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Fooling With Cooling

Thursday, September 05, 2002 by TheDoc

How do you tell a good heatsink from a poor one, and, more importantly, what makes a heatsink work? Is it the material used, the way it is built, or simply how powerful its fans are? These questions come up quite often; I get numerous emails on a regular basis from people asking if they should spend that extra cash on a heatsink made out of some special silver alloy or even copper. Is there any performance gain if your heatsink is made out of copper? Does this really affect performance that much? I’ll try to shed some light on things, and give you the lowdown on how to tell a good heatsink from a poor one. I’ll also throw in a review of some of the most used Pentium II / Pentium III / Celeron Slot 1 and Slot 370 / Socket 7 heatsinks as well, and suggest which is the best available at the moment.

Basically, a heatsink is designed to move the heat from the CPU, through one medium to another. In this case, from the heatsink material to the surrounding air.

This process is not limited by the material used in the heatsink--we only need a bigger heatsink if the heat transfer capacity of the material is lower. The resulting heatsink temperature will always be a product of the heatsinks C/W (K/W) value and the rated heat output of the CPU in watts.

Suppose we have a 0.5 K/W heatsink and a CPU generating 25 Watts of heat. The CPU's temperature will be:

0.5 x 25 = 12.5 degrees higher than ambient, no matter what you throw at it, even if the fan is spec’d at 100 CFM.

One of the most important, and often neglected, aspects of heatsink performance is case temperature. If you use a conventional heatsink this is the most important aspect of cooling your system, simply because a conventional heatsink with an infinite surface area will only cool the CPU down to case temperature.

But real heatsinks have a finite surface area, with typical performance of 0.1 K/W to 1.5 K/W. Suppose your case temperature is 40C, and your heatsink is similar to the one above, spec’d to have a 0.5 K/W thermal performance. Then a CPU dissipating 25 Watts will have a temperature of:

Case Temp. + Heatsink Perf. x CPU Dissip. = CPU Temp. : 40 + 0.5 x 25 = 52.5C

The only benefit of having a heatsink made out of copper or some silver alloy is that due to the better heat transfer, the heat is drawn from the CPU faster and distributed evenly across the heatsink's material. Thus you can use a smaller heatsink and get the same performance, because the heat spreads out across the heatsink more readily and can be removed by the fan more efficiently.

So you could use copper or a silver alloy as a heatsink if there's limited room for a big aluminum heatsink and if size is really the issue. To give you an idea of the heat transfer capacity difference and the related difference in size, silver has a 429 Wmk heat transfer coefficient and aluminum 237 Wmk. Hence a silver heatsink can be 45% smaller than an aluminum heatsink and have the same performance.

That might be worth considering if you plan on building some high-tech military gadget where space is limited and cost is not a problem. But in everyday applications and surely in the semiconductor field, CPUs and other heat generating semiconductors, we're better off using aluminum, due to cost, weight and ease of fabrication and handling.

On the other hand, without good design an aluminum heatsink can be worse than no heatsink at all. Thus the material alone is no guarantee of good performance. Remember, we need to get the heat from the CPU to the heatsink as quickly as possible. The use of aluminum can help tackle this problem. The heatsink must be designed so that heat is transferred rapidly to the surrounding air, which means heating up the air at the surface of the heatsink material. Thus the bigger the surface area the quicker a given amount of heat can be dumped.

A way to tackle that problem is to use a large aluminum plate. If we were to fold that plate into ridges, we'd have the same surface area, but in a much smaller space. This is the reason why heatsinks always have fins or pins. They increase the surface area of the heatsink, which leads to better performance.

This concept is known as the passive heatsink, found on many videocards and older CPUs. There’s no fan to worry about, so they will never stop working. The rule of thumb is the bigger the better, and the more fins or pins the better. It is all about surface area, and in order to cool a hot CPU, you’ll need plenty of it.

The next step is increasing the airflow across the CPU by use of forced convection--or in plain English, a fan. The number of Cubic Feet per Minute (CFM) determines how much air the fan pumps, and is used to rate its performance. If we add a fan to a heatsink formerly used as a passive heatsink, its performance can be increased tenfold. Thus if we use a fan we can use a smaller heatsink and get the same performance as a big heatsink without one.

One thing to keep in mind, though, is that the fan will be blowing air from inside the case over the CPU. This means that the temperature of the heatsink can never be lower than the case temperature. A good idea is to keep an eye on the case temperature when you’re thinking of buying a new heatsink. A single fan sucking cold air in from outside might double your current heatsink's performance.

So basically a good heatsink needs to have a large surface area, lots of fins or pins and a powerful fan or fans blowing over it. There are plenty of heatsinks that fit that description, so I’ll take a look at a couple of popular ones and two of the biggest and best Pentium II / Pentium III / Celeron / S370-S7 heatsinks on the market.

One way to make a good compact heatsink is to use a design where the airflow goes directly across the fins without being able to escape at the sides, and to allow the airflow to mix with the colder surrounding air. This is called air ducting. The air is forced over the fins and is forced to absorb its heat. The heated air will be then be dumped outside at the end of the duct. This principle is used in the Intel Retail heatsinks. Although small, they offer pretty good performance and will be sufficient for most overclocking with normal case temperatures.

Another way is to use a conventional finned heatsink with a couple of powerful fans. These fans will compensate for the not-so-efficient, finned heatsink. A good example is the popular GlobalWin FAB24/28 unit. But, unlike with air-ducting, there’s recirculation onto the fan, which means the hot air leaving the heatsink to be sucked back and blown over the heatsink. This obviously will decrease performance. The idea and construction is really simple and offers good performance. With good case ventilation, it will be sufficient for most overclocking.

The best way to make a heatsink is by using a very large, very fine pinned/finned heatsink and then add a couple of powerful fans sucking air in through an airduct. There are two ways of tackling the surface area problem: either you use many fine pins or a lot of very fine fins.

The Alpha P125C is an example of using the former, and the Alpha PFH6035MFC the latter.

This is the best approach to tackle the heat problem. CPU temperature can be as low as a few degrees above ambient if the fans are powerful enough and the airduct prevents the hot air from being sucked back into the fans, thus increasing performance. This principle is used on both the Alpha P125C as well as its little Socket 370 / Socket 7 brother, the Alpha PFH6035MFC. The fine pins/fins combination with the powerful fan(s) will transfer the heat very quickly to the surrounding air, creating a very competent heatsink. Powerful enough to put all of its competitors to shame...multifanned whoppers and sandwich heatsinks included. These heatsinks simply put all the basics of forced convection cooling to full use, and offer the best Pentium II / Pentium III / Celeron / S370-S7 cooling available.

Just for the record, the Alpha P125C will fit the Slot 1 Celerons and the SECC Pentium II. A little modification is needed to also fit the newer Pentium II SECC2 cartridge and the Pentium III. The Alpha PFH6035MFC will fit all Socket 370 or Socket 7 CPUs--but be sure your motherboard has enough room around the socket to be able to accommodate the Alpha.

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