Basic Knowledge of Heat Conduction

Heat exchange between heat sink and environment

When heat passes to the top of the heat sink, need as soon as possible the quantity of heat that sends to circumference environment to go, to air to air cooling heat sink character wants to undertake heat exchange with circumference air namely. At this time, the heat is passed between the two different medium, the following formula for Q = alpha X X Δ T, including Δ T for the temperature difference between the two kinds of medium, namely the heat sink and the ambient air temperature difference; While, is the thermal conductivity of the fluid, which is a fixed value after the heat sink material and air composition are determined; One of the most important is A heat sink and air contact area, on the premise of other conditions unchanged, such as the volume of A heat sink usually limited, the limited space inside the casing, will increase the difficulty of installation, and by changing the shape of the heat sink, increase the contact area with the air, increase the heat exchange area, is the effective measure to increase the cooling efficiency. To achieve this, the surface area is generally increased by means of fin-type designs supplemented by surface roughness or threads.

After the heat transfer to air, and air temperature of the cooling fin contact will sharply rise, at this time, the hot air should be around as much as possible and cold air by convection heat exchange way to take away heat, such as for air cooled heat sink, the main means is to improve the air flow speed, use fan to realize the forced convection. This is primarily related to the fan design and wind speed, the efficiency of the heat sink fan (such as flow, wind pressure) mainly depends on the fan blade diameter, axial length, fan speed and blade shape. Most of the fan flow is in CFM (inch, cubic feet per minute), and a CFM is about 0.028mm3 per minute.

Pure aluminum heat sink

Pure aluminum heat sink is the most common primary heat sink, its manufacturing process is simple, low cost, so far, pure aluminum heat sink still occupies a considerable part of the market. In order to increase the heat dissipation area of its Fin, the most common processing method of unalloyed aluminum heat sink is aluminum extrusion technology, and the main index to evaluate a pure aluminum heat sink is the thickness of the heat sink base and pin-fin ratio. Pin refers to the height of the Fin of the heat sink fin, and Fin refers to the distance between two adjacent pins. The pin-fin ratio is divided by the height of the Pin (excluding the thickness of the base) by the Fin. The larger the pin-fin ratio is, the larger the effective cooling area of the heat sink is, and the more advanced the aluminum extrusion technology is.

Pure copper heat sink

Copper has a thermal conductivity 1.69 times that of aluminum, so all other things being equal, pure copper heat sink can remove heat from a heat source faster. The quality of a material of copper is an issue however, a lot of flavor “pure copper heat sink” actually is not true 100% copper. In the list of copper, copper containing more than 99% is called acid-free copper, and the next level is red copper containing less than 85% copper. At present the copper content of most vibrant copper heat sink on the market is in between. And the copper content that a few inferior pure copper heat sink contains even 85% is less than, although cost is very low, but its heat conduction ability reduces greatly, affected come loose heat. In addition, copper also has the obvious disadvantage, the cost is high, the processing is difficult, the heat sink quality is too big mostly hindered the full copper heat sink fan application. The hardness of red copper is not as good as aluminum alloy AL6063. Some mechanical processing (such as cleavage) performance is not as good as aluminum; Copper has a much higher melting point than aluminum, which is bad for Extrusion and so on.

Copper aluminum bonding technology

After considering the disadvantage of copper and aluminum these two kinds of heat sink material respectively, at present part of the market high-end heat sink often uses copper and aluminum combined manufacturing process, these heat sink usually uses copper metal base, and heat sink fin USES aluminum alloy, of course, in addition to copper bottom, also has heat sink to use copper column and other methods, is the same principle. With superior thermal conductivity, the copper substrate can absorb the heat released by the CPU quickly. The aluminum fin can be manufactured into a shape that is most conducive to heat dissipation by means of complex processes, and provides a large space for heat storage and rapid release, which has found a balance point in all aspects.

Heat from the CPU core to the surface of the heat sink fan, is a heat conduction process. As for the base of the heat sink fan, since it is in direct contact with a small area of the heat source with high heat, it is required that the base can transfer heat rapidly. The selection of materials with higher heat conduction coefficient is very helpful to improve heat conduction efficiency. CRT can be seen through the heat conduction system, such as aluminium 237 w/mK, heat conduction coefficient of the heat transfer coefficient of copper is 401 w/mK, and compare the same volume of heat sink, the weight of copper is 3 times of aluminum, and aluminum heat was only 2.3 times that of copper, so the same volume, the copper can contain more calories than aluminum heat sinks, heat sink heating up more slowly. With the same thickness of the heat sink base, copper  can not only rapidly draw away the temperature of a heat source such as CPU Die, but also increase its temperature more slowly than aluminum heat sink. Therefore, copper is more suitable for making the bottom of the heat sink.

However, the combination of these two metals is complicated. The affinity between copper and aluminum is poor. If the bonding is not handled well, a larger interface thermal resistance (i.e. the thermal resistance caused by insufficient contact between the two metals) will result.  

The actual design and manufacture, the manufacturer always tries to reduce the thermal resistance of the interface as far as possible, to maximize strength and avoid weaknesses, which often reflects the design ability and manufacturing process of the manufacturer.