I recently purchased a Thermaltake CPU fan, and whenever it arrived in the mail today, I took it out of the box, and there was a thin layer of thermal paste on the underside of the CPU fan.
I had originally planned on applying thermal paste to the surface of the CPU, but since there is a thin layer of it on the underside of the CPU fan already, do I still need to apply thermal paste to the CPU surface?
If it is factory applied by Thermatake, and there's a cover of it to keep it from getting smeared in transit, I've had good luck with those.
Normally the compound will be a circle in the center of the area where the CPU will contact the heat sink, or two half-circles separated by a line with no compound.
Too much heat sink compound can be nearly as problematic as none at all; this paper looks at various compounds, their thickness, and their efficiency of transfer, with the optimum thickness of the compound ranging from 0.002" to 0.02".
Now, if the top of the CPU's can and the bottom of the heat sink were both as smooth as a Jo block, you would not need any compound; it's there to fill in air gaps between the surfaces. Air gaps of that size are typically about 10% effective at heat transfer, hence the need for compound; but the optimum layer is only about twice as efficient, and a thick layer can be ineffective. Here's a paper with explanations and pictures of a good application.
