Xigmatek opt for two 6mm and two 8mm diameter copper heatpipes swagged into the heatsink's aluminum base block, as you can see below. In the slim space between each heatpipe, some heat is conducted to the additional 6mm diameter heatpipes that are tucked in above.

stacked heatpipes

Exposed heatpipe base heatsinks work best with processors that have a thick integrated heatspreader and even heat distribution over the entire CPU IHS surface area. They can encounter problems when heat is localized too directly, such as if only one or two heatpipes receive the bulk of heat from the CPU.

Generally speaking, the four bottom heatpipes ought to have sufficient capacity to conduct heat from a 150W Intel/AMD processor to the aluminum fins of the Xigmatek SD128264 Aegir if the heatsink is designed properly.

The notches pictured above are for the rubber fan tabs to latch into. Xigmatek rely upon the same little rubber posts for all of its heatsinks. The fan is also illuminated with a couple tangentially located white LEDs.

Personally, we prefer wire fan clips with some sort of elastomer between the fan and fin tower. I'm afraid of ripping the rubber posts during repeated fan installations, or several months down the road for dust cleaning.