[SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity
- From: <jain.nitin@xxxxxxxxx>
- To: <PaulClarke@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 13 Dec 2006 02:17:40 +0530
Two things w.r.t "via" that affect the quality of High speed signals are-
1. Antipad size- The signal "via" that passes thru a ground plane in particular
sees parasitic capacitance because of the antipad. The bigger the size of
antipad, lesser is the parasitic capacitance. The effect of this can be seen
for Gigabit signals.
2. Stub- Stub (in simple terms-unused/unterminated portion of via) also leads
to SI problems due to reflection
________________________________
From: si-list-bounce@xxxxxxxxxxxxx on behalf of Salkow, Steven
Sent: Tue 12/12/2006 3:29 AM
To: PaulClarke@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity
Paul:
I will make this simple are seems reasonable. It does, however, seem to
me quite extraordinary that a mechanical fellow might be getting
involved with Gigahertz design of vias.
You're correct the effect does depend on speed. The "anti-pad" is used
when building plane layers (i.e.: solid layers) using negative planes.
It is the VOID area between the pad and the copper of the plane. The
effect is to provide a capacitive reactive effect given by the formula
Xc= 1/(2*pi*F*C) where f is frequency and C is capacitance. By the
formula, as F goes up, Xc goes up. The C capacitance is given by the
formula C = (Area*k*e)/length where length is really the distance the
two areas are apart (in this case the width of the anti-pad (the bigger
the gap, the smaller the capacitance). The effects of C is cumulative
for multiple planes.
If the anti-pad size is very large, are we out of the woods. NO!
All signals used in modern design as transmission lines have a certain
desirable impedance. The is the effective "resistance" of the line that
best matches the driver electronics. When effective "resistance" of the
line does not match the driver electronics one of two possibilities
happen:
The signal has energy reflected back to the source
Or excessive energy is absorbed by the circuit a too little gets to the
load.
Anti-pads are designed to maintain the required effective "resistance"
(impedance) of a transmission line at a matching value. What's that
mean?
If the line impedance and the driver impedance and the load impedance
are all 50 ohms, then the via should be tuned to the same value.
How do we tune via impedance? We use ground vias nearby and 3D Modeling
tools that exist to fufill this purpose but that is beyond the scope of
a short answer.
Steven Salkow
Lockheed IS&S
3130 Zanker Rd, San Jose
Ca. 94588
steven.salkow@xxxxxxxx
salkow@xxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Clarke, Paul
Sent: Monday, December 11, 2006 1:25 PM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] ROOKIE: Anti-Pad Size Effect On Signal Integrity
Hello,
Before you read the question please keep in mind that I am just a lowly
Mechanical guy that has better odds of selecting the right bolt than I
do
designing an LED circuit.
I have a question about how the size of an anti-pad can effect signal
integrity. The example application could be a backplane @ 5, 10, 20, 40,
or
80 [G] (I am asking for this range because I anticipate the answer may
depend on the speed).
If you have a BP via for a signal pair of .025" with a pad of .044", how
much impact can an antipad have on the impendance through a range of
sizes
of let's say .054-.060"? Center-Center distance could be 2.1 [mm].
In the case described above, would the antipad size range really have
any
effects or is it negligible?
Is an anti-pad just to keep solder off the pad if you flood the plane?
Or is
there an actual SI reason for those things?
How sensitive is the SI to changes in antipad size?
Any concerns regarding manufacturing tolerances on antipads?
Thank you for any information and your patience explaining any of the
above
questions to a mechanical guy.
Paul Clarke
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