BGA central ground matrix
Tim
Typically FPGA BGAs have a central square of balls which are all connected
to ground. What is the current wisdom on how to hookup the PCB tracking for
the central ground matrix.
The simplest, and lowest inductance, seems to be to put down a solid square
of copper covering the central ground ball pads, and pepper the square with
ground vias. But that looks like the dreaded 'Solder Mask Defined' pads. Is
it better to go with individual NSMD pads, tracking, and vias?
Thanks.
austin
That is so last year.
Howard Johnson showed that the loops need to be made small.
As it turns out all ranks of connections inside the square carry no
current at all. Solve Maxwell's equations, and surprise!
So, alternative power and ground is the best.
That is what our SparseChevron(tm) is all about.
Austin
Gabor
matrix is for thermal conduction to the ground plane. Check
with your assembly service, but usually placing a solid plane
under the BGA with solder mask defined openings will adversely
affect soldering. We usually use etch at about the pad size
going diagonally to one via per pad. No thermal relief where
the via attaches to the ground plane(s).
Central ground balls in these packages do carry DC current,
but were placed where they are for better thermal conduction to
a central die. Generally many more signal return grounds
appear in the outer sections of the same BGA's.
Also check with the chip manufacturer for possible application
information. I've also seen BGA packages with a single
heat slug on the bottom requiring a special pad and solder
paste pattern (Broadcom Gig ethernet PHY). A BGA that
has sufficient thermal conductivity to the top surface usually
works best with a heatsink rather than using the circuit
board for heat spreading (Xilinx flip-chip metal top packs).
Regards,
Gabor
Symon
an excellent post! We use a criss-cross pattern of smaller traces going
between the pads and vias, four narrower traces per gnd pad and per via.
Makes it look nicer around the edges of the central cluster, but works much
the same as yours!
Cheers, Syms.
colin
This all suggests that I can have an outer ring of vias in the center
of a device (next to every "outer" gnd ball) and a copper pour on the
top layer connecting the rest of the gnd balls with a few vias. I can
then easily put some bulk decoupling right in the center of the bga as
it is no longer peppered with vias. If this is the case then you have
my thanks for pointing this out.
By the way, you say no current flows on the inner balls but surely they
carry their share of the DC current?
Regards
Colin
Symon
news:1138876017.5...@g43g2000cwa.googlegroups.com...
>
> This all suggests that I can have an outer ring of vias in the center
> of a device (next to every "outer" gnd ball) and a copper pour on the
> top layer connecting the rest of the gnd balls with a few vias. I can
> then easily put some bulk decoupling right in the center of the bga as
> it is no longer peppered with vias. If this is the case then you have
> my thanks for pointing this out.
>
region. Works well on our FG676 parts with, IIRC, a 6x6 centre. The 0805s
are 2mm long so they fit nicely into the via matrix. The 0805 pads are
directly aligned with the FPGA GND pads, so the via fit into the gaps.
HTH, Syms.
Austin Lesea
You are making a classic mistake: not much DC current flows either.
The static magnetic field will force the static electric field to be
confined to the area adjacent to the current flow in the opposite direction.
This is not skin effect (where current flows on the surface at high
frequencies), but a very simple EM rule, that is completely ignored!
Use of any 2&1/2 D or 3D (Ansoft) modeling tool shows this.
The most famous (and true) story of this was with the SF Bay Area Rapid
Transit System (BART), in ~1974 or was it 1975?:
The design had a third rail, on the left of the train (from front of
train point of view) carrying 1000V DC for the train.
The return was the two rails.
Obviously(?) the Westinghouse engineers reasoned that the return current
would be equally divided among the two rails. 1/2 on right rail, 1/2 on
left rail.
They designed a "train in block" detector to show where they had trains
that detected when the two currents were balanced.
Day 1, they turn it on, and everywhere there is NO train, the light is
ON (giant status board in Richmond, Ca). Everywhere there !is! a train,
the light is OFF!
Westinghouse, Xerox (who did the computers?), and a host of consultants
descend on UC Berkeley to ask the E&M Professors "what the h***?"
As they (professors) laughed and laughed, they asked the grad and senior
students to figure it out with (for) the commercial engineers.
So, we all sat down, sharpened our pencils, got out our sliderules and
textbooks, solved it, and voila! 2/3 in the left rail (nearest to the
supply) and 1/3 on the right rail (furthest).
