Thermally-conductive phase-change interface materials nad heatsink effectiveness
29 June 2005
Manufacturing / Production Technology, Hardware & Services
Semiconductors can generate significant amounts of heat during operation which leads to an increase in junction temperature. If appropriate measures are not taken to dissipate this heat, the component's performance is critically affected, and can ultimately lead to its destruction.
In order to dissipate heat, power semiconductors are mounted on heatsinks. However, the surfaces of semiconductors and heatsinks are not generally flat and smooth, and a good heat contact cannot be obtained without taking 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.
Kunze, located in Munich, Germany, is a recognised leader in customised heat management solutions. The company manufactures a range of phase change interface materials that conduct a change of phase from a solid state to a soft state from a certain temperature - the so-called phase-change-temperature. Because phase-change materials expand after having passed the phase-change-temperature, if inserted between the heatsink and the component, they fill all air voids and expel the air pockets from the contact surfaces' micro-pores, wetting out the complete surfaces.
Through pressure and expansion of the material, this layer becomes very thin, resulting in minimal thermal contact resistance. Both the thermal contact and total thermal resistances remain then, forever very low - at all temperature cycles, even when the temperature drops under the phase-change-temperature point.
Kunze phase-change interface materials have no electrical insulation properties. They are made either of pure phase-change material or used in combination with highly thermally-conductive metals.
Aluminium with phase-change coating
One of the Kunze lines is the 'Heatpad KU-ALC and KU-ALF' products, which are both very thin aluminium foils, coated on both sides with the silicone-free, thermally-conductive polymer called 'Crayotherm'. This coating changes its state at about 60°C for KU-ALC and 51°C for KU-ALF, and becomes soft. Crayotherm expands in volume by about 15% to 20% once past the phase-change-temperature and complete wet-out of the contact surfaces takes place without outflow.
Figure 1. Picture on the left shows before first operation, unheated; picture on right shows the voids filled after first phase change
After the first phase-change has taken place and the material has expanded, it irreversibly remains in that condition throughout all following temperature cycles. Thus, a minimum thermal resistance and therefore a minimum total thermal resistance is permanently assured. The fact that Crayotherm is mixed with highly thermally-conductive graphite in the KU-ALF version, enhances this thermal result even more.
Types KU-ALC/S and KU-ALF/S have narrow acrylic adhesive stripes on the sides, to make mounting easier, and does not impair either the very good thermal flow effected by the Crayotherm, or the total thermal resistance.
Properties
This KU-ALC/ALF product demonstrates minimum thermal resistance through active wet-out of the interfaces by volumetric expansion of about 15-20% without outflow. It is silicone-free, with guaranteed coating thicknesses. Because of the aluminium substrate it is mechanically-stable, and can be replaced without special surface treatment. Cleaning is with isopropyl alcohol.
The product comes in all standard configurations, with or without adhesive strips on the edges. It can be stamped or cut according to customer specifications, or delivered in roll form.
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