Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <lihl905ul4iur03857kovvi1e02urhjclb@news.rcn.com>,
address@signature.blk wrote:
> On Sat, 01 May 2004 17:05:41 -0500, in <alt.comp.periphs.mainboard.asus>,
> nospam@needed.com (Paul) wrote:
> >
> [snip]
> >
> > The standoffs are _designed_ to touch the board and make contact.
> [snip]
>
> Only the conductive (typically brass, as opposed to plastic) ones; and there
> should only be one of these per mobo/case combination.
>
> > Each plated mounting hole on the motherboard is at ground potential,
> > as is the brass standoff underneath it.
> [snip]
>
> Correct. But this is because neither the mobo manufacturer or the case
> manufacturer know exactly *which* of these multiple potential grounding points
> will best line up with whatever case/mobo the user might mate with their
> product. It does *NOT* mean that you should use them all.
>
> > To further increase the ground
> > contact between the motherboard and the case, the "spring fingers"
> > around the I/O connectors are there, to try to make ground contact
> > between the I/O connector body and the case.
> >
> [snip]
>
> I'd have to see the particular I/O panel in question; but at least usually,
> this sort of thing is done more for shielding than for grounding.
>
> > If you use a fiber washer, not only will this "half insulate" things
> > (because the metal screw is still a conductor),
> [snip]
>
> This presumes a metal screw. It's also why nylon/plastic screws are used for
> this purpose.
>
> > ...it also screws up the
> > vertical axis mechanical alignment.
> [snip]
>
> You're kidding, right? The tolerances to which (at least most) cases are
> built are not anywhere near that close.
>
> > In cases where people used insulating standoffs, in order to make
> > a motherboard work, this indicates that there is something electrically
> > wrong with the install.
> [snip]
>
> Yeah, like having ground loops due to multiple grounding points.
>
> > Just because it fixes the problem, doesn't
> > mean the end result is a healthy install.
> [snip]
>
> If it gets rid of the ground loops, it does (at least in that respect).
>
> > What you don't want to happen, is for a brass standoff to be installed
> > where there isn't a matching plated hole in the motherboard.
> [snip]
>
> This is true.
>
> > If a
> > brass standoff lines up with and touches a copper track on the bottom
> > of the motherboard, that can short the signal.
> [snip]
>
> And kill the mobo as soon as power is applied (had this happen on an old T2P4
> when I "got creative" with my effort to install it in a case it really didn't
> fit well).
It took me a while to find a doc that even remotely addressed
the motherboard grounding issue. This one hints at it:
http://www.casereview.org/wtx/WTX-Spec-11.pdf (pg.33)
*****
5.2.1 Motherboard Grounding
The motherboard needs to tie into the chassis with the lowest
electrical impedance possible. Motherboard mounting features must
provide good ground paths into the adapter plate; this is the
responsibility of the board designer. In addition, the adapter
plate should be well grounded to the main chassis. Otherwise, the
motherboard can begin to "float" at frequencies in the RF range
due to the high impedance of the ground return to chassis, which
could enable the entire board/plate structure to act as an antenna
or cause excessive noise to be conducted to external cables. Good
plate-to-chassis contact is required at the five hook/slot locations
and at the screw locations defined for both the adapter plate and
the chassis. All metal components must be galvanically compatible
through the grounding path.
*****
My interpretation of this, is "EMI trumps single point grounding".
Better to have a little extra noise in the sound output, than having
your computer function as a radio transmitter.
*****
5.2.2 I/O Shield
The I/O shield for the motherboard connectors is one of the most
critical aspects of limiting EMI. Poorly grounded connectors will
allow noise to escape on their associated cables. Also, weak
connector grounding can make a motherboard much more susceptible
to functional errors or damage from electrostatic discharge (ESD).
The I/O shield should make secure contact to the metal connector
faces at multiple locations, with spacing between contacts not to
exceed 0.250 inches [6.35mm] at both the bottom and top of each
connector D-shell. Spring fingers formed from the bracket material
are the preferred method. Conductive I/O gaskets can work, but tend
to be more expensive and typically do not provide as low an
impedance as metal-to-metal contact. Make sure the finger length
and bend angle assure enough deflection when installed to guarantee
a solid contact. Some connectors, such as those for audio, LAN, or
USB require a more creative design of contact finger due to their
shape. These ports require a good ground just as the D-shell
connectors do. The I/O shield must provide a contact finger for
every port. The I/O shield must make contact to the chassis opening
around the entire perimeter of the part, with spacing between
contacts not to exceed 0.25 inches [6.35mm]. The chassis contact
area must be free of any paints, labels, or mechanical irregularities
that would prevent good metal-to-metal contact.
*****
Howard Johnston's web site has an article on single point grounding,
"moats and float" and the like. But, the PC already has a ground
path through the ground wire of the PS, and every connector on the
back of the computer is a secondary (and unacceptable to that theory)
path. As a consequence, I think the "ground flood" theory is being
invoked, where an attempt is being made to bring everything to a
common ground potential, with the majority of DC ground currents
being carried inside the motherboard itself. As this is a compromise
between analog noise performance (ground loops) and meeting FCC, and
the former has no legal ramifications and the latter does, it is \
easy to see which issue wins.
I'm not going to comment on whether it is better or worse, unless
there is an article somewhere showing how much difference in noise
floor there is between fully insulated mobo standoffs and using
brass ones. My motherboards have and will continue to use brass
standoffs. (BTW - the worst emissions now are coming from my
power supply, which has a conducted emission problem on the power
cable, at the switching frequency. Even the power brick for my
LCD monitor is kicking out emissions, even when the monitor is
switched off. By plugging both into my UPS, that seems to have
stopped it from leaking into the house wiring, stopping interference
with my TV. Problem solved. At least the UPS maker knows the
importance of filtering.)
As for the mechanical alignment issue, I mention it for people
having trouble assembling a computer, so they will at least
be aware that the level that the motherboard is lifted above
the case, can affect how easy it is to install AGP and PCI
cards. I didn't read about the issue, and discovered it by
trial and error during assembly.
HTH,
Paul
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