"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 are 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. Many of the 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 Figure 1 below, 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, 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.

Figure 1

  "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 with the ease of use and keeping it where it belongs. Such as spraying, screening, sticks and pads (pads that are dry to the touch, but 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.

Phase Change Materials

Phase Change Materials are generally paraffin based and have the characteristics of changing from a solid to a liquid at predetermined temperatures. The material is applied to a carrier such as aluminum foil or an electrical insulating material. This allows the product to be handled and applied as a thermal pad. The material has a high thermal resistance in its initial state. When the temperature is increased by the heat from the component or the ambient around it, the material will change state to a liquid and flow into the cavities of the heat sink and device surfaces. This flow fills the cavities with the material and therefore provides a low thermal resistance. The material will change back to a solid when the temperature is lowered by removing power from the device. There are many phase change materials with numerous additives to lower the resistance. They are applied in different thickness to different base materials. The thickness variations have more to do with the flatness issue then the cavities. Some of the material types are designed to flow easier when they melt in order to provide a thinner end result and therefore a lower thermal resistance. A compromise is to allow easy flow and still keep it in place (surface tension). The disadvantages of phase change materials is primarily in the replacement or changing of the component (device) mounted on it, due to its bonding capabilities. The material causes the device to stick to it and needs to be cut free as well as cleaned off the heat sink before a new device is applied to new phase change material. Relatively low application forces are required in mounting the device to these materials. Typical forces would be in the 5 to 30 pound area.

Elastomeric Materials (Compression type materials)

Elastomeric Materials generally consist of a thermoset elastomeric binder (usually silicone )containing highly conductive (thermally) ceramic fillers. These fillers are typically boron nitride, aluminum oxide and magnesium oxide. The material is reinforced with glass fiber, metal foil or a dielectric film. These materials come in a wide variety of thickness. The material has high voltage insulation properties for applications of device electrical isolation. These materials require a large compression force to reduce the thermal resistance. Typically this force is 300-500PSI. The materials are made in many colors which allows some recognition between material types.

The disadvantage of these materials is the high compression force required and the lack of flowing into the micro cavities preventing the lowest thermal resistance they could otherwise obtain. These materials are available with and without adhesives applied to the surface (one side and both sides). One should remember that the addition of adhesives to the material increases the thermal resistance value.

Gap Fillers

Gap fillers are thermally conductive materials of many different thickness' that are used to act as a thermal interface between two items. Helping to cool the circuit side of pc boards by placing a Gap filler between it and a metal bracket or heat sink is one of many uses. The gap fillers are often very pliable and conform to irregular surfaces. Considerations in selecting a Gap filler is the thermal conductivity rating as well as its ability to conform to the surfaces of the source and sink. Some gap fillers are able to be formed like play dough (putty) and others are flexible but not like putty.