Ever wonder why most heat sinks come with a tiny thermal pad on the bottom? Does this pad help to improve your computer's performance, or is it really just covering a problem like a Band-Aid?

To answer this question we took a Celeron 366 MHz and put it through a battery of tests. We tested the top of a Celeron for flatness, general processor operating temperature, and on several other points, modified it, then put it through the tests again. The results surprised even us...

What we worked with:

• 366 MHz Intel Celeron PPGA (week 15, s-code: SL36C, Malaysia)
• Cooler Master DP5-5022B heat sink with a 1.44W BB brushless fan
• Test system: Abit BX6-Riv2.0, 64MB RAM, running Win 98
• Silicone heat sink thermal grease

Cooler Master, 52x52x34 mm

If you were to look closely at the top of most Celeron processors you would see a piece of metal about 20 mm square. On one side of this piece of metal the silicon die is attached, and on the other, the heat sink. This metal plate on the processor allows heat from the core to be transfered to a heatsink, where the heat is disapated to the surrounding air by convection or fans. For that transfer of heat to occur, the two surfaces in question must make direct contact. If they don't, the heat from the processor can't flow to the heatsink, and the heatsink won't be doing its job. The heat that can move across this barrier is directly related to the amount surface area in direct contact. But when you look at your CPU, that surface area may not necessarily be what you think it is.

What happens if that CPU-plate or heatsink is slightly out of whack? If either surface is not flat, then the amount of surface area in direct contact will decrease, and with it, the effectiveness of the computer to shed heat. So the question becomes; is your chip really flat?

Examining our test chip:

First we looked at the surface of the cpu-plate closely. What we saw were lots of small checks, scratches, and depressions - basic surface flaws. Combined together, all these minute flaws can be a significant barrier to how cool your processor runs. Each one of those little depressions can act like a very tiny air bubble when covered by your heatsink, even with thermal grease-though to a lesser extent. Air makes an excellent insulator (just think about those double-pain windows keeping your house warm in the winter), and in this case, will help to keep your processor running warmer .

This is the CPU-plate of an average Celeron. At this angle it looks pretty flat, and the surface looks fairly smooth, although some minor imperfections in surface finish are visible (looks like static on a TV screen).

With the CPU on a slight angle, our scanner picks up the size of those flaws a bit better (the colour is from the scanner due to the angle of the chip). Now the surface appears rough and bumpy. (looks similar to that of a road surface)

Secondly, we looked at the flatness of the CPU-plate on our test chip, and the bases of several OEM heatsinks we have. We found that most of our heatsinks had good surface finishes (ie the bases were quite smooth in most cases), but were generally bowed out at the center. Of those we testesd, heat sinks which had their bases machined were very flat. Our Celeron, on the other hand was not absolutly flat! The corners were raised up by as much as 0.0024". Other Celeron's we tested were a bit better, but most had their edges slightly higher then their centers. That may not sound like much but picture what happens when you strap on a heatsink.

If either processor or heat sink are not absolutly flat, mounting them together can produce areas that have direct contact and areas which are separated by a small void. While themal grease usually fills these voids, your computer will run cooler if you can get rid of them!

What does all this mean?

What does this mean to your system's performance? All those little flaws added together have a big effect on your computer - they decrease how much the processor comes in direct contact with the heat sink, and that means a higher running temp, slower clock speed if you overclock, and more cooling requirements.

Here's basically how the problem comes into play

• Rough CPU surface means you need lots of thermal grease or a thermal pad
• Lots of grease or using a thermal pad decreases the conductivity of chip-to-heatsink heat transfer
• Decreasing the rate at which your processor cools, means it runs hotter
• Running hotter as we all know can mean performance and overclockability suffer.