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