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.