A semiconductor can generate significant heat during operation, which leads to an increase in the junction temperature. If appropriate measures are not taken to dissipate this heat, the performance of the components will be critically affected.
In extreme cases, this can ultimately lead to the destruction of the components. In this article, Kunze Folien, a leading provider of customised heat management solutions, provides some technical information on the issue, and describes materials that are used to solve the problem.
In order to dissipate the heat generated during operation, power semiconductors are mounted on heatsinks. However, the surfaces of the semiconductors and the heatsinks are not generally flat and smooth and a good heat contact cannot be obtained without undertaking special measures such as grinding the contact surfaces. In many applications, the component must also be electrically insulated from the cooling surface. This requires an electrically insulating material having a low thermal resistance
Fundamentals
Heat dissipation from the source (semiconductor barrier layer) goes through many layers of different materials before the heat is finally transferred to the ambient environment through free or forced convection. The heat flow H (per time unit of dissipated heat mass Q) through a layer in thermal equilibrium is given as:
H = dQ/dt = -kA x (dT/dx)
A designates the contact area, dT/dx the temperature change over the thickness of the layer and k is the specific thermal conductivity of the material.
For a homogeneous material of uniform thickness and in thermal equilibrium, the formula simplifies to:
H = kA x [(T2 - T1)/d]
where the temperature T2 is greater than T1 and d designates the thickness of the layer. The specific thermal conductivity is a material constant. Assuming the same geometry, the higher the value of k, the better the heat transfer. Some values of k are:
Aluminium (99%): 220 W/mK
Graphite: 169 W/mK
Steel: 45 W/mK
Air: 0,0026 W/mK
Analogous to the formula for electric current, the above equation can be written as:
H = ΔT/Rth
H x Rth = ΔT
where Rth is the heat resistance. Substituting the previous formula results in the following expression for Rth:
Rth = d/(k x A)
Rth is generally given in °C/W.
As can be seen, the thermal resistance of a layer is dependent on the dimensions as well as on the thermal conductivity of the material. The thermal resistance decreases with an increase in the contact area or a higher thermal conductivity of the interface material and with a decrease in the layer thickness. It is therefore known as Rth material.
Another important impact on the heat transfer between two contact surfaces is the thermal contact resistance: Rth contact. Surface areas are never purely even and the bigger the surface is, the more ruggedness occurs caused by concave, convex and other imperfections, hence the less contact there is. These imperfections or 'micro pores' impede the heat flow, as the thermal path is then limited to the actual contact points between the two surface areas. The thermal contact resistance therefore depends on: the surface, the surface quality, the evenness, the flexibility of the interface material and the pressure applied.
The total resistance of heat flow between two surfaces is the sum of the material thermal resistance and the thermal contact resistance:
Rth total = Rth material + Rth contact
In practice, the contact area is pre-determined by the component outline. Likewise, the thickness of the material is kept within practical limits with respect to the dielectric strength of the insulating material as well as by the unevenness or burrs that must be smoothed out by the thermally conductive layer.
Materials
In power electronics, there are currently a variety of thermally conductive insulating materials in use. These materials include elastomers with thermally conductive fillers, polyimide films coated with thermally conductive soft materials, ceramic insulators, mica insulators in combination with thermally conductive grease.
Elastomers: The time-consuming process of applying thermally conductive grease and the problems of soiled components with the grease ingredients led to the development of elastomer insulating materials. They are made of an elastomer binding agent, together with a thermally conductive filler.
When pressure is applied, the elastomer insulator adapts very well to the contact surfaces resulting in a low thermal contact resistance between the surface areas.
The most common elastomer binding agent is silicone. In addition to high tensile strength and good chemical stability, silicone has high temperature stability. The total thermal resistance can be optimally adjusted by regulating the application pressure. Because the degree of cross linking in silicone is very high, there is practically no danger that the silicone molecules will leak out over time and dirty the components.
