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Cooling systems. Part 1: Processor Coolers

August 21, 2002






First microprocessors appeared over 30 years ago. The microelectronic technology made a step forward, and while in the beginning a computer was affordable only for elite, today it's our "integral part". But together with this transition the computer industry acquired a lot of problems. 

No secret that high-performance processors heat up much, i.e. dissipate a lot of thermal power. And a modern high-speed silicon heart of a computer can't do without additional cooling devices. A problem of obtaning the optimal working temperature is very urgent today - it is a corner-stone in creation of a reliable, ergonomic and high-performance computer system. The widespread and universally recognized processor cooling means is a cooler (heat-exchange apparatus of forced air-cooling). Usually they combine a metallinc finned plate (heatsink) and an air pump (fan); it supports a processor's working temperature within the permissible frames providing its correct and reliable functioning. Well, let's take a closer look at such devices. 

Heatsink

Heatsink is a device that facilitates heat exchange between a processor and environment. A surface area of a processor's die is usually rather small (several square centimeters today) and is not enough for effective carrying off of thermal power of dozens of Watts. Thanks to its finned surface a heatsink installed on a processor makes its area of contact with environment much greater, and, thus, it makes heat exchange much more intensive and a working temperature much lower. 

Thermal resistance of a heatsink relative to a surface of a processor's die is its fundamental technical characteristic; it estimates its cooling effectiveness. 

Thermal resistance has the following formula: 

Rt = (Tc - Ta)/Ph

Rt - thermal resistance of heatsink,
Tc - temperature of die's surface,
Ta - ambient temperature,
Ph - thermal power dissipated by processor. 

Thermal resistance is measured in  °C/W. It shows increase of a die' temeprature relative to temperature inside the system unit with thermal power carried off via a given heatsink installed on the processor. 

Let's take the VIA Eden platform as an example. A typical thermal resistance of a processor's heatsink is 6°C/W, a typical thermal processor's power is 3 W and a typical temperature inside a system unit is 50°C. The product of the heatsink thermal resistance and processor's thermal power is 18°C. Now we know that the temperature of the die's surface is higher than the temperature inside the system unit by 18°C and it will be maintained at 68°C. This temperature complies with the "medical" standards for the VIA Eden ESP processors, and we have nothing to be worried about. 

Now let's consider one more example. If we take a heatsink from VIA Eden ESP and couple it with the AMD Athlon XP processor whose thermal power is 40-60 W, the result will be quite sad: the processor's temerature will reach 300°C and more which will cause its death from "heat stroke". Such thermal power needs a heatsink (or even a normal cooler) with much lower thermal resistance so that the processor's temeprature doesn't exceed 75-90°C. 

There is a clear principle of thermal resistance: the less, the better. If we know it, we can easily find out whether we can use a certain heatsink (or a normal cooler) in our certain conditions. Also, we can avoid mistakes which can cause failure of a computer system or of your wallet :). 

Thermal resistance of a heatsink depends not only on a finned surface but also on its design and technology of production. At present the market offers 5 archetypes of heatsinks involved in mass production. 

Extruded heatsinks. The cheapest and most popular on the market; they are made of aluminum. The method of extrusion allows for making a complicated finned surface and good heat-removing properties. 




Folded fin heatsinks. A folded metallic sheet is attached (soldered or glued with special adhesive heat-conducting grease) to the heatsink's base; its folds make something like a finned surface. The basic materials are aluminum and copper. As compared with extruded heatsinks this technology allows for smaller devices of the same of even higher effectiveness. 



Cold-forged heatsinks. The cold pressing technology allows making not only rectangular fins but also pins of an arbitrary section. Such heatsinks are made mostly of aluminum, but quite often its base contains copper plates to improve heat-removing properties. The cold forging technology is known for its rather low performance, that is why such heatsinks are more expensive than extruded or folded ones, but their thermal effeciency is not always better. 



Bonded/fabricated fin heatsinks. They are very similar to folded fin heatsinks, but there is a considerable difference: the finned surface is made not of a sheet but of separate thin plates soldered or welded to the heatsink's base. The main material is copper. As a rule, such heatsinks have a higher thermal effectiveness than extruded or folded ones, but only if quality of the production process is strictly controlled. 



Skived fin heatsinks. Today these heatsinks are the most advanced and expensive as the production involves high-precision mechanical processing of solid blanks (they are processed on special CPU-based high-precision machines), and their thermal effectiveness is the best. Aluminum and copper are the main materials. Such heatsinks are able to press out all other archetypes from the market provided that the cost of this technology is reduced to an acceptable level. 



Now, when we have covered the heatsinks, let's take a look at the fans. 

Fans

Modern processors need cooling devices with thermal resistance as low as possible. Today even the most advanced heatsinks are not able to cope with this task: in case of free air convection, i.e. when an air mass speed is low (for example, vapor over an asphalt road in a hot summer day), the rated thermal effectiveness of heatsink is not enough for maintaning an acceptable working temperature of a processor. There is the only way to cardinally reduce heatsink's thermal resistance - to ventilate it thoroughly (i.e. to create conditions of forced convection of a heat carrier (air)). That is why almost every processor's heatsink is equipped with a fan which blows through its inter-fin space. 

Today processor's coolers mostly use axial fans which form an air flow in a direction parallel to an impellar's axis of rotation. 




The fan's rotating part can be built on a sleeve bearing (the cheapest and most fragile construction), on a combined bearing (one sleeve - one ball bearing (the most widespread one), and on two ball bearings (the most expensive, reliable and durable one). And an electrical part of a fan consists of a miniature direct-current motor. 

How can we know whether a fan is good or bad? What are its technical characteristics and working parameters? Let's see! 

First of all, a fundamental characteristic of any fan is its performance, i.e. the value that shows a volumetric air flow rate (cubic feet per minute, CFM). The higher the fan's performance, the more effectively it pumps air through a heatsink reducing its thermal resistance. A typical performance is 10-80 CFM. 

Secondly, an impellar's speed is a very important characteristic of a fan (rotations per minute, RPM). The faster the impellar rotates, the higher the fan's performance. A typical speed is 1500 - 7000 rpm. 

Finally, dimensions should be accounted for as well. As a rule, the bigger the fan, the higher its performance. The most popular dimensions are 60x60x15 mm, 60x60x20 mm, 60x60x25 mm, 70x70x15 mm, 80x80x25 mm. 

As far as working parameters are concerned, a noise level and a service life are the most considerable among them. 

A noise level is measured in decibels and shows how loud noise can be in subjective perception. A noise level can vary from 20 to 50 dBA. A human considers a fan quiet if its noise level doesn't exceed 30-35 dBA. 

At last, a service life of a fan is measured in thousands of hours and it objectively shows how reliable and durable a certain fan is. An acual service life of fans based on sleeve bearings doesn't exceed 10,000-15,000 hours and of those based on ball bearings - 40,000-50,000 hours. 

So, that's all for today. Next time we will disassemble fans and take a closer look at some technical details. 
 
 

Vitaly Krinitsin (vit@ixbt.com)
 

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