Originally posted by: Matthias99
Big Lar makes a good point, even if I had plastic standoffs, the I/O shield touches the metal parts of the mobo where the cables get plugged in. So there is still metal contact between the mobo and the case, right? Or is the I/O shield there for that reason, to shield the mobo from making contact?
I think the I/O shield is there mostly so that you don't have a big gaping hole in the back of your system.
It's for EMC compliance - i.e. prevention of electro-magnetic interference. It should be a tight fit into the case, and ideally the case should have spingy clips, or metal coated rubber seals, which ensure a reliable electrical contact. The better the electrical contact, the more likely it is to contain any interference within the chassis.
As it provides electrical shielding, it will also help with static protection - a solid metal shield will prevent the static charge from entering. It simply allows the charge to flow around the sensitive circuits - rather than through them.
That's also how modern anti-static bags work. The older ones used very slightly conductive plastic (these are black, or blue) - these would prevent static building up on the chip from the chip moving around inside the bag, by constantly allowing it to discharge. Modern PC chips are too sensitive for these bags, and can still be damaged from an external static discharge. Modern bags, use the conductive plastic, but surround it with an aluminium coating, which is then insulated from the outside. The metal coating acts as a shield and prevents external static charges being conducted in.
The spacers are metal for a reason - it's too ensure that the mobo gets an excellent ground connection.
No, the spacers are metal because it's cheaper to make tiny little threaded metal standoffs than tiny little threaded plastic ones. The motherboard does *not* get its ground connection from the case/spacers. It's wired to ground in the PSU through the ATX power header. Thus why you can run it in a nonconductive case, or sitting outside a case on a piece of cardboard, etc... Now, a particular motherboard might tie those standoffs to ground, but it's certainly not required. In fact, I suspect it would cause problems if the case wasn't actually grounded well.
Yes, the motherboard is grounded through the power connector - but there are problems associated with a single ground point, and ESD (electro-static discharge) is one of them.
ESD is, very characteristically, extremely brief (nanoseconds), high-voltage, and often relatively high current. It can therefore cause significant voltages to be generated across even very small resistances or very small inductances. If an ES discharge enters a motherboard via a port, and the only ground point is the ATX connector, then there could be substantial transient voltages generated within the mobo's ground planes - and could very easily cause malfunction or damage.
A well recognised technique is to ensure that there are multiple connection points throughout the ground plane - where the internal ground plane can be bonded to chassis ground: the best way to protect against large jumps in potential, is to ensure that all points jump together.
Of course, this assumes that the case is well grounded. The PSU should ground the case, via its attachment - and all parts of the case should be electrically connected to each other. Unfortuately, aluminium is a potentially a problem in PCs - because of the surface coating. Most Al is 'anodised' - a process which greatly thickens the insulating oxide layer on the surface. This is a disaster in terms of EMI and ESD protection - unfortunately, most Aluminim PC cases I've seen are made from anodised aluminium, instead of the more appropriate chromated aluminium.
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