General guide in heatsinks and cooling
Function and purpose of a heatsink
At the junction of semiconductor components and resistances the dissipation power (Pv) is converted into heat (Q) and causes a rise in temperature. If the temperature of the junction (Tj) exceeds a certain maximum level, stable operation can no longer be guarantee and the semiconductor may be damaged. Details of the junction's maximum permissible temperature can be obtained from the data sheet supplied by the manufacturer of the semiconductor. In cases where it is not possible to dissipate the heat adequately through the semiconductor casing into the surrounding medium (normally air) it will be necessary to mount the semiconductor on a heatsink. This enlarges the surface area of the casing, improving heat dissipation and, consequently, the reliability of the semiconductor and the whole circuit and also their useful life. A heatsink is made of materials with good thermal conductivity, generally aluminum alloys, and has a geometric structure and surface designed to optimize its functional efficacy.
The materials used in heatsinks are:
AlMgSiO.5 F22 for extruded aluminum profiles,
AlSi8Cu3 for aluminum die-castings
Al 99.9% pure for aluminum strip
How a heatsink works - heat transfer and types of convection
Transmission of heat from a heat source (e.g. the junction of a semiconductor) via the heatsink into the surrounding medium takes place in four successive steps:
- transfer from heat source to the heatsink
- conduction from within the heatsink to its surface
- transfer from surface into the surrounding medium by either free or forces convection
- radiation depending on the nature of the heatsinkīs surface
Thermal resistance and thermal equivalent circuit of a heatsink
Thermal resistance is defined as the degree of a temperature increase resulting from a power input and is used as a measure of determining and comparing the heat transfer capacity of heatsinks. Thermal resistance is expressed in K/w [Kelvin / watt]. The heatsink and the semiconductor form a single functional unit which can be portrayed graphically as a thermal equivalent circuit in the same way as the Ohm's Law for an electrical circuit.
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The above circuit is subdivided into the following sections:
- Dissipation power (Pv) input converted into to heat flow (Q)
- Heat conduction from junction to casing
- Heat transfer from casing surface to heatsink surface
- Heat dissipation from heatsink into surrounding medium
Calculation of heatsink needed
Calculation of thermal resistance required for a given dissipation power and the permissible temperature gradient.
| Rthh-a | Thermal resistance of heatsink | K/W |
| Tjmax | Maximum junction temperature of semiconductor as per data sheet | °C |
| Ta | Temperature of environment | °C |
| Pv | Dissipation power input into heat source | W |
| Rthj-c | Internal resistance of junction to casing in the semiconductor | K/W |
| Rthc-h | Heat transfer resistance at the surfaces | K/W |
| DT | Temperature difference | K |
Any heatsink with a lower thermal resistance than the one calculated by this formula will be suitable for the relevant application.
What data for a heatsink looks like
In the pic above taken from the ALUtronics heatsink catalog we see first of all what the heatsink looks like. Actually you see the cross section because you can order different length of the same pattern depending on the thermal resistance needed. In this catalog the company gives a lot of info like how much power we will need to dissipate and so on. On the left chart you see for different power dissipation values and different lengths what the thermal resistance will be. This is a very accurate method compared to what most companies give. On the right chart you see for 3 given lengths the temperature difference versus power dissipated.
This is a simpler form of data from the Fischer elektronik catalog. You can see the heatsink code, the available dimensions, the heatsink cross section and dimension, and at last a chart that shows thermal resistance versus length of heatsink. Itīs very important to notice that the thermal resistance doesnīt decrease linearly for an increase of length. Here we have to be careful when designing to find a compromise in length because heatsinks do cost a lot.
