Question

Discussion in 'Electronic Design' started by Ko, Jul 29, 2013.

1. KoGuest

I am making my first through plated PCB.
How big do holes in VIA have to be for electrons to feed through without much friction?
This is for high speed logic you know.

2. Bill SlomanGuest

The OP asked a very silly question, but this is an even sillier answer.

In practice via size is limited by the drill sizes available to create the via hole, and any practical via size gives you enough surface area that thecopper layer laid down has very little electrical resistance.

With very high speed logic, you probably should worry about the extra inductance inserted by the via, and make sure that the ground planes defining the other sides of your transmission lines are properly linked at the via, sotha the return current flowing in the ground-planes doesn't have make a long diversion.

3. Syd RumpoGuest

You should not have holes in your vias. Holes represent a shortage of
electrons and as such will capture passing electrons.

Cheers

Can we use them to fix what got broken?

5. Guest

Don't forget the skin effect, the high frequency current moves only
very close to the surface, causing additional losses.

But does the current flow on the outside of a plate through hole
(between metal and PCB) ? How about the inner side of the hole ?

Of course, with sufficiently high frequencies, the RF will pass
through the hole in some circular waveguide mode

6. Robert BaerGuest

You want friction so that they can reproduce..

7. Phil AllisonGuest

"Ko"
** Best thing is to make all the holes black.

Then you never have to worry about seeing electrons again.

..... Phil

8. mpmGuest

I was going to offer that the lower bound for the size of the via would be the size of the electron itself (otherwise, it wouldn't fit through!).

And then, the upper bound could be set by how many electrons can safely pass through simultaneously. For if they all fell through at once, there wouldn't be any left over for your circuit to work.

Just try to snatch the pebble from my hand, grasshopper...

10. Spehro PefhanyGuest

These days microvias are approaching minimum sizes seen in design
rules from early IC manufacturing. 15um are apparently possible.
50-100um is pretty common. Laser, plasma, chemical methods.

Aside from just saving space occupied by the annular ring etc.,
via-in-pad can allow you to fan out a big BGA with a smaller layer
count.

Reducing the via diameter through a 1.5mm board from 0.5mm to 0.1mm
increases the inductance from ~0.6nH to ~1nH-- pretty much negligible
in comparison to that from typical trace lengths.

Best regards,
Spehro Pefhany

11. Bill SlomanGuest

It's while since I read up on laser drilled holes, and back then they were always conical, and couldn't be more that fifteen times deeper than they were wide. That's perfectly compatible with a 50um diameter hole in individual layers of a multilayer board.

Getting that sort of aspect ratio out of plasma etching or chemical etchingsounds more difficult, and expensive,
Well worth paying for.
Sure, but when you have to treat traces as transmission lines, you also have to worry about the extra inductance in the in the return path. Sadly, youcan't often have your via flipping the trace from one side to the other ofthe same ground plane, so you mostly have to figure in the impedance of the via's linking the two ground planes, often including the capacitors that allow the two ground planes to be at different DC voltages.

My experience with very fast pulse edges was that they didn't like vias andthis was with a pulse which we could narrow down to 500psec wide. Modern "very high speed logic" can go a bit faster.

12. Bill SlomanGuest

Vias aren't lossy - losses mostly happen in the pcb material - but they do represent impedance discontinuities. You also have to worry about dispersion in microstrip traces (on the surface of the board). Buried strip-lines are non-dispersive.
Not everything that Howard Johnson fusses about is nonsense. Winfiled Hill did recommend his book when it first came out - not because it was good, but because it was pretty much the only book available that got into the details of the problems of high speed logic layout.
The "huge" capacitance is between substantial areas of ground plane. If your return path has to spread itself over a couple of square inches of groundplane to find enough capacitance through which to move between the ground planes you are going to see a significant impedance discontinuity, giving your brutal active equalisation a bit more work to do.
Probably not a useful observation.

13. John DevereuxGuest

[...]

I made a directional fluid / air flow meter, using an interleaved
pattern configured as a bridge. Worked really well, although the
resistances were a bit low of course

I also tried a capacitative sensor for fluid water content, made of PCB
tracks. That sort of worked although it turned out the base fluid was
conductive which messed up that particular design.

There are umpteen other capacitative things, touch sensors, keys,
sliders. I think the digital micrometers work like that too.

All the microwave stuff which looks like a speck of transistor
surrounded by weirdly shaped PCB foil patterns, half of them not even
connected to anything.

I have seen what can only be spark gaps in commercial offline switcher
and telecom designs. Pointy ended tracks cleared of solder resist.

Planar transformers.

I have used them for various essentially mechanical parts, due to
familiarity and the low per-square-inch pricing model.

Someone here suggested making front panels out of them. (How is it I can
get a beautiful fully finished, double sided PTH board for \$20 or
\$30. Yet a printed plastic overlay or panel can be 10-20 times that,
made by what could surely be fundamentally analogous processes. Where
are the low cost chinese prototyping shops for these?)

14. Guest

Radio amateurs make RF-tight boxes from double sided PREss.

The top and bottom panels are as long as the long side walls, but
about 2 cm wider than the short side walls. The short side walls are
soldered about 1 cm from the bottom panel long side and as well as
centered along the long axes, leaving about 1 cm on both ends. The
short side walls are soldered to the base plate at both sides.

The long side walls are installed against the short walls (extending
the whole length of the base plate but slightly less from the sides.
The long side panels are soldered to the bottom plate on both side as
well as soldering the short side panels to the long side panels on
both sides (about 1 cm from the bottom panel end).

Finally after installing the electronics within the box and testing
it, the top panel is soldered to the long and short side walls. This
is the only seam that is soldered only on a single side. Of course, to
improve shielding, fingerstock can be used on the inside to improve
conductivity between the inside of the top panel and the inside of the
side panels.

To complete the job, put some copper tape over the exposed PCB edges
and solder the tape to the top and bottom side of the PCB.

15. Guest

Also various forms of microwave patch antennas can be made with PCBs.

On lower frequencies, double side teflon boards are quite reasonable
high voltage capacitors for transmitting magnetic loops.