Austin
Paul Johnson
wrote:
>The most famous (and true) story of this was with the SF Bay Area Rapid
>Transit System (BART), in ~1974 or was it 1975?:
I was there in '70 or '71: I remember that the line and stations were
finished, but there was some technical problem that prevented the
trains running.
dp
> .....
> The static magnetic field will force the static electric field to be
> confined to the area adjacent to the current flow in the opposite direction.
>
Austin,
sure you did not really mean that? Static magnetic fields do not
affect electric field(s) according to physics.
Dimiter
------------------------------------------------------
Dimiter Popoff Transgalactic Instruments
http://www.tgi-sci.com
------------------------------------------------------
austin
Perhaps these folks say it better:
"Proximity Effect" As true at DC as at any frequency
http://www.cda.org.uk/Megab2/elecapps/pub22/sec4.htm#Proximity%20Effect
Commonly misunderstood.
You tell me what is happening?
I say the DC magnetic field affects current flow.
Austin
austin
This link is also confused. They talk about ferrous vs non ferrous, and
skin effect, too. The basic effect has nothing to do with ferrous or
skin effects.
austin
austin
Anyone who can point to a clear and simple explanation, please do.
When I first mentioned this to our packaging group, the lead engineer
said "oh yes, I see this in the EM simulations..."
So, I know I am not imagining it!
Austin
dp
Austin,
the only way a simulator can see DC current
resulting from a static magnetic field is a software bug
or, worse, misconcepted basics behind the software.
If physics would allow that we would have unlimited
energy for free... just put a magnet next to a conductor
and off you go... :-)
Perhaps you meant moving (mechanically) a static magnetic
field relative to some conductors? This would of course
do the job.
austin
I know you do not belive me. And you haven't ever solved Maxwells
equations for this case (or else you would see it).
I am not going to convince you, so I will not try, but it is a real
effect, and it really happens.
I also admit that it is greatly misunderstood (after all, Westinghouse
believed as you do, util they made a million dollar mistake by building
it, and experiencing it first hand).
Austin
dp
> dp,
>
> I know you do not belive me. And you haven't ever solved Maxwells
> equations for this case (or else you would see it).
>
Austin,
I also think it is some kind of misunderstanding, of course.
Perhaps (if you refer to the railroad story) the motion came
from the train moving, or something else they just did
not take into account initially, things like that do happen.
However, for the case of the BGA socket, this cannot
apply. If there is no DC current through the central pads
it can only be because of higher active resistance or,
more likely, because there is little if any (leakage only,
I guess) DC current to talk about. Come to think of it,
it should be that last one.
BTW, my (1.27 mm pitched) BGA designs all have a
via hole in the center of each pad, this is OK if you run
small quantities.
The most important drawback is the necessity to once
fry the BGA chips belly up with some flux, before you use
them on the board, lest some of the balls get detached
(come coldly soldered from chip vendor) and flow through
the board forming a bubble .... (I had this several times until
I figured out how to deal with the problem).
The most important advantage is obviously having access
to all the BGA pads with the scope etc.
Jim Granville
> dp,
>
> I know you do not belive me. And you haven't ever solved Maxwells
> equations for this case (or else you would see it).
>
> I am not going to convince you, so I will not try, but it is a real
> effect, and it really happens.
>
> I also admit that it is greatly misunderstood (after all, Westinghouse
> believed as you do, util they made a million dollar mistake by building
> it, and experiencing it first hand).
Take a magnet near the front of a Shadow mask CRT, and you can
clearly see the effect a magnet has on moving (dc) electrons.
DC current requires electrons to move, even if the ammeter does not.
-jg
austin
As I said, you do not believe me.
Go run the simulation for a 6X6 array.
The outer wall of 6 is + (6+6+3+4, the perimeter), and all the inner 5X5
(25 conductors) are -.
Then look at the distribution of current at DC.
I did find one article on furnaces, which showed the proximity effect on
carbon electrodes, but it also made mention of frequency effects, and
seemed unclear on what they saw. They clearly saw what I describe in
the plots of current. But, they also attributed it to the 50 Hz AC
field (which is pretty absurd....skin effect at 50 Hz is negligable!).
As I said, well misunderstood. Even after looking at the answer, they
explained it wrongly.
Austin
dp
High school physics is sufficient to know you can deflect the beam of
electrons because of the two interacting magnetic fields, the one the
electrons produce when moving with the one you apply with your
magnet.
The DC current value remains unchanged, I hope you are aware of
that.
dp
Austin,
I do believe you know what you are talking about.
