Rough or smooth?

The surface of a thermal interface pad is one aspect that will improve heat transfer.

A rough surface up-close will look like mountains and valleys, with deep troughs and high peaks. These variations in the surface randomly reflect light, and hence don’t give a good reflection when looked at. These surfaces effectively have very few points that will be in direct contact with your CPU as only the peaks will make contact.

In order to minimise the effect of this, heat paste is used to fill the troughs so that a larger area touches CPU. Obviously heatpaste won’t be as conductive as pure metal, but it’s better than an air space.

A shiny surface will instead have shallower troughs and shorter peaks. This reflects light much better in a more uniform manner, resulting in a better reflection. In this case, more peaks will make contact, and as the troughs are shallower, less heat paste is needed and so there is less distance that ‘heat’ has to travel through that isn’t pure metal.

Flat or lumpy?

A much more important aspect is where the surface of the contact pad is flat. If it isn’t then no matter how much you polish the surface, only a small portion will make contact with the CPU.

If the middle is domed, then only that part will make contact with the CPU, unless an excessive amount of heatpaste is used which is far less efficient.

Nearly 100% of the time, a rough, flat surface will be far better than a shiny, lumpy/domed surface.

A simple way to check whether your heatsink pad/CPU heatspreader etc. is flat is to get a knife blade (be careful not to scratch the surface of your heatsink/CPU) and roll it across the surface. It should lay completely flat with no gaps either side.