[SI-LIST] Re: PCB Trace impedance algorithms - Free trace calculator
- From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 02 Mar 2007 10:45:48 -0800
Dear Sam:
What is important is that the trace widths you initially throw down on
your PCB meet the following concurrent requirements:
1. Satisfy your board's house minimum trace width when the trace is
over etched by 10%
2. Are lower in initial impedance so they may be etched to your
required impedance accuracy
3. Meet your companies' manufacturing standards for reliability
4. Use different trace widths for differential or single ended
signals on the same layer having the same target impedance
No matter which ECAD tools or calculator you use to determine the cross
section of your traces, these initial values will be adjusted by the PCB
fabricator. When the Gerber files get to the fabricator, there are no
net names that relate back to your original design in a meaningful way,
hence, you fabrication drawing will be forced to make statements such as
"all 9.75 mils line on microstrip layers are differential and 50 ohms
+/- 10%." This allows the pcb fabricator to grab the 9.75 mils traces on
all surface layers and tweak their widths according to their process
adjusting for etch and dielectric constant and variation is lamination
thicknesses.
Tools you may have may not account for the soldermask when calculating
microstrip. Other calculators such as
mine(http://www.bychoice.com/stripline.exe), defaults to a soldermask
thickness of 1 mil and er = 4.7. When evaluating calculators, compare
several to Cadence Allegro over the RANGE YOU INTEND TO USE.
I have made a comparisons of several tools over a limit range
http://www.bychoice.com/microstrip_calculations_for_embedded_circuits_su
mmary.xls
Any tool that is within 2% should meet your needs fine. My calculator is
not only free but shows the equations.
In the above comparisons, I am attempting to create a new equation for
embedded microstrip that will do a better job with varying thicknesses
of soldermask. If any one want to help, its fine with me.
Steven Salkow
Lockheed IS&S
3130 Zanker Rd, San Jose
Ca. 95134
steven.salkow@xxxxxxxx
salkow@xxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Sam Sam
Sent: Monday, January 22, 2007 7:35 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] PCB Trace impedance algorithms
Dear si-list members,
I am learning tool support for pcb designs. I have some questions
regarding calculating impedance of a traces in PCB. I use allegro's
built impedance calculator. I am also aware that there are various other
calculator tools from UltraCAD, Polar Instruments etc. I am wondering
how efficient and accurate these calculations are. I guess most of them
use some kind of assumptions and have simplified closed form formulas to
qucikly extimate the impedance. But can you people guide me as what is
the exact technique or algorithm to calculate the impedance of a pcb
trace say for a microstrip structure. Any papers or links to this study
would be appreciated. In specific, since i am using allegro 's
calculator i would like to know how they calculate the impedance and
what are the assumptions they take. I have seen most calculator allow
single ended and differential trace calculations. Is it possible to
extend these techniques to multiple traces. More importantly the
accuracy of the
formulas is of concern to me. When compared to full wave results these
formulas from different tools give different result. So i am looking to
learn what is the background behind these? Please advise me on this.
Thanks in advance. Looking forward for your answers....
Sam
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