I just do not accept the explanation - physics, as we know it,
says it must be different.
I am pretty sure you don't just assume there is little,
if any DC current flowing through the central BGA pads,
you know it is so. It just cannot be explained by any
static magnetic field, that's all. My assumption
is that there just is no DC current, it can be measured
as DC once it has been summed up in the power/ground
plane capacitance, decoupling capacitors etc. With CMOS
chips, you actually have only leakage DC current, which
is orders of magnitude lower than what I believe we are
talking about. The rest is only AC.
Peter Alfke
different path.
I look at it this way:
Everything else being equal, the dc current would take the path that
puts the least energy into the magnetic field. In other words, it
minimizes the rea of the current loop.
But opposing that is the resistive drop if all current were to use the
smallest loop. So the current finds the right balance. Nature is smart,
and consistent.
Peter Alfke
al82
dp ha escrito:
>
> I am pretty sure you don't just assume there is little,
> if any DC current flowing through the central BGA pads,
> you know it is so. It just cannot be explained by any
> static magnetic field, that's all. My assumption
> is that there just is no DC current, it can be measured
> as DC once it has been summed up in the power/ground
> plane capacitance, decoupling capacitors etc. With CMOS
> chips, you actually have only leakage DC current, which
> is orders of magnitude lower than what I believe we are
> talking about. The rest is only AC.
>
I think that you are calling AC current to what actually is Switching
current, but CMOS transistors switching current is DC.
Try to add a DC ammeter in the VCCAUX and you will see several hundreds
of mA. That's why you need a large DC voltage regulator.
The result is that there is a large DC current flowing through the
central BGA pads as well as some AC current (that originates from
changes in the DC current value).
colin
It seems to me that when I learnt Kirchoffs there should have been a
caveat that these laws are almost completely pointless (even at DC!)
Any decent pointers as to where to start reading would be appreciated.
Colin
Uwe Bonnes
> Further,
Perhaps put some simulation results online, with an explanation of the
input data. With simulation GIGO (garbage in, garbage out) easily comes
into play.
--
Uwe Bonnes b...@elektron.ikp.physik.tu-darmstadt.de
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
Paul Johnson
>Austin Lesea wrote:
>> .....
>> The static magnetic field will force the static electric field to be
>> confined to the area adjacent to the current flow in the opposite direction.
>>
>> .....
>
>Austin,
>
>sure you did not really mean that? Static magnetic fields do not
>affect electric field(s) according to physics.
I think the confusion is in the use of the word 'static'. A DC current
produces a magnetic field (Ampere's law, and Maxwell's 4th eqn). Line
up two conductors next to each other, run a current through them, and
the two resultant magnetic fields will interact. An electron in bar A
will move in the component of the magnetic field produced by bar B,
and so will feel a force due to it. This is the proximity effect.
Perhaps not all 'static', but all DC.
Paul Johnson
a) The central balls "carry no current at all", are isolated from the
die GND, and are just for thermal conduction, or
b) They are connected to the die GND, they do carry some return
current, although less than the GND pins at the edge, and their
decoupling is still important, but not very important?
Brian Drummond
>austin wrote:
>> Further,
>>
>> Anyone who can point to a clear and simple explanation, please do.
>>
>> When I first mentioned this to our packaging group, the lead engineer
>> said "oh yes, I see this in the EM simulations..."
>>
>> So, I know I am not imagining it!
>>
>> Austin
>
>Austin,
>
>the only way a simulator can see DC current
>resulting from a static magnetic field is a software bug
>or, worse, misconcepted basics behind the software.
I don't think he's talking about the magnetic field generating a DC
current; but modifying the path of one that exists from other causes
(the PSU).
Think about those moving electrons (beta particles) in a particle
detector; a static magnetic field certainly modifies their path.
(Thus you can determine their velocity from its radius)
Also think about the force on two busbars carrying a _DC_ current; on
what does the force act? On the electrons, i.e. on the current, which
(in modifying their path) apply force to the busbar.
The same will apply in a BGA package; it will not generate any current,
but it may redistribute it between conductors.
- Brian
- Brian
dp
> ...
> >the only way a simulator can see DC current
> >resulting from a static magnetic field is a software bug
> >or, worse, misconcepted basics behind the software.
>
> I don't think he's talking about the magnetic field generating a DC
> current; but modifying the path of one that exists from other causes
> (the PSU).
