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One popular way to achieve maximum thermal transfer is by lapping the heatsink's base contact surface. The term lapping describes the process of sanding the heatsink-to-processor contact surface to eliminate microscopic air pockets caused by machining or extruding in the heatsink fabrication process. In addition, external factors such as rough handling or poor shipping practices can lead to scratches in the contact surface. By smoothing out the surface of the heatsink, you can maximize the surface area that will be in contact with the processor. Even the smallest imperfection can create the opportunity for an air pocket to interfere with contact. This will mean a loss in cooling efficiency, because air is a poor thermal conductor.
The process of lapping can be tedious, especially with poorly machined heatsink coolers. The base surface must be sanded with fine-grit sandpaper to obtain a mirrorlike finish and eliminate as many imperfections as possible. Disassemble the fan cooler from the heatsink before you begin lapping to prevent damage to the fan or electrical connections during the procedure. Clean the heatsink thoroughly to remove any deposits or filings introduced during the lapping process. Alcohol is the best cleaning solution and it won't damage finishes on the heatsink surface.
The best way to begin lapping a heatsink is to start with a low-grit paper (400 to 600), then slowly progress to higher grades until you reach the desired results, usually at the 1200- to 1600-grit level.
The most accurate method for sanding is to move the heatsink's base contact surface across the sandpaper in an alternating circular motion, with the paper firmly affixed to a perfectly flat surface. A small piece of extruded glass, such as a windowpane or mirror, works well as the flat surface. Glass will not scratch the heatsink surface if you make an error. Regular scotch tape or even a mild spray adhesive works best for attaching the sandpaper to the glass. Lapping requires great patience. Some heatsinks can require up to an hour of fine sanding before the desired surface consistency is reached. The procedure is described below.
With the sandpaper firmly attached to the glass, move the heatsink across the paper in a figure-eight motion.
Apply even pressure throughout the length of the motion to guard against introducing deformed flat spots or upraised regions.
Alternate the pattern of movement at regular intervals.
Polish and clean the heatsink once its base is sanded to a uniform finish. Cleaning with alcohol will preserve any finishes applied to the surface.
Apply a quality metal polish evenly and buff it as specified on the package. A rotary tool with a cloth-polishing wheel operating at a low speed works well for many materials.
Once the heatsink is polished, clean it with alcohol again. As with air gaps caused by scratches, polishing materials left on the surface reduce thermal conductivity.
A properly lapped heatsink will result in a processor that runs cooler and with more stability, thus improving your likelihood of achieving overclocking success.
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