Motherboard Mounting

[Psycho_Logical]

I'm having a bit of an e-mail "war of words", and I need some "ammo" to fight back with.

BACKGROUND

HIM - Old time electrician who has a store-bought system, but has "poked around" inside the case, adding RAM modules, etc.

ME - Hobbyist who has built three systems from scratch, but with no formal educational background in electronics.

HE SAYS:

Metal standoffs and screws are absolutely required because they ground the mobo to the case.

I SAY:

This is not true. You can get a system to boot and run fine with the mobo entirely out of the case, sitting on a piece of cardboard.

Motherboards run fine in a case if you use plastic standoffs or fibre washers under the screw heads, both of which I have done.

HE SAYS:

The mounting holes have those silver circles around them for electrical continuity.

I SAY:

The silver circles are there to prevent the underside of metal screw heads from chewing into the PCB layers.

HE SAYS:

The mobo needs to be grounded to the case to eliminate interference.

I SAY:

Interference is caused by a cheap power supply or faulty shielding, but can be contained by using a good case.

If you ground the motherboard to the case, and if there is the right kind (well, really the wrong kind) of major fault, the case would become part of the circuit and a shock hazard.

WHAT I NEED

Some definitive way to prove that the mobo is not grounded to the case by using metal standoffs, that they are just one way of mounting, not necessarily the only way.

Ideally, this would involve using a voltmeter, but I'm not sure how or where to hook one up. Would I check for voltage with the power on, or for continuity with the power off?

I'd hate to touch the wrong meter leads to the wrong thing at the wrong time and short something out just to try to prove a point.

But I would like to win the war. :D

Any thoughts or opinions?

[Dudster]

I'm not 100% but I'm pretty sure that he has it backwards :) I think the metal stand-offs are to prevent the motherboard grounding on the case and the mounting holes aid this. Some people also use little rubber rings so the screws in the mounting holes don't touch the motherboard. I think the machine wouldn't work if the motherboard was grounded on the case.
If you ground the motherboard to the case, and if there is the right kind (well, really the wrong kind) of major fault, the case would become part of the circuit and a shock hazard. I don't think it would be a shock hazard, just a hazard to the motherboard if anything.

Edit: I can't think of a totally safe way to prove it. Personally, I'd just leave it, safely in the knowledge that I was right :)

[Psycho_Logical]

Maybe I wasn't clear enough, or you misunderstood.

I understand there has to be some kind of standoff to keep the bottom of the motherboard, where the solder joints are, from touching the case. But if it/they did, that would be a short, or an unintentional grounding.

I mean that he says the metal standoffs ground the mobo on purpose.

[jadison]

He's wrong...your right!
You don't need to have the motherboard in the actual case, for it to post.
This is not true. You can get a system to boot and run fine with the mobo entirely out of the case, sitting on a piece of cardboard.
Check this out: http://totl.net/Spud/gallery.html


-=jd=-

[Dudster]

No you were perfectly clear enough :) Just me not really thinking before I posted. Sorry about that :o

[Mntsnow]

You are correct...the motherboard actually receives it's "grounding" thru the powersupply. In this fashion it also helps eliminate "grounding loops or fields".

[Welsh Wizard]

out of the ark needs grounding, new stuff does not. the use of metel normally Brass stand off is better than plastic for stability only in a new system,

Makes adding cards and ram easier, also plastic often fails to hold the mobo well when used with MD CPU based system, ( plastic don't like warm conditions.

Brass cost more than plastic, if you want to cheapskate use plastic, if you want quality use brass.

other than that his arguments a Crapolla in this day and age, tell him to go back to wiring houses.

WW

[Axel]

Having built several systems over the years myself, I have never seen a motherboard correctly installed with "just" nylon / plastic stand-offs.

The stand-offs primary function is to hold the motherboard high enough off the chassis frame so it's protruding conductors do not short on the metal case and to hold the board in place. A secondary function is to further ground the motherboard to the case metal frame, which, in turn also grounds to the metal housing of the power supply which then grounds to the ground plug in the wall.