>
> Think about those moving electrons (beta particles) in a particle
> detector; a static magnetic field certainly modifies their path.
> (Thus you can determine their velocity from its radius)
I really regret I have to go back to this thread. I suggest everyone
posting more on this nonsense makes sure to consult at least
some high-school physics books first.
The electrons (or beta particles, the origin does not matter)
can be moved in vacuum or in a gas because they, when moving,
produce a magnetic field, which interacts with the static
magnetic field, exactly in the same way as two magnet pieces
interact with each other (i.e. results in a force applied to the
freely moving electron).
When the electrons move inside a conductor (metal), this effect
is seen as a mechanical force applied to the _conductor_. It takes
electric rather than magnetic field to move electrons inside the
conductor. This is how electric motors work. You _cannot_ affect
the path of the electrons inside the conductor by a static magnetic
field, just as you cannot force them to exit the conductor.
> Also think about the force on two busbars carrying a _DC_ current; on
> what does the force act? On the electrons, i.e. on the current, which
> (in modifying their path) apply force to the busbar.
>
> The same will apply in a BGA package; it will not generate any current,
> but it may redistribute it between conductors.
>
> - Brian
No. It cannot. I am tired of this thread, I would have hoped all
electronics engineers would have at least some fundamental
understanding of physics. Apparently there are some who don't.
Anybody who has doubts please consider taking some basic
course of physics.
Phil Hays
> You _cannot_ affect
>the path of the electrons inside the conductor by a static magnetic
>field, just as you cannot force them to exit the conductor.
Hall effect.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html
--
Phil Hays
Symon
news:1138977225.2...@z14g2000cwz.googlegroups.com...
> Brian Drummond wrote:
> When the electrons move inside a conductor (metal), this effect
> is seen as a mechanical force applied to the _conductor_. It takes
> electric rather than magnetic field to move electrons inside the
> conductor. This is how electric motors work. You _cannot_ affect
> the path of the electrons inside the conductor by a static magnetic
> field, just as you cannot force them to exit the conductor.
>
> No. It cannot. I am tired of this thread, I would have hoped all
> electronics engineers would have at least some fundamental
> understanding of physics. Apparently there are some who don't.
> Anybody who has doubts please consider taking some basic
> course of physics.
>
> Dimiter
>
> ------------------------------------------------------
> Dimiter Popoff Transgalactic Instruments
>
> http://www.tgi-sci.com
> ------------------------------------------------------
>
Did you ever learn about the Hall Effect in your physics classes? Check
out:-
http://en.wikipedia.org/wiki/Hall_effect
There's even a nice picture for you! ;-)
HTH, Syms.
Symon
news:q8r6u1h32albkt9oq...@4ax.com...
>
> Hall effect.
>
>
> --
> Phil Hays
>
Phil, you beat me by 3 minutes. Dammit!
Cheers, Syms.
dp
Hall effect has negligible values in conductors. On the other hand,
it may take place somewhere on the die, that might well be.
If the current does not flow out of the chip into the wires there
is no need to redistribute it. I hope everyone is happy now.
Dimiter
Phil Hays
>Phil Hays wrote:
>> "dp" wrote:
>>
>> > You _cannot_ affect
>> >the path of the electrons inside the conductor by a static magnetic
>> >field, just as you cannot force them to exit the conductor.
>>
>> Hall effect.
>>
>> http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html
>>
>>
>> --
>> Phil Hays
>
>Hall effect has negligible values in conductors.
Not always. Same as with skin effect, it isn't always negligible at
power line frequencies. Best to do the calculations before, rather
than after.
--
Caution: Contents may contain sarcasm.
dp
It has nothing to do with frequencies, and we are talking
DC. And yes, it is negligible inside the conductors
in the context ot the thread, this is why I did not
mention it at all. If you want to pursue this further,
you are welcome to do the maths and prove me wrong,
with all the sarcasm included. I shall not do it,
I have more practical things to do.
Dimiter
Phil Hays
>Phil Hays wrote:
>> Caution: Contents may contain sarcasm.
>with all the sarcasm included.
Oh dear. Please excuse me.
I use several different signatures depending on the topic at hand.
Please accept my statement that I didn't intend to use this signature
for this discussion.
--
Phil Hays
Brian Drummond
>Brian Drummond wrote:
>> ...
>> >the only way a simulator can see DC current
>> >resulting from a static magnetic field is a software bug
>> >or, worse, misconcepted basics behind the software.
