One thing remains a Crucial
factor in any cooling solution and is frankly often overlooked, neglected or not given the attention it requires; the thermal interfacing of the actual CPU-die with the base of the heatsink. This is of the utmost importance as any defect or slight imperfection of the mounting hardware, the finish of the base of the heatsink or simply the misalignment of the heatsink and the CPU-die will cause for a huge decrease in cooling performance. You can mount the biggest heatsink featuring the most powerful ducted fan approach available but if there's a slight air gap in between the CPU-die and the base of the heatsink, or if the heatsink is slightly tilted and not properly aligned, it will quickly drive the CPU-die temperatures over its functional limits and if no action is taken could mean destroying your newly bought CPU.
To even out any misalignments or small heatsink base imperfections, it has been common practice in electronics to use thermal paste, or thermal 'grease' as it's often referred to, although we're really not 'greasing' anything, as there's no moving parts. It is used both as a 'gap filler' and to ensure that the whole CPU-die is actually in thermal contact with the heatsink base. Apply too little and you might trap air underneath and decrease thermal contact, but apply too much and it could act as an isolator. Applying thermal paste is something that deserves some attention, but if done properly has no better alternative other than chemical or mechanical bonding of the CPU-die and heatsink.
Fig 4. The correct application of thermal paste, notice that the whole surface area of the CPU and the heatsink base is covered with a thin layer.
Other alternatives are available too, such as thermal interface pads, but those should be avoided at all cost, they're simply too thick and not suited to handle the heat load. Another option is to use a TIM pad, which is no substitute for quality thermal compound, but better than a thermal interface pad. A TIM softens with rising temperature and forms a more or less permanent bond with the CPU-die after first use, however the pressure applied to it by the mounting hardware determines the thickness of the remaining layer of interface material. Insufficient pressure or die-temperatures that are too low for the TIM to reach an acceptable amount of fluidity could thus create an insufficiently thin layer of interface material.
Using thermal paste is no sinecure either, normal 'electronics grade' zinc-oxide is often used and yields good results if applied properly, but others are available too, such as the excellent Artic Silver and ShinEtsu thermal paste. A paper-thin layer of thermal paste is usually all that is needed to guarantee a good result. By sliding the heatsink back and forth, or making small circles after it has been mounted onto the CPU is a good way to evenly spread out the thermal paste. Once the friction starts to increase the layer is thinnest and the excess thermal paste will have been spilled over at the edges of the CPU-die, the heatsink is now in best thermal contact with the CPU-die.
One other thing that is important and should actually need no explanation is the fact that even the biggest heatsink featuring the most powerful ducted fan approach available can only cool a CPU down to a few degrees above case temperature. For a heatsink to keep a CPU at case temperature it would need to be of infinite size, so no heatsink or heatsink and fan combination will run cooler than your case temperature. If your case temperatures in excess of 40...45C you'll quickly run into thermal issues if your CPU is dissipating 70-watts of heat into your case. Case ventilation and airflow is then an important factor to consider, and one that deserves attention.