BGA central ground matrix

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...

They're the same thing

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.

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.

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.

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...

Aust> Aust>

Austin Lesea schrieb:

;-) 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).

"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!

Falk,

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!

Aust> Austin Lesea schrieb:

austin schrieb:

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

Falk,

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 ...

Aust> austin schrieb:

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??

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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Argh/blush. Stupid example. How about: What's the currrent distribution in a pair of tightly coupled striplines?

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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.

Yes, but in your first example, you claimed DC Current diffences, not forces (kg) on the wires ?!

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

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.)

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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.

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

Well sorta .... it's all about definitions. Reversal is one component of the definition of things that are not pure DC current.

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?

Isn't that pretty basic?

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

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