>>
>> I don't think he's talking about the magnetic field generating a DC
>> current; but modifying the path of one that exists from other causes
>> (the PSU).
>>
>> Think about those moving electrons (beta particles) in a particle
>> detector; a static magnetic field certainly modifies their path.
>> (Thus you can determine their velocity from its radius)
>
>I really regret I have to go back to this thread. I suggest everyone
>posting more on this nonsense makes sure to consult at least
>some high-school physics books first.
> When the electrons move inside a conductor (metal), this effect
>is seen as a mechanical force applied to the _conductor_.
Think about HOW that force is applied to the conductor.
Think about whether the presence of current (motion of electrons) is
significant in this process, or whether, as your version suggests, the
magnetic field and the conductor alone are sufficient.
>It takes
>electric rather than magnetic field to move electrons inside the
>conductor.
That was never in debate.
But once they are in motion, what happens?
- Brian
Austin Lesea
I am working on that.
Thanks.
Austin
Austin Lesea
The latter (b).
They do carry current, but it is falling off as 1/r or 1/r^2 (I just
can't remember which).
The BART rails had 2/3 nearest the power rail, and 1/3 in the rail
furthest. Which makes me think it was 1/r, not 1/r^2.
Also, BART has shorting links every X meters that ties the two rails
together (now) to lessen the return resistance (improve efficiency).
I think I was told that the inner 2X2 balls had 1/8 to 1/16 the
current...but it may have been more (or less).
As I already said, I will post some results (when I find them).
Austin
Jim Granville
> When the electrons move inside a conductor (metal), this effect
> is seen as a mechanical force applied to the _conductor_. It takes
> electric rather than magnetic field to move electrons inside the
> conductor. This is how electric motors work. You _cannot_ affect
> the path of the electrons inside the conductor by a static magnetic
> field, just as you cannot force them to exit the conductor.
If true, then why do superconductors have a critical magnetic field level.
Moving electrons are influenced by magentic fields, the key
question, is how much ?
-jg
fpga...@yahoo.com
Austin Lesea wrote:
> As I already said, I will post some results (when I find them).
I've given seminar talks for the last 20 years pressing designers to
constantly reevalutate the underlying assumptions in a design, as they
frequently change, and with small invalidations in the foundation, the
whole design can, and does, fail. Actually to document them, and well.
As a consultant tackling failed projects, one reoccuring theme when
I started probing the design/architecture was asking questions about
the assumptions and getting the "everybody knows ..." answer. The
"we have always done it that way, and it works ..." answer. Well, why
is it now broke?
This is another case of "everybody knows", that will be fun to add to
my
on going talk, "It's not what you know that will hurt you, it's what
you
think you know" as a case study :)
Jim Granville
> Paul,
>
> The latter (b).
>
> They do carry current, but it is falling off as 1/r or 1/r^2 (I just
> can't remember which).
>
> The BART rails had 2/3 nearest the power rail, and 1/3 in the rail
> furthest. Which makes me think it was 1/r, not 1/r^2.
>
> Also, BART has shorting links every X meters that ties the two rails
> together (now) to lessen the return resistance (improve efficiency).
Hmmm....
> I think I was told that the inner 2X2 balls had 1/8 to 1/16 the
> current...but it may have been more (or less).
Hmmmm....
> As I already said, I will post some results (when I find them).
Please do, we can agree there is an effect, my antennae just question
how much of an effect at DC ?.
You still have to satisfy ohms law, so any push effects that favour
flow, have to model somehow as mV(uV) generators....
To skew Ball DC currents 7/8 or 15/16, frankly sounds implausible, and
maybe the models there forgot to include resistance balancing effects ?
[ ie do not believe everything you are 'told' ]
-jg
Symon
news:1138992533.5...@g49g2000cwa.googlegroups.com...
>
> As a consultant tackling failed projects, one reoccuring theme when
> I started probing the design/architecture was asking questions about
> the assumptions and getting the "everybody knows ..." answer. The
> "we have always done it that way, and it works ..." answer. Well, why
> is it now broke?
>
http://www.wowzone.com/5monkeys.htm
Monkeys are funny. :-)
Cheers, Syms.
Paul Johnson
>Brian Drummond wrote:
>> ...
>> >the only way a simulator can see DC current
>> >resulting from a static magnetic field is a software bug
>> >or, worse, misconcepted basics behind the software.
