OCModShop has posted a Thermalright XP-90 Heatsink review

In addition to the fins there are four heat pipes. Heat pipes are closed metallic tubes that generally contain a phase change liquid in side. Heat is applied to one end of the pipe, which in turn changes the liquid to vapor where it moves to the other end of the pipe where heat can be removed. In this design the heat pipes are enclosed into a cast aluminum base where heat from the processor will be applied and then whisked away and distributed to the top of the fin array. Here a fan will remove the heat from the fins through forced convection. It seems the theory behind this design is to remove heat as quickly as possible (higher heat transfer rate - aluminum dissipates heat faster than copper using forced convection) as opposed to having a high heat capacity (large copper bases - copper has a higher heat capacity and conduction rate than aluminum). This tends to explain such a drastic change from what many would consider a usual design from Thermalright.

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