop that you want 50 ohms or 60 ohms for a particular width lets say 4mil or 5mill trace with 6mil or 5 mil gaps.

u see the point 4mil go with 6 mil airgap and 5mil with 5 mil airgap.

or any decent layout tool (cadence, mentor) will have an in built 2d field solver u can play with the numbers

or go to ultracad.com and download their impedance caluclator. and just play with it enter different numbers and get a feel for it. read some books regareding high speed designs.

my suggestion read "High-Speed Digital System Design: A Handbook of Interconnect Theory and Design Practices"

now the impedance also depends on frequency but it will be showing its effect at RF or mulit gigahertz, you as well as so many of us have a long way to learn it. also it is much more complex.

regarding the via size.
it depends on the board thickness

conservative board house will accept an aspect ration of 1:8

that is if your board thickness is 100 mils then any board house will be able to manufacture your board if you tell them 13 mils as the Finished hole size of the via

a new board house with latest tech will be able to achieve 1:10 aspect ration
that is for a 100 mil thick board they can manufactire upto 10 mil FHS

certain hi-tech board house will allow u to use 8 mil FHS but its gonna cost u dearly

regards
sukohi_berkut

#### mckinson

##### Full Member level 2
that is very good for suggestion. I layout 8 layer board

#### Budda

##### Newbie level 5
Re: Traces and Via's

Im designing a mixed signal board with a micro running at 200Mhz, but it is more the rise time im interested in? which i think will be 6ns. Will this put any limitations on my design?

And if im desinging analoge traces then the only thing i have to worry about is current carrying capacity?? oh and also keeping them away from other dirty noise sources??

Thanks for help
Regards Kevin

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