>>
>> I don't think he's talking about the magnetic field generating a DC
>> current; but modifying the path of one that exists from other causes
>> (the PSU).
>>
>> Think about those moving electrons (beta particles) in a particle
>> detector; a static magnetic field certainly modifies their path.
>> (Thus you can determine their velocity from its radius)
>
>I really regret I have to go back to this thread. I suggest everyone
>posting more on this nonsense makes sure to consult at least
>some high-school physics books first.
I've got a Physics degree, and I expect and I'm not alone here.
Although, I have to admit, that was over 20 years ago...
> The electrons (or beta particles, the origin does not matter)
They're the same thing
>can be moved in vacuum or in a gas because they, when moving,
>produce a magnetic field, which interacts with the static
>magnetic field, exactly in the same way as two magnet pieces
>interact with each other (i.e. results in a force applied to the
>freely moving electron).
What is a 'static' magnetic field? This is, I think, your confusion;
see my other post. Two things: (1) - Maxwell's equations are concerned
with the rate of change of a magnetic field; not whether they're
'static', or 'moving', which is meaningless, and (2) magnetic and
electric fields are exactly the same thing; it just depends on you
frame of reference. One observer sees only an electric field; another
observer moving relative to the first observer sees a magnetic field.
> When the electrons move inside a conductor (metal), this effect
>is seen as a mechanical force applied to the _conductor_.
What's the difference? Certainly, a conductor is different from free
space; this is what the skin effect is about. But, in this case,
nothing happens when moving from free space into a conductor: there
are moving charged particles either in the conductor or in free space;
a force is applied to them. They respond to the force, and not by
leaking out of the conductor.
Austin Lesea
It has to do with current creating a magnetic field, and how the
magnetic fields interact.
Imagine I have a rectangular loop (tall and skinny), divided down the
middle by a sheet of glass.
On either side of the glass I have a scale (made of plastic) to see how
much the wire pulls away from the glass as the current increases in the
loop.
At some point, I add a third wire on one side of the glass in parallel.
It is some distance away from the glass, more so that the first set of
wires.
What I claim is that the force of the third added wire will be less than
that of the first wire, and the force of the first on the same side of
the glass wire will be somewhat less, but will not be 1/2. In fact with
the BART rail spacing, it would be 2/3 and 1/3.
At DC.
Guess what? Current creates a field, a field tells current how to flow.
I think Faraday discovered this?
This works by the way for superconducting wires, resistance has no part
in this. R does not appear in the equations to show this is true.
QED for this "Gendanken" Experiment...
Austin
Falk Brunner
> What I claim is that the force of the third added wire will be less than
> that of the first wire, and the force of the first on the same side of
> the glass wire will be somewhat less, but will not be 1/2. In fact with
> the BART rail spacing, it would be 2/3 and 1/3.
>
> At DC.
>
> Guess what? Current creates a field, a field tells current how to flow.
>
> I think Faraday discovered this?
;-) This is really funny. A very basic effect of physics is "forgotten"
by the highly trained specialists.
Maybe I can jump in and help to enlight the non-belivers. The effect in
question is called Lorentz-Force (Lorentz-Kraft in german). Its the
effect that makes every electrical engine spin. Just have a look at
those small toy motors, they use a permanent magnet to greate a static
magnetic field and a DC current inside the rotator loop. OK, the current
gets reversed by the commutator every fraction of a revolution, but this
is not the point. Another example is the good old CRT TV set. A (quasi)
magnetic field is used to deflect a electron beam (moving charge carriers).
> This works by the way for superconducting wires, resistance has no part
> in this. R does not appear in the equations to show this is true.
>
> QED for this "Gendanken" Experiment...
"Gedanken"Experiment. Just a small typo. (yeahh, germans are known to be
real pedantic ;-)
Regards
Falk
P.S. To be onest I never thought of the magnetics stuff before when
looking at the GND/VCC balls on a package. Interesting!
austin
Thank you.
I only had three years of high school German, so forgive my spelling.
Just think how surprised those Westinghouse Engineers were when they had
10,000 trains....and 6 empty train blocks light up on the 10 meter by 3
meter map display!
Austin
Falk Brunner
> Just think how surprised those Westinghouse Engineers were when they had
> 10,000 trains....and 6 empty train blocks light up on the 10 meter by 3
> meter map display!
Hmm, but why didn't they have a small prototype for testing? Or did they
think this is sooo trivial no need for testing?
