Essential Guide to Thermal Management
Heat transfer is vital for creating superior products.
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Thermal ManagementOverview
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.
Electrical Isolation
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.
Compression Force
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
Dielectric breakdown voltage is the potential where a non-conducting material becomes conductive.
Testing Guidelines
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).
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Using ThermalInterface Materials
As surface mount components replace through hole designs, there is a need for thermal management solutions that offer better thermal conductivity. Case mounted tabs are no longer the norm. Instead, surface mount packages transfer heat directly to a circuit board. Although the board is flat, there is often a difference in heights for the various packages of the board-mounted components. As for surface mount packages that do have heat sinks, there is also a need for a manner in which to transfer the heat from the package to the heat sink.
Air Gaps Thermal Interface Materials (TIMs) are designed to fill in air gaps and microscopic irregularities, resulting in dramatically lower thermal resistance and thus better cooling. Unlike still air, which has a low thermal conductivity, TIM is purposely made to conduct heat.
Thermal Interface Solutions Typical TIM ratings are between 1 and 5 W/mK, with the more advanced materials hitting numbers above 7 W/mK. TIMs are hitting these numbers while exhibiting a shore hardness that conforms to the component’s package in a manner that offers the lowest combined thermal resistance.
A TIM is one element of the cooling chain. Thermal resistances determine the amount of heat that is transferred from the heat source (in this case, a semiconductor chip) to the ambient environment (usually air or in some cases, a liquid cooled plate).
Common IndustryApplications
Thermally conductive materials are used in a variety of applications. Printed Circuit Boards (PCB) often dissipate power, which results in a temperature rise among various components. TIMs are used to wick the heat away from the components to a heat dissipating media, such as a liquid cooling plate or heat sink.
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