This is a back-up ground. The primary ground is the ground wires coming off the power supply and in 99.9999% of the cases - that more than enough.

The PCB does, in part, ground through the chassis. It will run on a sheet of cardboard, but is much more vulnerable to shorts and static discharge there.

The eletrician is correct in that the case and stand-offs are part of the "grounding system" - You are correct that the board can run without the case - but would be foolish to do so as a working system.

[NDC]

As a matter of fact, I had a very strange incident some time back on my Dual Tyan S1832-DL mainboard where I bumped into the case with my knee fairly hard and the system wouldn't power on! I tried just about everything possible. I tried pulling out all PCI devices, CPU, memory and out of frustration even the IDE devices! LOL.. After about an hour of wresting, I pulled the mainboard out of the case and it just fired right up! I said cool! Now, I remounted the mainboard to the case and then again, no fireup! I ended up putting non-metallic washers between the mounting screws and the mainboard and all was good.

[jnas]

I had a similar experience to NDC with my last build. The case I bought did not come with any plastic standoffs so I used the brass screw in ones that were provided. I couldn't get the computer to come up at all. I tried assembling everything outside the case and guess what. It worked fine. Reinstalled motherboard with plastic standoff and insulating washers on the screw in post and have not had a problem since.

John

[FalcomPSX]

to prove it, just run a mobo out side a case, if the grounding was required it shouldn't run properly right? :devil:

[MechEng]

The power supply ground is known as a live ground.

The motherboard grounding is called the earth ground and the pads are used for grounding. In fact in many multilayer printed circuit board, one of the layers is entirely a grounding layer.

Indeed earth grounding is a backup and also a requirement to get UL approval when designing a PC. The thought process is if you loose your live ground, the earth ground will take over. Much like the fact that your house wiring consists of a metal stake driven into the ground. i.e. 3 prong vs. 2 prong. The 3rd prong is earth ground. By running your PC outside the chassis, you might as well cut the 3rd prong off of your cord.

With or without, your PC will run. One rule of thumb is that the better grounding you have, the less EMI (electromagnetic interference) you will have.

So who cares...well if you have a monitor it will be effected. EMI interferes with it resulting in a degradation in picture quality. Hold a hair dryer next to your TV and you will see. Wireless phone's or any kind of antenna in the room will also be effected. I was given an example about a problem with computer screens in a retail environment. There was a light line toward the bottom. It was returned to the manufacturer time and time again with this problem. They couldn't simulate it when it got there. You guessed it...EMI from the PC running it. On a side note, EMI is reduced by minimizing the openings in your chassis. My company uses the bus speed to calculate the allowable venting hole size and also does FCC testing to meet the requirements to get FCC approval. Everything no matter how well insulated leaks EMI, even you power cord. FCC sets a limit.

To sum it up EMI and grounding are important. That's why the FCC and UL are in business and why company's like mine spend a fortune and a ton of time getting our products tested. Look on the bottom of any product and you'll see the label. It eliminates problems and liability down the road.

[mookymooky]

heres my two copper pieces :) look at your power plug to the board :) black ground yeloow is 12 volts red 3.3 i think green is yyour on and off switch orange is your 5 volt wires :P so your right he is just a jaw flapper :P

[mookymooky]

yep a few beers and ya type bad ... sorry :P

[DustyBottoms]

HIM - Wrong

YOU - Correct

If you look at the standoff mounting holes you will find that none of them actually connect to any of the motherboard traces. Therefore it makes no electrical difference to the operation (or safety) to the computer. It is impossible for these isolated mounted points to serve any function beyond mounting points.

As others have mentioned, the PC case is connected to earth ground via the power supply to prevent a shock in case of internal short. An internal short circuit will find a path to ground via the case and blow a supply fuse/circuit breaker rather than just make the case "hot" and possibly shock the consumer. All electrical devices with metal cases fall under the same UL category.