Mobo design question - processor location

[Axel]

Looking at dozens of motherboards and knowing a little about thermo-dynamics, I thought I'd ask this question and see what broke loose -

Why do mobo manufacturers place the processor over the AGP and PCI slots so the heat from those boards as well as the I/O chipset go up and "bathe" the processor up a few degrees?

It would seem to me that, on a full ATX form factor, the best place for the processor chip or chips would be to the right of the PCI slots. That way, if the fans are placed properly, the heat from the expansion slots would rise up and away and the processor chip would be in the coolest position in the case - near the bottom front of the case away from the power supply and drives which also generate heat and out of the rise path of the heat coming off the expansion boards......

You usually don't have anything near that position except an internal speaker in most mid-tower cases.....

So - do we have anyone who has done any mobo PCB design work? What am I missing?

I know - all PCB designers live in the Colorodo river valley and cannot get flood insurance, so they put all the important stuff up higher...... :D

[Mntsnow]

You know...I've always wondered that as well.

[surrealchereal]

So the processor fan coats it with dust?

[SalaTar]

Good question
May have to do with the limits on how much resistance the traces can have to the slots and bus??????

[Welsh Wizard]

If you have used the extra System fan then this should be close to the AGP card and extract air out from the case, cool air in theory should come from the front of the case and no heat should therefore go up to the CPU, trouble is a lot of case manufacturers don't supply the system fan and a lot of OEM's don't fit them to save on cost,

It does work when set up correctly and no heat from the AGP card get to travel up to the CPU, if you don't believe get a good case fan and a digital heat probe and bluetac the probe between the AGP and the CPU close the case and then monitor the heat when its fired up. ;)

WW

[jadison]

the way the circuits are mapped out on the motherboard put the CPU above the PCI slots. It's just a matter of the route the information takes on a mobo. that allows for CPU's location. Over many years manufacturers have found that putting the CPU in its current location works best. Heat wasn't a big consideration due to the fact that there were and are many desktops on the market. In desktops the heat rises upwards, and doesn't rise through any components.

-=jd=-

[Axel]

WW - I'll be the first to admit that good case fans do wonders for a system - no arguments there - no need for a probe -

J - I don't buy it - desktop PC's often come with feet or stands to set them on edge for space considerations. If 90% of cases lay flat and did not stand the mobo on edge - you've have a legitimate argument - but not here - I'd guess-timate that upwards of 45% of modern PC's are mini to full tower models that often, as Welch Wizard points out, do not come with the proper case fans and do cause heat to rise over the processor from the other components. Some new case models that can be set on edge even come with an annoying plastic ridge on one side so the user cannot set the case on that side in effect blocking the cooling slots.... I've set a few hundred of these up - you notice such things.....

So - still looking for a mobo pcb designer that might be able to give us a definitive answer as to why the CPU is placed above and not to the right of the PCI slots on a motherboard.

[jadison]

well, I do agree that ~45% don't come w/proper cooling, DELL as well as other major manufacturers include 2 fans (1-PS, 1-case), but sometimes that's not enough.

As for the reason for the placement of the CPU in its current location, I can't provide a valid answer. My weak attempt in the previous post was obviously not helpful, but you've sparked an intriguing question.

I'll read up on it.

-=jd=-

[hawkeye177]

I think that if you put it to the right more it would not matter. It will still be exposed to the heat because it raises and it goes in many directions. Also you would not want a cpu blockin the way of you Ide cables.