PHASE CHANGE PAD

6/7/2003

Phase Change Pads are pads used to attach heat sinks to components requiring heat sinking. They are made of engineered materials that are intended to change phase (melt) at some point above room temperature and below the temperature at which the device would be damaged. The phase change allows them to flow and establish better contacts with the surfaces than a simple heat conducting pad. They are purported to be superior to thermal grease in that they will achieve uniform thickness and are not subject to thermal pumpout wherein grease may eventually creep out of the space between the sink and the hottest parts of the device when subjected to repeated off (cold) on (hot) thermal cycling.

Manufacturers assert that the best pad materials have thermal conductivities equivalent to thermal greases. They also assert that even though the pad material melts, it will not pump out as grease might do because it is thixotropic. Thixotropicity is the property of remaining soft until subject to stress then flowing freely. Tomato ketchup is thixotropic. Thus. it is claimed that the materials in heat pads will flow only when and where the user wants.

Phase change pads are moderately expensive and are not reusable. Non conductive thermal greases are generally used instead of phase change pads for short term evaluation and other temporary usages.

CPU manufacturers sometimes recommend thermal pads and some CPUs ship with them attached. The pads often have a plastic sheet on top that must be removed prior to installation. As of 2003, AMD is recommending the use of thermal pads for all their current PC CPUs. Intel ships some CPUs with pads, others with thermal grease.

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