What is the Best Way to Apply Thermal Grease? - Part 2
By Rafael Coelho on September 29, 2011
A few months ago, we tested seven methods for applying thermal compound in order to determine which was the best. At the time, we concluded that a small dot in the center of the CPU is the best method to apply thermal compound.
But this conclusion may be applied only for the hardware we used, and, since we ran our tests using a CPU cooler with a mirror-like base, we couldn’t extrapolate that result for coolers with different characteristics, such as direct touch heatpipes coolers. In those coolers, there are gaps and irregularities on the base of the cooler which may need a different quantity or procedure to apply thermal compound.
That’s why we are testing again. This time we will try to answer the question, “What is the best way to apply thermal grease on direct-touch heatpipes CPU coolers?”
Figure 1 shows the base of the cooler we chose for this test: the Corsair A70.
Each different method is presented in three pictures: one of the thermal compound applied to the cooler base or to the CPU before installing the cooler, one of the CPU after the test with the cooler removed, and one of the base of the cooler after the test.
Figures 2, 3, and 4 present the test with a tiny drop of thermal compound in the middle of the cooler base. As you can see, the amount was not enough to cover the entire CPU surface.
Figures 5, 6, and 7 illustrate the test with slightly more thermal compound applied at the center of the cooler base. The CPU surface was totally covered.
Figures 8, 9, and 10 demonstrate the test by pouring the thermal compound on a single line, transversal to the heatpipes. Although the CPU was not correctly covered, a small quantity of compound leaked.
In Figures 11, 12, and 13, you see the test using one stripe of thermal compound at each heatpipe. Notice that the thermal compound leaked.
Figures 14, 15, and 16 illustrate the test done by spreading the thermal compound on the CPU heatspreader surface.
We tested the thermal compounds using the same testbed system that we currently use to test CPU coolers, which is fully described below. Our Core i7-860 (quad-core, 2.8 GHz) CPU, which is a socket LGA1156 processor with a 95 W TDP (Thermal Design Power), was overclocked to 3.3 GHz (150 MHz base clock and 22x multiplier), and we kept the standard core voltage (Vcore).
We used a Corsair A70 CPU cooler, which is a typical high-performance cooler where the heatpipes touch the CPU directly. The thermal compound we used was the Arctic Silver Céramique, which we tested some time ago. We chose this compound because of the large sample supply that we have, enough for making all the tests with the same thermal compound. Note that this thermal compound is very thick and viscous; therefore, a more fluid compound can behave differently.
Room temperature measurements were taken with a digital thermometer. The core temperature was read with the SpeedFan program (available from the CPU thermal sensors), using an arithmetic average of the core temperature readings. During the tests, the left panel of the case was open.
Operating System Configuration
Since both room temperature and core temperature readings have 1 °C resolution, we adopted a 2 °C error margin, meaning temperature differences below 2 °C are considered irrelevant.
The table below presents the results of our measurements.
|Thermal Compound Quantity||Room Temp.||Core Temp.||Difference|
|Tiny dot||15 °C||55 °C||40 °C|
|Small dot||15 °C||50 °C||35 °C|
|Transversal stripe||15 °C||51 °C||36 °C|
|Parallel stripes||15 °C||51 °C||36 °C|
|Spread||15 °C||51 °C||36 °C|
In the following graph, you can see how many degrees Celsius hotter the CPU core is than the air outside the case, with the CPU at full load. The lower this difference, the better is the cooling performance achieved.
We concluded at the end of our first article on this subject that “Putting a small drop of thermal compound at the center of the CPU is the best way to apply thermal compound.”
The tests we made this time corroborate this conclusion with one small adjustment. On a cooler with a mirror-like, perfectly flat base surface, “less is more,” i.e., applying less thermal compound than the ideal is not a big problem. However, if the base of your cooler is not smooth because of the gaps and irregularities typically found on coolers where the heatpipes touch the CPU directly, you will need a little more thermal grease.
The worst result we achieved was when we didn’t apply enough thermal compound to cover the entire CPU surface after the installation of the cooler.
On the other hand, the best cooling performance was measured when we used a small drop of thermal compound in the center of the cooler base. (It should be at the center of the CPU, and the result should be the same.) This means that, when using a CPU cooler where the heatpipes touch the CPU directly, you will not need to use a special procedure to apply the thermal compound, such as applying a stripe of grease at each heatpipe.
You will need to apply only a drop of thermal compound at the center of your CPU. If you want to be sure that it was enough for best performance, remove the cooler and check if the grease spread correctly. If not, just apply a little more.