Regards
Falk
austin
I have no idea. But they (Westinghouse) had never done a modern urban
transit system before, and had been given the contract.
Politics, pork, etc.
There were many at the time who said that this was yet another example
of incompetence, and that the contract should have been given to the
experts in electric traction urban transit...who were Germans (at that
time).
All in all, the system was well engineered, and was very modern (when
introduced). IBM did the payment system, which was so easy to clone
that students bragged about how they could make copies of their $20
cards with nothing but a flat iron. (Do not know if this was true or not).
Presumably that got fixed ...
Austin
Hal Murray
>When I first mentioned this to our packaging group, the lead engineer
>said "oh yes, I see this in the EM simulations..."
I think the key idea is that the return current is flowing close
to the forward current. Or rather closer to parts of the conductor
than it is to the rest.
What's the current distribution in the center conductor of a coax
carrying DC?
Where are the balls carrying the "return" current relative to
the central clump of ground balls??
>So, I know I am not imagining it!
Simulations never get the wrong answer?
[I used to be reasonably good at this stuff, but that was a long
long time ago. This feels like a good question for PHD orals.]
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Hal Murray
>What's the current distribution in the center conductor of a coax
>carrying DC?
Argh/blush. Stupid example. How about:
What's the currrent distribution in a pair of tightly coupled
striplines?
David Brown
> austin wrote:
>> dp,
>>
>> I know you do not belive me. And you haven't ever solved Maxwells
>> equations for this case (or else you would see it).
>>
>> I am not going to convince you, so I will not try, but it is a real
>> effect, and it really happens.
>>
>> I also admit that it is greatly misunderstood (after all, Westinghouse
>> believed as you do, util they made a million dollar mistake by
>> building it, and experiencing it first hand).
>
> Take a magnet near the front of a Shadow mask CRT, and you can
> clearly see the effect a magnet has on moving (dc) electrons.
> DC current requires electrons to move, even if the ammeter does not.
>
> -jg
>
That's a different situation, in that the electrons are moving quickly.
The average speed of the electrons moving through a conductor is,
AFAIK, very slow.
Jim Granville
> Jim,
>
> It has to do with current creating a magnetic field, and how the
> magnetic fields interact.
>
> Imagine I have a rectangular loop (tall and skinny), divided down the
> middle by a sheet of glass.
>
> On either side of the glass I have a scale (made of plastic) to see how
> much the wire pulls away from the glass as the current increases in the
> loop.
>
> At some point, I add a third wire on one side of the glass in parallel.
> It is some distance away from the glass, more so that the first set of
> wires.
>
> What I claim is that the force of the third added wire will be less than
> that of the first wire, and the force of the first on the same side of
> the glass wire will be somewhat less, but will not be 1/2. In fact with
> the BART rail spacing, it would be 2/3 and 1/3.
Yes, but in your first example, you claimed DC Current diffences, not
forces (kg) on the wires ?!
>
> At DC.
>
> Guess what? Current creates a field, a field tells current how to flow.
>
> I think Faraday discovered this?
I await your real examples, with hard data.
So far, I have placed this in the urban myth box.
Some reality checks, from my old, trusty University Physics book:
Force on wire = Current * Length * B(Field)
B near a long wire = MUo * Current / 2*Pi*R
so yes, B falls off inversely with distance.
MUo is small, at 4*pi*10e-7 weber/amp-meter
( that's why you need many turns, and small air gaps, in a motor )
Motors start with a force, and then the 'moving wire in magnetic field'
law (Lenz's law to some) creates a back-emf, that reduces the current,
by reducing the apparent voltage.
Now to the Hall effect, (some have quoted as the cause) :
Vxy = Current x B(Field) / n * e * thickness
Their worked example applied a massive 1.5 weber/m2 to a 20mm x 1mm
copper strip and the resulting Hall voltage, across the copper strip
was 22uV
So, yes, it is an effect, but no, I cannot see it causing a large shift
in DC current balance due to the field set up by a single wire.
Seems time and the urban myth effect have confused the B field
variation ( which DOES fall off with 1/R ), with the DC current, and we
are still unclear on the details of what exactly failed westinghouse.
So, as to DC current in the inner BGA Balls being a fraction of their
outer neighbours, show me some proof. [and remember, this is DC, not AC ]
Perhaps a high quality thermal image, good enough to show the ball
temerature profiles, due to DC current ?
-jg
Hal Murray
>( that's why you need many turns, and small air gaps, in a motor )
You can get motion from single turns as long as you use a enough current.
