Interaction Of The Heat Spreader & Heat Sink
The Heat Spreader
When you cut a CPU in half, you notice that the chip (die) itself is much smaller than the CPU package, and thus the die touches only part of the heat spreader. The spreader’s job is to distribute the CPU die’s heat across a larger area, which allows it to pass to the CPU cooler's heat sink.
The drawing illustrates two little-known facts. First, the CPU manufacturer fills the gap between die and heat spreader with a heat-conducting material. While AMD (just like Intel did in the past) fills the void with some kind of solder, Intel now uses a thermal compound on most of its chips, which has a higher thermal resistance, but probably saves a few pennies in production. This explains why cooling overclocked Intel CPUs has become more difficult since the Ivy Bridge architecture.
Heat Spreaders, Hot-Spots & Dire Consequences
The drawing also shows that, due to the size difference between CPU die and heat spreader, there are some areas on the heat spreader that will be cooler than the area directly above the die. The area above the die is called the hot spot because it is directly heated by the die underneath. The two images below illustrate what a hot-spot is, albeit in an over-simplified way. Reality is not as simple; CPU cores may be loaded differently, and there is also the issue of on-die graphics, which may be more or less active than the processing cores. But let’s just look at the die as a whole and the heat spreader on top of it, viewed from above.
In this example, the Intel CPU has a narrower hot-spot due to the smaller die width seen from above. You should take this into account when choosing a heat sink. After all, you need to dissipate heat from the hot spot first and foremost.
Benefits & Drawbacks Of DHT Coolers
CPU coolers with exposed, ground-flat heat pipes are the latest fad. They certainly save some money during production, which marketing departments then sell to customers as a performance-enhancing feature. But there are drawbacks to this mechanical design. Consider a cooler with, say, four heat pipes, like the Xigmatek Achilles overlaid on the CPU picture below. The outermost heat pipes miss the hot spot completely. Even the two innermost heat pipes only partially cover the narrow hot spot of an Ivy Bridge-based CPU. Adding insult to injury, the cooler typically cannot be turned 90 degrees.
If we could turn the heat sink around we'd ameliorate this situation. AMD CPUs are typically not as affected due to their larger die area and CPU orientation; in most cases, all heat pipes cross the rectangular hot spot. If you want a DHT-based cooler, consider one with five heat pipes for more modern Intel CPUs, and try to avoid designs with large gaps between the ground-flat pipes.
Just by choosing a poorly-suited cooler, you can lose more thermal performance than the most expensive compound could ever gain back! But there is more bad news. Let’s take a look at what happens between the heat spreader and the heat sink.
A microscope will show you that neither the surface of a heat spreader nor the surface of a heat sink are really smooth. What looks even to the bare eye is full of pits and grooves.
When you press both surfaces together, only parts of the metal touch each other. Without a thermal compound, air fills the gaps. But air is a bad heat conductor. It's more of an insulator, actually. Thus, without thermal paste, much of the engineering that goes into heat spreaders and CPU coolers is wasted, as heat is only conducted where the metal surfaces touch.
Heat-Conducting Materials To The Rescue! Pastes & Pads
Clearly, the insulating air needs to be displaced by some thermal compound. Obviously, any thermal paste, pad, or liquid metal will conduct heat less effectively than the two metal surfaces involved. So, you want the application to be thin enough to not impose a lot of thermal resistance, but thick enough to overcome the surface imperfections of the heat spreader and sink.
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