Importance of thermal transfer resistance
Close attention must be paid to the question of thermal contact between the semiconductor casing and the surface of the heatsink. It is influenced by the surface quality (peak to valley height) and flatness, contact pressure and insulation and filling material used. Peak to valley heights vary from RZ=2.5 to 5.0 unprocessed extruded profiles and RZ<1.5 for faced surfaces. The flatness variance of a 100 by 100 mm surface ranges from 0.5 to 1.0 mm for unprocessed extruded profiles to DIN and 0.1 mm or better for faced surfaces. Air pockets (sink holes) between the adjacent surfaces can be filled with heatsink compound. This reduces heat transfer resistance (Rthc-h). The compound should be applied as thinly as possible in order to avoid formation of air bubbles. Newer materials now in use are heated under pressure to above their melting point. This fills the tiniest cavities and allows any trapped air to escape. There are also models with electrically conductive and insulating carrier foils. Conventional screw mountings are nowadays being increasingly replaced by inexpensive clip mountings with a clip nut already inserted in the profile. These are quick to mount and the contact pressure is applied at the right point on the semiconductor.
Surface color of heatsink
The influence of a heatsinkīs radiation component (black surface) on its thermal resistance is frequently miscalculated because there is a general rule to allow for this. Heat is radiated mainly from the outer surfaces of a finned heatsink because the spaces between the fins are normally too narrow to permit radiant heat to escape and heat exchange by radiation consequently takes place only between the surfaces of the opposing fins. The radiation component does not therefore increase in direct proportion to the area available for heat convection. The percentage share of radiation in total heat transfer is substantially higher from a flat cooling surface than from a fully finned heatsink. Most commercially available heatsinks are designed for heat transfer by convection and not by radiation. The share taken by radiation is highly temperature-dependent and increases by the fourth power as temperature rises. If the surface temperature is kept low, for example by forced ventilation, radiation transfer is negligible because the heat is constantly being transported away. The anodized layer is more likely to have an adverse effect on transfer resistance because it has a thermal insulating effect. In cases forced ventilation is installed, especially when it has a string air flow, an untreated or chromated heatsink is most suitable.
A black heatsink can also absorb more radiation from the environment. Consequently, if there are any components in the vicinity with a fairly large radiating surface which could attain higher temperatures than the heatsink itself, the effect can be reversed and the heatsink heated up from these sources (radiation exchange). A black anodized heatsink is generally most suitable from the technical point of view:
- for natural convection and higher surface temperatures
- where there are no other radiation sources in the vicinity
- where external thermal resistance is fairly high (insulating effect of anodized layer is only slight in comparison with the radiation component)
Protection against surface damage is obviously important as well.
A chromated surface retains its electrical conductivity and also provides surface protection. This feature is particularly important in situations where casings or casing components have a comply with EMV requirements.
Different types of convection
Forced ventilation can reduce a heatsinkīs thermal resistance. In cases where the thermal resistance to free convection is known and the temperature gradient remains constant, it is possible to calculate the thermal resistance of a heatsink of a given length when subjected to forced ventilation at various air flow speeds. The following diagram shows the curve for a heatsink 100mm in length and a temperature gradient of 80°K.
Factor f is the ratio of thermal resistance with natural convection to the thermal resistance at a given air flow speed.
Static and dynamic properties of a heatsink
All the information given so far applies only to a steady state operation. Thermal capacity and running time are the two main factors that can temporarily affect the behavior of heatsinks. Sudden high loads can quickly generate large amounts of heat that have to be stored somewhere temporarily. This calls for high thermal capacities and as low a thermal resistance as possible. This problem is solved by the use of either an aluminium or copper block or a heatpipe.
Heatpipes
Thermal transmission resulting from changes in the state of the aggregate is a special type of convection. One example of this is the heatpipe, a closed tube containing a liquid (e.g. water) which is evaporated at one end and condensed at the other. The liquid is a returned by capillary flow through either a mesh network or a system of internal fins within the pipe which transports the heat of evaporation. This is a much more efficient way of transmitting heat than by conduction through a solid. A heatpipe can only operate within a specific temperature range and has to be specially designed for a given application.
Heatpipes are not a substitute for heatsinks. They are used to transmit heat and to improve a system's dynamic properties by better heat distribution. They are ideal:
- where there is a lack of space in the vicinity of the heat source
- for effective heat distribution where there are clusters of heat sources
- for extracting heat through the seals of closed cases
- for coping with a sudden, short term heat surges
- where there are moving parts or assemblies
- where there are weight problems and for light structures