Mounting a Pentium 4 CPU

Introduction

Have you ever wondered about what it meant by careful mounting of a modern CPU on a motherboard? Everyone mentions thermal grease and how you shouldn't use too much ... or too little. Never fear, the confusion has an end. While I don't claim to be perfect ;-) I thought it might be handy to snap a couple of shots the next time I mounted a new CPU and post those.

Disclaimer:I'm posting this because I took some photos while re-mounting one of my CPUs. Don't take this as gospel, don't flame me if you do things differently, please use common sense and don't blame me if you break your CPU :-)

Why Use Thermal Grease ?

Modern CPUs consume lots of power much of which is radiated out as heat. Simply put, during operations the chip surface may hit temperatures of several hundred °Celsius or more which will damage the chip. Basically it might fry your precious CPU ;-)

So that the chip remains within defined operating temperatures, heatsinks are placed on top. Fans on, in or near the heatsink ensure waste heat is vented as quickly as possible.

If you're curious about what the thermal limits for your CPU are, check out the Intel and AMD technical documents, for example for the Pentium-4 family and the Athlon family.

Thermal grease is a heat-conducting paste which is smeared between the top of the CPU and the bottom of the heatsink to smooth out the microscopic ridges which exist even in such apparently smooth materials. To do so only a tiny amount is needed, commonly we say an amount the same size as a grain of rice. This is then smoothed over the CPU, excesses cleaned away carefully and the heatsink mounted on top.

As an example of thermal grease, here is a photo of Arctic Silver 3 which is what I used recently. (This product is continually evolving and at time of writing Arctic Silver 5 had been released).

This compound has real silver in it (hence the name) and this gives it excellent thermal conductivity.
Like many products of its kind, it is supplied in a small syringe with sufficient compound to use on close to a hundred CPUs.

NB: since it contains silver it is also electrically conductive! It is thus vital that excess thermal compound is cleaned away, especially at the edges of the CPU core since it may cause hard to troubleshoot shorts if it contacts conducting wiring.

We'll now take a look at what the CPU looks like when it is being prepared with the grease.

The Motherboard & Socket

Here's the motherboard I used for these shots, a Gigabyte GA-8I875 Pentium-4 socket 478 motherboard:-

And here's a close-up of the socket. The ZIF (Zero-Insertion Force) socket is unlatched slightly which is why the brown handle is pointing slightly upwards. Note the compononts right underneath and to each side of the socket, stray blobs of grease could easily cause an unwanted short.

Pin 1 is indicated by the absence of a couple of pins of the regular array on the bottom right corner (click photo to zoom).

Preparing the CPU

Photo of the Pentium-4 with a layer of older grease on it

Here is my Pentium-4 CPU the moment I removed it from its previous motherboard and took off the heatsink. The vendor had done a pretty fair job in that thermal grease was applied rather than a less efficent (but cheaper) thermal pad and enough grease was present to have formed some form of layer.

I'm not entirely happy with the grease layer though - note the thickened areas which seem to have prevented more uniform contact between CPU and heatsink (the CPU had nonetheless performed well for several months prior to this operation).

Here's the cleaned processor.

I wiped off the older thermal grease with a kitchen towel and then a softer tissue. The older grease had not hardened or dried and there was no need to use any other solvents like iso-propyl alcohol or the like.

Be gentle with your CPU - it is fragile and there is no need to exhert large force when cleaning.

Photo of the cleaned Pentium-4, all shiny and new!

Applying Thermal Grease

I put quite a lot on this time (too much pressure on the syringe!). This is what it looked like. To get rid of the excess I used an old credit card to scrape across the CPU to get as even as possible a surface.

Note how I've stayed away from the edges. This is because the material will spread naturally anyway and we don't want any to drop off and cause a short.
Second, the center is the most important bit because that is where the CPU core resides. ; the silver area we see is the head spreader which rests on top of the core.

Here's the CPU again, mounted in the socket. Remember to align Pin 1 on the CPU with the correct hole on the socket. You can see this in the photo:- the top-left edge of the CPU has the edge ground off at an angle, correspondingly, the socket has a little triangle silk-screened on it.

Don't force the CPU into the socket!

You might have to gently wiggle it a few millimeters each way for all pins to line up correctly but it ought to just drop into the socket with a slight thump!

All Done !

CPU with heatsink mounted in the socket, side-ways view

Here's the final product:- the CPU with the heatsink on top mounted in the socket, viewed side-ways on.

It's difficult to see but the metal brackets to the left and right of the heat-sink fins are slotting into the yellow CPU mount (best visible on left). You will have to use a certain amount of pressure to push the second bracket in once the first is in place. Again, be sensible - you need controlled pressure, not insane force.

Things to Remember

Note that thermal grease is grease, which means it will not set nor will it help hold the CPU against mechanical stress. If your heatsink is missing one (or more!) mounting clamps, replace it since the grease will not hold it.
This was demonstrated nicely by a Toms Hardware Labs test where several modern CPUs were run without heatsinks. (Some good but large videos of CPUs going up in smoke).

Second, let me re-iterate the previous comment about conductivity: thermal greases tend to be electrically conductive. Clean away excess grease to prevent electrical shorts !

Third, be aware that not all thermal grease compounds are created equally. Beware of those which are only silver coloured. Some Googling will help.

Fourth, you can have too much of a good thing. The amount of grease you see above was too much (sorry for no better photo, it turned out to be out of focus!) so I wiped some off. Remember all you need is a thin layer.

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