Thermal pumping is an effect wherein thermal grease used to enhance heat flow from CPUs to heatsinks may creep out from the space between the CPU and the heat sink, This is caused by repeated heating/cooling cycles that cause the grease to expand and contract. Expansion forces some grease away from the hottest part of the CPU. Contraction does not fully restore it to its original position. The net result is that eventually the grease layer over the hottest parts of the CPU can become inadequate to conduct the heat away from the CPU. The CPU will then overheat.
That's the theory anyway. Thermal pumping does occur. It has not been fully established that thermal pumping at normal operating temperature ranges really is sufficient to damage CPUs. Nonetheless, AMD at least does not recommend the use of thermal grease for long term thermal transfer and will not replace overheated CPUs under warranty. AMD recommends the use of phase change pads (thermal pads) instead.
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Copyright 1994-2002 by Donald Kenney.