Material Thixotropy

Thixotropy is a term which is sometimes used to describe the property of pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. In layman's terms a thixotropic material is thin and manageable when static and undisturbed, and thickens as the material is manipulated or spread.

As we see progression in the area of Thermal Interface Materials, there has become a substantial increase in the number of manufacturers who embrace thixotropic TIM's. Many of these products make up the newest names on the market, and can be compared to the revelation that the Heatpipe Direct Touch technology has seen in the CPU cooler industry.

Even though nearly all of our TIM products were new, some had been sitting on the test shelf for a many months. Before each test the TIM material was mixed and agitated to ensure proper consistency. Silicon-based TIM's are more susceptible to the breakdown because of dissimilar compound bonds, and should always be mixed prior to use regardless of product age. Newer carbon-enriched silicon compounds seldom exhibit breakdown because of their thixotropic consistency (see definition above). Regardless, all test samples were aggressively mixed prior to application onto the CPU cooler. Once the product was prepared, it was applied onto the surface in a very thin coating and spread evenly.

Round Surface Application

If there's one thing our recent 33-Way Thermal Interface Material Comparison article has taught me, it's how the advice freely handed-out in discussion forums can often be wrong. After we wrote the article, many enthusiasts argued that spreading out the TIM with a latex glove (or finger cover) was not the best way to distrubute the interface material. Most answers from both the professional reviewer industry as well as enthusiast community claim that you should use a single drop "about the size of a pea". Well, we tried that advice, and it turns out that maybe the community isn't as keen as they thought. The image below is of a few frozen peas beside a small BB size drop of OCZ Freeze TIM. The image beside it is of the same cooler two hours later after we completed testing.

Decidedly, the BB size seems to be a much more appropriate amount for the PGA-478 application since it easily covered the one-inch round copper core of this Intel cooler and then some. Our images above have done sufficiently for depicting how much TIM should be used in a socket 478 cooler, but that's old news.

To begin our experiments, we have started with the most basic of all designs. Circular coolers are very common equipment, with the product list spanning from small Intel-supplied stock cooler up to the larger Thermaltake MaxOrb aftermarket cooler. But while the round outer design may be popular to designers, it is much less common to find a circular base as the contact surface. To the best of my knowledge, the only coolers I have ever seen with this finish are those from Intel, but I digress. The design has its pros and cons, and regardless of cooling performance the round base is among the easiest to properly apply thermal paste to.

Because the raised copper center core is circular, the most logical application pattern is going to match the shape. A single drop of thermal paste roughly half the size of a BB is placed at the center of the cooler, which will theoretically spread evenly in every direction as pressure is applied.

In all of our experiments, the images showing the depressed material once it has been spread out were all taken only moments after mounting the cooler. The thermal material is not allowed to cure, and the system is not powered on to begin thermal cycles. This is all done so that you can see the initial impact of mounting pressure and the direction that Thermal Interface Material travels. Please note that allowing the system to complete a few initial thermal cycles will thin the viscosity of the material and level out the paste, which will also allow it to bleed out towards the edges.

Judging from our cold test, a single round drop of thermal paste roughly half the size of a BB is more than sufficient to cover the entire mating surface of a stock Intel-included cooler. Ideally, you will want to use slightly less material than the amount shown above. Just remember that once heat is applied to thermal grease the viscosity will thin and spread out towards the edges just a little more, allowing the surfaces to come into closer (or direct) contact with each other. Keep in mind that thermal paste is only meant to fill the gaps, not coat the surface; perfectly flat metal on metal with no material in-between is your ultimate goal.

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