Thermal Conducting Compound/Tape
To allow the most effective conduction between the heatsink and the CPU, a thermal conducting compound is used between the CPU die and the heatsink. This is because the more contact between the die and the heatsink, the better the transfer of heat will be.
Even flat objects that appear to be in full contact might have limited points of contact, due to the roughness of the surfaces at a microscopic level. Its estimated that only 1% of the surfaces may actually be in contact when two flat metal parts touch each another! The remaining space is filled with air, which is a poor thermal conductor. Thermal compound fills in these gaps of contact and greatly increases the efficiency of the heatsink.
Proper use of a thermal compound between the CPU and heatsink is absolutely necessary for proper cooling of the CPU. If your CPU and heatsink instructions tell you to use a thermal compound, do not omit this step.
Thermal compound comes in two forms. First is thermal grease, which looks just like any other thick liquid. If thermal grease is used, you simply place a drop of thermal grease on the die before installing the heatsink. Use a drop just about the size of a small pea and place it at the center of the die. As the heatsink is installed, it is pressed down and the thermal grease will compress and flatten out.
Second, and a better, less messy method, is a thermal tape applied to the heatsink that comes with the heatsink (Figure 29 shows the thermal compound on the bottom of the heatsink). Examine your heatsink and your heatsink instructions to see which method is used. If your heatsink has a strip of thermal tape on it, you dont need to use thermal grease. The tape is used instead of the grease.
If your heatsink has a thermal tape applied to it, remove the cover of the tape just before you install the heatsink. Dont allow the thermal tape to be exposed for a long period of time before doing the installation. You dont want it to attract dirt.
If you ever need to remove the heatsink from the CPU, which originally had thermal tape and then reinstall the same CPU and heatsink (you probably will never need to do this), youll need to scrape off all of the thermal compound from the heatsink. Because the material will fill in the pores at the microscopic level, youll never remove all of the old material. But, try to remove all visible material. Then, youll apply new thermal material. For the Athlon, AMD.com has a list of approved thermal materials, including Bergquist HF225UT (See AMDs Builders Guide For Desktop/Tower Systems, Document 26003A for other thermal materials).
Incidentally, AMD only approves phase-change thermal material. So, dont use ordinary thermal grease of an unapproved type. If you purchase your CPU in a retail-box version, it will come with a proper heatsink and an appropriate thermal compound inside the retail box.
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How to Build Your Own PC (http://www.PCGuide.com/byop/) on PCGuide.com
Version 1.0 - Version Date: May 4, 2005
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