The Exploratorium has (had?) an exhibit with several 1 inch dia wires
running vertically reasonably close to eachother. They were attached
at top and bottom but not constrained in between. 6 or 8 feet high.
You step on a switch and it dumps a lot of current into the wires.
They move.
(I forget the details. It's been a few years since I saw it.)
fpga...@yahoo.com
Jim Granville wrote:
> Please do, we can agree there is an effect, my antennae just question
> how much of an effect at DC ?.
>
> You still have to satisfy ohms law, so any push effects that favour
> flow, have to model somehow as mV(uV) generators....
> To skew Ball DC currents 7/8 or 15/16, frankly sounds implausible, and
> maybe the models there forgot to include resistance balancing effects ?
> [ ie do not believe everything you are 'told' ]
The problem is that there may not be ANY DC component. Consider slowly
decreasing the clock period so that all logic paths settle well before
the next
clock edge. In this case the current goes from zero, to one or more
peaks, and
back to zero for each clock cycle.
Given that the clock in this stable case will be in the megahertz
range, it's
quite justified to say that the DC effects may just completely vanish,
or be
insignificant at best.
With some very careful design, using multiple clock domains and phased
clocks, to time spread with overlap the distribution of dynamic
currents to
create some DC component based on minimal filtering effects of the on
die
capacitances.
Async designs, have a might better chance of creating some DC component
out of the dynamic currents.
There might be a DC path in the I/O's from pull ups, pull downs, and
slower
clock rates.
Peter Alfke
Well, well. It does not take a genius to find out that all current (or
power) consumption ends up as a (pulsating) DC current through the
chip, from Vcc to ground.
Just imagine the transistors as simple switches, and the loads as
capacitors. When driving High, charge (current) flows in from Vcc. When
driving low, that same charge gets dumped into the ground leads.
I call that dc current. there isn't even any reversal of the current
direction.
Isn't that pretty basic?
Peter Alfke
fpga...@yahoo.com
Peter Alfke wrote:
> I call that dc current. there isn't even any reversal of the current
> direction.
> Isn't that pretty basic?
Well sorta .... it's all about definitions. Reversal is one component
of
the definition of things that are not pure DC current.
http://en.wikipedia.org/wiki/Direct_current
http://www.school-for-champions.com/science/dc.htm
Continuous is the other. The models for steady continuous DC are
different than modeling transient (plused DC) and AC circuits, are they
not? When does a high voltage RF AC waveform with a high DC offset,
become DC? The rapid changes in fields, and interactions of fields,
produces the effects of unbalancing the currents in the ball array do
they not? Very low current, low voltage, steady continuous DC should,
as suggested, not have much of an imbalance at all since the field
strengths will be low.
And, one of the problems about pulsed DC, is that it frequently turns
into AC due to ringing,
aka undershoot.
The fact here is, part of the discussion here is how much the current
distribution is influenced by traditional continuous DC effects, and
how much the distribution is influenced by the transient effects
(pulses), is it not?
Jim Granville
> Jim Granville wrote:
>
>>Please do, we can agree there is an effect, my antennae just question
>>how much of an effect at DC ?.
>>
>> You still have to satisfy ohms law, so any push effects that favour
>>flow, have to model somehow as mV(uV) generators....
>> To skew Ball DC currents 7/8 or 15/16, frankly sounds implausible, and
>>maybe the models there forgot to include resistance balancing effects ?
>> [ ie do not believe everything you are 'told' ]
>
>
> The problem is that there may not be ANY DC component.
Two issues there:
i) This discussion ( with Austin) was explicitly about DC current spread
and what splitting effects there may, or may not be, and their values.
ii) You have seen the latest FPGA data sheets :) ?
I see the 90nm device from lattice, can draw 1.5 AMPS, at 105'C Tj
That's just static Icc.
Thus, the latest FPGAs are a long way from your classic CMOS...
DC current is there, and at not insignificant levels...
-jg
Jim Granville
Probably dangerously basic, if you are talking with a novice :)
An expert would warn the novice that this basic DC current, actually has
many elements :
- The average value determines the average voltage, via the IR drop.
- The RMS value, determines the heating in the PCB traces
- The AC component, superimposed on the DC, is what causes the
inductive ringing effects, via V = -LdI/dT, and the skin effects,
that further increase the resistance...
The AC component also determines the decoupling cap sizes, to
prevent local short-term supply sag.
-jg