[SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity: By the formula, as F goes up, Xc goes down (was up by typo)
- From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
- To: chand basha <chand_374@xxxxxxxxx>, PaulClarke@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 12 Dec 2006 15:19:17 -0800
These links will help
characterization_of_a_printed_circuit_board_via
http://www.coe.montana.edu/ee/lameres/vitae/publications/1_thesis/thesis
_002_msee.pdf
Dr. Johnson constructs a working large scale model of a via, large
enough so the he can reach into the board and modify the structure from
within while observing, in real-time, the electrical behavior of the
via.
http://www.sigcon.com/SiLab/Via_clip.wmv
3D field solvers versus Network Analyzer and real models
Many of you are already using coupons to assess the quality of your
boards. For those they are not familiar, Test coupons are typically
small sections around the periphery of a board with exactly the same
layers stackup as the main PCB that are fabricated at the same time as
the PCB. Coupons are or may be used to test a number of PCB features
that determine impedance, design integrity, etc.
The opportunity exists to assess via-model-designs at practically no
cost to the project other than the via design time and lab assessment to
characterize the results:
I envision a series of coupon with the various vias and anti-pads as
well as guard grounds place so these may be connectorized with surface
mount sma or sna connectors compatible with your Network analyzer. This
will allow an engineer to extract an S parameter model from each
physical module which may be useful for further simulation. (Most high
speed circuit simulators will be able to use S parameter model in
simulations.) This may seem, on the face of it, an expensive approach.
In reality, it is cheaper and quicker than 3D field solving but DOES NOT
produce an exact solution.
Understand this:
The optimum via design is rarely the one used as board space is not
available to contain all the signals and all the grounds in the same
area. As density of vias go up, the ground planes are literally carved
away near the BGAs where circuit density is the highest. What also goes
away is exact prediction of circuit behavior without exact 3D modeling
which is time intensive and uses expensive software tools.
Steven Salkow
Lockheed IS&S
3130 Zanker Rd, San Jose
Ca. 94588
steven.salkow@xxxxxxxx
salkow@xxxxxxxxxxxx
________________________________
From: chand basha [mailto:chand_374@xxxxxxxxx]
Sent: Tuesday, December 12, 2006 12:03 AM
To: Salkow, Steven; PaulClarke@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal
Integrity:By the formula, as F goes up, Xc goes down (was up by typo)
Steven Salkow,
its an excellent presentation, very simple really very simple,
I have a dought in the last para i.e
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.
if you can explain a littile bit about tuning the impedance with ground
vias will be very much
help full.
Thanks in advance.
chand
"Salkow, Steven" <steven.salkow@xxxxxxxx> wrote:
-----Original Message-----
From: Salkow, Steven
Sent: Monday, December 11, 2006 1:59 PM
To: 'PaulClarke@xxxxxxxxxxxxx'; 'si-list@xxxxxxxxxxxxx'
Subject: RE: [SI-LIST] 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 down (was up by typo). 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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