how much is too much

hi...i have this non-standard mounting procedure for thermaltake big
water SE water cooling...you have to tighten the nuts to get the
waterblock secured...how tight do i have to go? is there a rule of
thumb...i wand the waterblock to have a good contact, but dont want to
damage the cpu...tnx

 

I'll assume this is an Intel LGA775 processor. You can go to
developer.intel.com and in the processor section (technical documents),
you can get a datasheet for your processor. For example, the
600 series processors (the ones with 2MB cache - doc 306382), the
datasheet says the "normal force" may range from 18 lbf to 70 lbf.
There is a higher number for dynamic force, which I'm guessing has
something to do with a shock specification (if you bump or drop
the computer). "Normal force" is the force pressing down on the
heat spreader, and the force is assumed to be uniform across the
surface of the spreader.

The amount of force you apply, actually depends on the thermal
interface material used. If you use thermal grease, the amount of
force pressing down on the IHS can be lower, than if using other
materials. Grease does not need quite as much force as other
materials.

Part of the downward force is intended to prevent the heatsink
from separating from the CPU, if the computer is bumped or dropped.
So, you want enough force to prevent the water block from moving
out of place, plus just a little more to give it some shock
resistance. The water block probably has less mass than your
average high performance air-cooled heatsink, so again, that
means you don't need quite as much shock resistance as you would
need with a more massive heatsink.

If you are using the "H-plate" to hold the water block in place,
and there are four screws, then 70lbf divided by 4, gives a
_maximum_ of 17.5lbf for each screw. The H-plate will probably bend
a bit, and the legs of the plate will function as a spring
(f = k * delta_x, but we don't know the value of k for the plate).
All I can suggest, is imagine what 17.5 pounds of force would be
like on each screw, and make sure you use less than that amount
of force

Also, make sure you attach your hoses to the block, before fitting
the block to the processor. You don't want to be cranking on
that assembly, except to do up the nuts on the H place.

Paul