Wrong screws makes montherboard non-bootable, why?

[mxnerd]

As title.

Can anyone explain why? I have a new motherboard that won't boot because I used smaller screws. When I replaced with larger screws the motherboard finally boots.

Why is that? Isn't that the purpose of screw to fasten the motherboard to the case? Why wrong screws make the motherboard non-bootable when it boots fine outside of the case?

 

[kranky]

Possibly the smaller screws allowed a part the motherboard to touch the case and cause a short.

 

[jdjbuffalo]

Possibly the smaller screws allowed a part the motherboard to touch the case and cause a short.

This.

 

[mxnerd]

I checked, but I'm pretty sure the screws were not short circuited.

Funny thing is these wrong (smaller, round top) screws comes with the case. I have to use my existing, but larger screws with flat top to make it bootable.

Go figure.

 

[AndroidVageta]

Are you sure you dont have motherboard stand-off posts installed on the case that are touching the back of the motherboard where there arent screw holes?

 

[MrCreosote]

I have heard stories about this and have wondered what could be the cause.

First, if the mobo was to be insulated from the stand offs, using a screw would be crazy without an insulating flanged bushing and insulating washer - both required to keep mobo from touching screw and standoff. That NEVER happens so I question the insulation requirement theory.

What I have seen are radial conductive traces around bolt holes intended to cause a good grounding to the case at that bolt hole. A small head bolt could miss those radial races and perhaps not form a good ground. With high frequency circuits, ground planes and grounding are extremely critical.

But then, I indiscriminately replace brass standoffs with plastic ones which allow me to mount mobo as easily as possible. So much for all those grounding screws.

 

[mxnerd]

Are you sure you dont have motherboard stand-off posts installed on the case that are touching the back of the motherboard where there arent screw holes?

On a second thought, that could be the cause.

========================

But I also have the exact same thinking as MrCresote's.

If motherboard will boot without grounding outside of box, why it's necessary inside of box?

Anyway, will use larger screws or plastic standoffs in the future.

 

[MrCreosote]

If motherboard will boot without grounding outside of box, why it's necessary inside of box?

That is definitely true.

So could it be that the PC case is not at ground potential - maybe off a couple of volts due to bad caps in PSU so that some mobos cannot tolerate this?

I think I would scrutinize each of the bolt holes in the mobo. It should be readily discerned if grounding is intended. But if I saw a nice, heavy layer of coating on both sides, I would not rule out that screw should be insulated.

I think this may be a problem of older mobos also perhaps?

Tom

 

[Mwing]

i had similar experience in the days of althon thunderbird

my asus a7v wouldnt boot after everything set up in place, then i took out mobo and put electrical tape on each standoff, and screw the mobo thru the tape to provide insulation, that thing booted up no problem again