Thermal Interface Materials
In electrical products where heat is generated, it is necessary to conduct heat away from vital components. In applications where direct forced air or liquid cooling are not possible, a Thermal Interface Material (TIM) is often used to transfer heat without altering a device’s physical properties or affecting product performance.
In some cases, electrical isolation is also necessary. Electro isolation with heat transfer requires a material that has high thermal conductivity with high electrical insulation properties. It is essential that the material used to transfer heat meet certain criteria, including compression force and heat transfer according to U, K and R values.
Increasing the compression force effectively reduces thermal resistance. An analogy would be the case where a pan is on an electric stove. Pressing harder on the pan reduces thermal resistance, thus transferring more heat from the stove to the pan.
This situation is repeated in the common heat sink that is used on the tab of a semiconductor device, such as a TO-220 package. For the TO-220 package, the metal tab on the package is compressed to the metal heat sink through torque using a nut tightened on a bolt.
In situations that require the tab to be isolated, a TIM and nonconductive bolt are used.
Thermal Conductivity Testing
Dielectric Breakdown Thermal Interface Materials are specifically designed to transfer heat while exhibiting a compression force characteristic that accommodates both the component that is creating the heat as well as the device that is being used in order to dissipate the heat.
The rate at which heat passes through a material is its thermal conductivity.
- We typically talk about the conductivity of Thermal Interface Materials in terms of Watts per Meter Kelvin. This is expressed as W/mK.
- The thermal conductivity of a material is not affected by the thickness of the product. However, the overall thermal resistance of a product is generally affected by the TIM thickness.
- It is best to have the shortest distance between the heat source and heat radiator while having the TIM cover the maximum area of the heat source.
Thermal conductivity can be measured using equipment such as the Hot Disk 500S. This instrument uses a sensor that applies a constant power and simultaneously measures the temperature of the probe.
Dielectric breakdown voltage is the potential where a non-conducting material becomes conductive.
Dielectric testing consists of placing electrodes on both sides of a TIM and applying an increasing amount of voltage until a current starts to flow.
- Common test equipment measures up to 10 kV.
- Normally, the TIM should be electrically isolating, but dielectric breakdown voltage depends on the application.
- Air has 3kV/mm dielectric breakdown voltage.
- Silicone and ceramic fillers have -20kV/mm.
- Dielectric breakdown voltage is typically between 1kV/mm and 20kV/mm.
- A dielectric breakdown less than 8kV/mm is usually adequate for most applications.
Compression Force Testing
Compression force is closely related to material hardness. Material hardness is measured using a Durometer that determines the resistance of a material to indentation.
RELATIVE DUROMETER SCALE:
COMPRESSION FORCE CONSIDERATIONS:
- Amount of area that is compressed (example 1x1 cm, 2x2 cm).
- Maximum force which is speed dependent.
- Relaxation of material (applied force is reduced over time).