Grounding a Rack-Mounted Motherboard?

MadCow-ard asks: "Here is one for the Electrical Engineers among us: I have a rack-mount case that I had installed an Intel D845GBV and P4. I had to switch them out with an Asus A7V333 because of incompatibility with my other hardware (which is a long story for another day), but they were functioning fine otherwise. In doing so we found that the new motherboard would not work in the system. We checked everything: multiple motherboards, video, RAM, power supply, cables, you name it. We were getting AOK POST sometimes, others not. It would randomly boot and other times it had no video, or a partial boot. After going nuts (in the field with a couple of clients) we switched out the risers the motherboard sits on. Voila. The risers appear to have grounded the system board. Not the Intel, just the Asus. I spoke with another tech associate who claims to have seen the same issue recently. Now grounding I understand, but it seems that it wasn't the risers specifically. It was their height. We tested two, the bad ones were 2mm smaller. It could be the alloy, but I thought that motherboards would shield the screw points from grounding. It appears to have been a field that built between the case and motherboard due to the smaller risers. It wasn't actual motherboard contact with the case, I checked. Does this make sense? Has anyone else seen this? Is it some sort of capacitance with the case that is generated specifically from the board design and layout?"

Comments

Did you try putting the old ones back?

[Eagle7]

I'd be curious if you actualyl fixed it with the higher risers, or if the real fix was some sort of connection problem that was rectified by the act of reinstalling the motherboard when swapping the risers.

Just find it hard to believe (and it sounds as if you're skeptical as well) that anything could have been grounded without touching. If this was the case, there would have been arching, and you would have seen marks. Even some sort of magnetic or eletrical field generated between the board and the case (Is this possible? I have no idea) doesn't seem like it should cause such a problem - I'd imagine that there are fields of all sorts overlapping everywhere in the average case.

Lower risers - cards not fully in their slots ?

[Sherloqq]

Have you made sure that with the lower risers all expansion cards (particularly the video card) were fully going into their slots? Sounds to me like your problem is right there, especially since you say that with the new risers the motherboard is 2mm higher than previously. 2mm is a *big* difference, and very well could have prevented your video card from working and/or made it work only some of the time.

Just my CDN$0.02

Shorting out?

[redelm]

Normally, the mounting holes are surrounded by solder rings and obviously supposed to be grounded. Additional capacitance is negligable because the board usually has a ground plane [layer] that is much closer. 2mm is a big difference -- are you sure the motherboard wasn't shorting against the tray with the short ones? Mobos frequently have some thru-soldered devices where the pins can protrude 2-3mm. Short one of these, and you're hosed.

The other thing to check is if the holes on the mobo matched the case perfectly. It isn't unusual to have the holes 0.5mm off, and then the standoff or screwhead could short a trace.

Right Solution, Wrong Problem

[GoRK]

What you have done here is fix your problem without realizing what it was. Don't worry, though, the Higher-Ups will probably buy your grounding/capacitance bullshit anyway if you have to justify the downtime to them. It sounds impressive.

Motherboard replacement involves handling, reseating, and resocketing every single component in the system, except sometimes drives. Anyway, here are the problems you might actually have been having:

1) Grounding something that wasn't supposed to be grounded: If you didnt get the new board on exactly square, or you accidentally left a metal riser where the old board needed one but the new board didn't, you might have been grounding something that shouldn't have been. How many times did you take the new board out and check before changing the risers?

2) Card seating. Your Asus board might have had PCI/AGP slots manufactured by a different company than your old motherboard. Perhaps the 2mm difference made a huge difference in the way cards seated in them (The AGP slot would be a likely contender to exhibit weird behavior on a 2mm height difference since it has two rows of pins.) This happened sometimes to me on EISA boards where I would not get a card seated all the way (You had to push HARD) This explination seems the most likely

3) Regular gremlins. Reseating everything: processor, cards, cables, etc. can sometimes do wonders for a system that's causing you to bang your head on the wall. If it just so happened that whatever device was giving you the problems was twiddled in just the right fashon when you switched the risers, then it would seem only natural that the risers fixed the problem, only they probably just forced you to unplug everything looking for it, and as a side effect you got some contact to scrape up against some peice of metal just so..

Hope that answers your question.

~GoRK

Grounded by other means just as bad

[kriston]

Having built over 100 systems by hand this is nearly always the case of the chassis coming in contact with pin lugs that are hanging out the back of the motherboard, or the motherboard mounts contacting circuitry.

I have built a number of systems which have stamped motherboard mounts which are quite wider than the usual screw-in mounts--so wide that they contact the motherboard well past the screw holes and into the circuitry. Not good! Worse yet, these stamped mounts can't be removed (obviously) and if the motherboard doesn't have a mounting hole where the mount is located you have to insulate it with something that cannot be punctured by the pins hanging out the back of the motherboard.

This has been a problem more than I like to say but it is always corrected by using non-conductive washers on the mount points and using rubber feet or cut-off plastic standoffs for spacing. The problem for me was prevalent on many MicroATX chassis, and especially thin steel cases of the sub-$30 variety which had a tendency to flex and bend in normal use.

Of course others have mentioned cards not seating properly (AGP cards are terrible for this) so to solve it you don't use the screw mounts adjacent to the AGP slot. This allows the motherboard to flex towards the AGP card and not "pull back" on it and unseat it. Newer AGP cards have retainer clips but I haven't run into those yet.

Kris