Material

The lightness (2,7 g/cm³), the thermal conductivity (220 W/m•K) and the plasticity are the main properties that make the aluminium the most suitable metal for the manufacture of our heat dissipation systems. Where otherwise indicated, all the profiles in this catalogue are extruded in aluminium alloy UNI 9006/1, numerical designation 6060 AlMgSi 0,5.

Special products

Meccal makes and provides the following mechanical supports for the electronic industry:

• Extruded commercial bars (flat bars, angles, square bars, L and U profiles).
• Extruded boxes and containers.
• Enclosures for high frequency technology obtained from a full aluminium base.

HOW TO SELECT A HEAT SINK

In the electronic devices, due to the Joule’s law, the electric current flow causes a temperature increase led from the following relationship  

where T_j  [°C] is the temperature achieved from the device due to the power dissipation P_d   [W]T_a,   is the surrounding ambient temperature and R_THja  [°C/W], called thermal resistance, represents the device temperature increase due to the application of an electric power of 1 Watt. The power limit of an electronic device is therefore closely related to the device temperature limit.

In an electronic system, a heat sink, enhancing the heat dissipation from the device (hot surface) to the cooler surrounding ambient, allows to decrease the thermal resistance of the entire system and therefore the temperature achieved from the device. In the same way, fixing the maximum working temperature of the device, a heat sink allows to dissipate a higher power.

The thermal resistance of an electronic system with heat sink is equal to the sum of all the single thermal resistances met on the heat flow thermal path: the junction to case device resistance (R_THjc ) usually provided from the device manufacturer’s data sheet, the case to sink resistance (R_THch ) to account for the heat transfer for conduction between case and heat sink, and the sink to ambient resistance (R_TH ) to account for heat transfer for convection and radiation from the heat sink to the surrounding ambient. The last two thermal resistances are the design variables on the chosen of a heat sink. 

The thermal resistance between the case and the heat sink depends on the thermal resistivity of the material used in the interface case/heat sink – usually silicone grease for obtaining an homogeneous contact surface – on the material thickness and the contact area.

The performances of a heat sink are related to its thermal resistance   provided from the heat sink manufacturer. The thermal resistance depends on different factors: material (thermal conductivity), shape and size, colour and surface finishing (radiation efficiency and contact resistance), convection and heat sink mounting position (natural or forced convection). Obviously, smaller is the thermal resistance and better is the heat sink performance.

Knowing the ambient temperature, the maximum power dissipation of the device, its thermal resistance and maximum temperature reachable, it is possible to calculate the maximum thermal resistance value allowable for the heat sink as

Therefore you need to select from the catalogue a heat sink having a thermal resistance value at least less than the one calculated.

MEASUREMENT OF THERMAL RESISTANCE

In the catalogue, the heat sinks are shown divided for kind of product, shape and increasing order by size (in millimetres). For each profile, the following parameters are indicated:

• KG/MT: Profile weight (kilograms per meter)
• L: Heat sink length in millimetres related to the shown thermal resistance
• W: Heat sink width in millimetres related to the shown thermal resistance (only for the HIGH PERFORMANCE product)
• R_TH,N : Thermal resistance [°C/W] in natural convection calculated with sink to ambient temperature difference of 70°C (ambient temperature of 25°C)
• R_TH,F : Thermal resistance [°C/W] in forced convection calculated with air speed of 3 m/s and sink to ambient temperature difference of 50°C (ambient temperature of 25°C). For air flow with other speed, refer to “AIR SPEED CORRECTION FACTOR” graph to determine the multiplication factor to apply to the thermal resistance.

The thermal resistance values come from tests made in Meccal’s air conditioned laboratory in the following conditions:

• One heat source in centre of heat sink using silicone grease between them
• Temperature measured on the heat sink surface under the heat source through miniaturized thermocouples
• In natural convection tests, fins arranged vertically. In horizontal use, the thermal resistance value R_TH,N has to be increased by around 20%
• Raw surface heat sink. For black anodized heat sink in natural convection, the thermal resistance value R_TH,N has to be decreased by around 10%
• The thermal resistances values are related to the shown profile length. Increasing the heat sink length, the thermal resistance decreases with a non-linear law.
• For assembled HIGH PERFORMANCE products, the thermal resistances are related to assembled heat sinks around 100 mm wide (W). Varying the heat sink width, the thermal resistance curve can be approximated with a linear law, that is doubling the width the thermal resistance is halved.

The technical data on this catalogue come from laboratory tests and simulations accomplished in accurate way, so they can be considered reliable. Since the real heat sink working conditions could be different from the laboratory ones, we suggest to make a practical verification using the same final conditions.