Thermal Paste: How It Works & How You Should Apply It
Convex & Concave Heat Spreaders
To make matters worse, heat spreaders are not merely rough, but due to the way they're manufactured, they aren't level, either. The following diagram exaggerates the problem for the purpose of illustration:
AMD’s heat spreaders are slightly higher in the center and Intel’s have higher edges. From our perspective, AMD's approach is better-suited for cooling. Due to the mounting pressure of the CPU cooler, the thermal compound is thinnest where most of the heat needs to be transferred. Thus, Intel CPUs deserve perhaps a touch more thermal paste, and you should take care that no air gaps form in the center.
How Thermal Paste Spreads Under Pressure
The following animation illustrates how thermal paste escapes to the sides when pressure is applied. Later, we’ll discuss the relationship between a paste’s fluidity (how "liquid" it is, the opposite of viscosity) and the maximum mounting pressure in detail. For now, let’s just say that a paste with low viscosity is more suitable for low-pressure mounting methods, like Intel’s push-pin method, than a "hard" paste.
The technical specifications of a thermal paste’s thermal resistance do not necessarily predict its real-world performance for a specific combination of CPU, paste, and cooler. A good heat sink can be crippled by a mismatched thermal compound. A good match between cooler and paste can achieve more than just blindly choosing an expensive thermal compound.
A Philosophical Debate: The Application Method
It’s tough to pick a technique for applying paste. Any method only works well if paste quantity and viscosity is absolutely correct for the particular application. In light of the hot-spot discussion, however, we believe that smearing paste on the whole CPU is quite pointless and a thing of the past. Instead, we want to focus on the particularities of the CPU, its heat spreader, the heat sink, and the mounting method (in particular the mounting pressure).
Brushes & Low-Viscosity Pastes
Liquid pastes like the Revoltec Thermal Grease Nano can be applied with a brush, and are consequently the easiest to use. However, low viscosity comes at the price of a high silicone content, which impacts thermal conductivity. These pastes typically fall to the bottom of our performance charts. When you try to apply semi-liquid pastes by brush, typically you wind up with too much, and that isn’t optimal, either.
Drop, Sausage Or Wall Painting?
Trying to spread a high-viscosity paste with a credit card is a fool’s errand. You'll waste a lot of time and won’t achieve a thin, smooth layer. Yes, you can try to put a latex glove on your hand and use your index finger. But even with this method, the risk of applying too much paste is significant, especially if you have no practice. The higher the viscosity, the less successful you can anticipate being trying to "paint the wall."
The Strip Method: It's All About The Sausage
When you imagine CPU die under the heat spreader, it may seem smart to put a strip of paste over that area. But don’t apply too much. Otherwise, the paste will ooze out on all sides. If your paste is electrically conductive, you can almost be assured of hardware damage.
When you apply the paste strip frugally, the result is better. Don’t worry too much about bare spots. The edges of the heat spreader don’t contribute much to thermal transfer anyway. If your cooler sports a back plate and applies lots of mounting pressure, the paste will spread further. As a rule of thumb, the lower the viscosity of the paste and the higher the heat sink's mounting pressure, the more your compound of choice will spread.
The Ideal Blob
The "drop" or "blob" method can be used by both newbies and enthusiasts, and it even works with high-viscosity pastes, assuming you are using a quality cooler that applies plenty of mounting pressure.
Don’t apply too little paste for fear of overdoing it. The compound might end up not covering the hot spot, hurting thermal conductivity and leading to an overheated CPU.
Take the type of cooler into account, too. An aftermarket heat sink with a back plate, which is screwed down, can tolerate less paste than AMD's "hook a clip and flip a lever" or Intel's "four push-pin" sinks. When you use pastes with higher viscosity, you want a cooler able to apply more pressure, and it's alright to use more paste. Of course, when we say more, we mean a little, not an extra-generous slathering.
The picture above shows a near-optimum spread; we wound up with a thin layer that completely covers the die. Since it didn’t reach the edges, we know we didn't use too much paste, and that it wasn't applied too thick. Beware of literally using a pea-sized blob. A paste ball about 2mm to 4mm in diameter should be enough; don’t use more than that! We're talking about a lentil-sized ball here.
Last But Not Least: Don’t Panic!
CPU manufacturers also believe in a less-is-more philosophy, as evidenced by their boxed coolers. For instance, AMD’s heat sink only touches about two-thirds of the heat spreader. The stencil-printed paste sports a high viscosity. It's almost solid, and it doesn't spread outwards (the sink's mounting pressure is relatively low). But this method obviously gets AMD’s blessing.
Why do we bring up this cheap boxed cooler? To allay fears and to encourage a healthy do-it-yourself spirit. Yes, two decades ago I also had my doubts about mounting aftermarket CPU coolers. But I encourage you to try it with an ounce of preparation, a sprinkle of can-do attitude, and a pinch of carefulness. Nothing will go wrong.
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