Elastomer binding agents only have low thermal conductivity, but they can absorb large quantities of thermally conductive fillers. The thermal conductivity can be changed by mixing in ceramic powders. The least expensive ceramic filler is aluminium-oxide. Compared to mica and ceramic insulators, elastomer insulators filled with aluminium-oxide have medium thermal conductivity. This is adequate in most cases.
Another filler is boron nitride. It is finer grained and is not as thick as aluminium-oxide. This makes the rubber softer and more pliable so that boron nitride filled elastomer insulators adapt more easily to uneven surfaces, hence decreasing the thermal contact resistance. Boron-nitride is more expensive, but has a thermal conductivity that is distinctly higher than that of aluminium-oxide. These elastomer insulators filled with thermally conductive ceramic can be reinforced with fibreglass.
The thicker the fibre, the higher the material's tensile strength and the lower the thermal conductivity. The reinforced fibreglass also gives the insulator its mechanical loading capacity. Polyimide films are also used as a carrier.
The thermal conductivity of mica with thermally conductive grease lies somewhere between that of aluminium-oxide and boron-nitride filled elastomers. The thermal resistance of mica insulators coated with thermally conductive grease depends to a great degree on the care with which the grease is applied and on the thickness of the mica insulators.
The thermal resistance of elastomer insulators is not dependent on the assembly process and varies negligibly due to tight manufacturing tolerances. For the production, this means a guarantee of process reliability and reproduction capability.
Polyimide films: Polyimide films can also be used to insulate components. They have high tensile strength and are at the same time tough and flexible, so that they are resistant to punctures and tearing. Polyimide films have relatively low thermal conductivity and are therefore usually very thin and coated. Coating with a good thermally conductive material such as thermally conductive silicone or polymers with phase change properties guaranties a good thermal contact.
Phase change materials: Phase change materials consist of a special thermally conductive compound that improves the heat flow by changing to a soft state and back to a solid state at pre-determined temperatures reached during the operation cycles of the component to be cooled. During the change to the soft state, the compound expands in volume thereby filling all interstitial voids and micro pores between the mounting surfaces. This eliminates all air pockets between the interfaces thereby allowing maximum heat transfer from the semiconductor to the cooling surface thus minimising thermal contact as well as total thermal resistance. For mechanical stability and depending on the application, phase change materials can be applied either on electrically conductive metals like aluminium for instance or on electrically insulating substrates like polyimide films.
Ceramic insulators: Ceramic insulators are used as insulators for semiconductor components. These are predominantly made of aluminium-oxide, beryllium-oxide, boron-nitride and aluminium-nitride.
They are hard and are therefore usually used with thermally conductive grease to effect a good thermal contact. By mechanical handling of the surface (eg, grinding), the heat transfer resistance can be considerably reduced. The thermal conductivity of a ceramic insulator is distinctly higher than that of mica. Therefore, greater thicknesses of ceramic insulators can be used in order to achieve a high tensile strength. They are very brittle and expensive so their use is relatively costly. Moreover, beryllium-oxide requires careful processing to avoid mechanical damage as the inhalation of beryllium dust can lead to chronic pneumonia.
Mica insulators: Mica insulators have long been used in a similar way. The dielectric strength of mica is excellent, however the thermal conductivity is not particularly high. Like the ceramic insulators, mica is very hard and therefore does not make very good contact with the component to be cooled.
For this reason, mica has a high thermal resistance and is generally only used in combination with thermally conductive grease. When the contact surface is uneven or burred, the brittle mica insulators can easily break during mounting causing a short circuit. The use of thermally conductive grease requires a costly manufacturing process. The components must be cleaned and soldered before the grease is applied. With silicone-based greases, the danger arises that the silicone will leak out over time, thereby dirtying the electrical contact. This can lead to corrosion of the contacts and consequently to a loss of conductivity.
Solutions
Kunze Folien offers the full scope of heat-management solutions, focusing on highly thermal interface materials, heatsinks and semiconductor clips.
| Tel: | +27 11 458 9000 |
| Fax: | +27 11 458 9034/5 |
| Email: | [email protected] |
| www: | www.electrocomp.co.za |
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