13.07.17
13.07.17
The efficiency of a thermal management system can be considered to be a measure of how well the system conducts heat away from the source(s) to the outside world. Commonly used materials in heat sink construction, such as Aluminium and Copper, have defined thermal properties, which help to identify the best solution for a given application.
Thermal Conductivity is the measure of heat flow through a material under a stable temperature differential. Materials with the highest Thermal Conductivity properties are shown in Table 1.
THERMAL CONDUCTIVITY = k = W/(m k)
MATERIAL @25C
Diamond 1000
Silver 429
Copper 401
Gold 310
Lithium 301
Beryllium 218
Aluminium 205
Tungsten 174
Graphite 168
Pinchbeck 159
Magnesium 156
Clearly we are not going to be able to use Diamond, or in most cases justify the high cost of precious metals, to build our thermal management systems. The most common and lowest cost material for construction of power systems is steel, but this has a thermal conductivity of only about a quarter of that of Aluminium, which leads to the use of Copper or Aluminium to improve the flow of heat in thermally efficient solutions.
The density of the material is an important feature of the cost v. efficiency calculation. Copper may have a thermal conductivity twice that of aluminium, but it is three times as dense – which means that a heat sink which offers like for like heat flow will weigh around 50% more in Copper. Add to this the relative costs per tonne (Copper is currently around three times the cost of Aluminium), and you find Copper will work out at well over four times the cost of an Aluminium solution.
Aluminium can be extruded relatively simply, and although Copper can be extruded it is very difficult to extrude the shape of a heatsink because it has to be heated to such high temperatures, potentially damaging or greatly reducing the life of the extrusion tooling. Original die costs for Aluminium extrusions can be amortised over many tonnes of material, but machining copper into extrusion shapes from solid material is costly. This put Aluminium extrusion firmly in the lead for most applications.
From Table 1 it can be seen that pure Aluminium has a Thermal Conductivity of 205 W/m.K. When alloyed with other elements (primarily Manganese, Silicon, Magnesium, Copper & Iron), Thermal Conductivity varies depending upon the temper of the alloy.
Commonly used alloys for extrusions include 6060, 6061, 6063. Alloys are selected for particular applications based on their mechanical properties, including corrosion resistance, tensile strength, and hardness. Alloy 6060 has a Thermal Conductivity of 209 W/m.K. It is commonly selected for complex cross sections and responds well to anodising processes, making it a good choice for heat sinks and electronic modules. Alloy 6063 has a slightly lower conductivity of 201 W/m.K, but is also commonly used for heat sink extrusions.
For high performance liquid cooled plates different aluminium alloys are more appropriate; those primarily used are 1050A (TC 222W/m.K) and 6082 (TC 180W/m.K).
Simple extruded heat sinks may not be efficient enough to deal with the heat flow requirements of many modern high power applications, including cooling battery pack & inverter equipment being used in the latest hybrid and all-electric vehicles. The same aluminium alloys, combined with leading edge technologies for inserting copper plates & tubes into aluminium structures, can stretch thermal performance to the highest possible level.
Power Products International, working closely with our technology partner Mecc.Al S.r.l., can offer equipment designers access to the latest thermal management solutions, including welded and assembled high performance heat sinks, liquid cooled plates, thermal simulation and precision machining. Contact our technical sales team to assist with all of your thermal management requirements.
