"UNDERSTANDING HEAT TRANSFER SURFACES BEFORE SELECTING INTERFACE MATERIALS"

The importance of understanding heat transfer surface conditions is very often over looked. The surface finish and the surface plane of both the heat sink and the device to be cooled should be understood in order to select the proper interface material. The options for interface materials are coming to the market fast and furious. It is easy to misunderstand which material should be used for each application. Most of the materials are presented to the user with design data for that particular material as tested in a controlled laboratory environment. Test standards have been implemented over the years that do not always relate to the user's conditions or is misunderstood by the user. One of these examples is the use of standard thermal pads with specified thermal resistance. The user should realize that the thermal resistance for most pads are tested when compressed 300 to 500 PSI. More recent materials have been developed to meet the needs of lower pressures applied to the material by the device. In order to select the proper material for the interface, one should have an understanding of what the interface presents for challenges.

Heat Transfer Surfaces

The contact of two surfaces being pressed together (the heat sink and the device) is imperfect at its best when it comes to the Thermal Resistance developed across the connection of the two materials. The heat transfer area of the joint without a thermal interface material is only a fraction of the total apparent area. As shown in Fig.1, the peaks and valleys of the surfaces allow only a few points of contact to be made. The remainder of the area is filled with air and therefore results in a high thermal resistance between the two surfaces. The air space between the two surfaces is created by the surface finish and the surface planes. The problem of surface planes is greater as the interface area gets greater. This is easier to visualize and understand knowing that an area of 1 square centimeter with 100 micro inches of separation results in a thermal resistance of 1 degree centigrade per watt. Standard Extrusion heat sinks (without a secondary fly cut) will have surfaces from 1 to 4 mil inches per inch of non flat surfaces. As can be seen in Figure 1 below, the point to point contact is a small part of the total area. The task for interface materials is to fill the microscopic cavities as well as fill the large spaces due to the non flat mating surfaces. The next task is to fill them with the best and most cost effective material. The best as far as thermal conductivity goes and the best as far as filling all the space. This will result in a low Thermal Resistance for the application.

  "Interface Material Selections"

Thermal Compounds

Thermal Compounds are available in a wide variety of formulas from Silicone and Non-Silicone bases filled with metal oxides. The metal oxide particles provide the high thermal conductivity to the compound. The metal oxide particle sizes will depend upon their ability to fill the tiny cavities. The particles are designed to give the highest thermal conductivity to the compound. The lowest thermal resistance is a combination of high thermal conductivity and the ability of the material to penetrate all of the cavities and fill all the spaces created by any non -flat areas of the two mating surfaces. Thermal Grease provides the lowest thermal resistance interface available (not including a soldered type connection). The disadvantage of thermal grease is the inconsistency of application and the problem of keeping it from being messy to use..

There are many grease application products available today to help the ease of use and to keep the product where it belongs. Such as spraying, screening, sticks and pads (pads that are dry to the touch, but is grease). A lot of work is being done in this area due to the excellence of grease as a product for the lowest thermal resistance.