Thermal Management Terms
Terms are from the Parker Chomerics Thermal Catalog.
Thermal interface materials can be complicated – especially with so many different terms and technicalities. Let’s break it down.
Thermal interface materials (TIMs) are used to fill air gaps within an electronic interface by adhering to the device’s uneven mating surfaces. This enables the component’s junction temperature to be reduced, as the thermal interface material’s heat conductivity is much greater than that of the air in between the two surfaces, thereby reducing the heat resistance.
Thermal interface materials offer several material properties that are important to consider for material compatibility purposes, but the main benefits relate to thermal impedance and thermal conductivity. Thermal impedance is the measurement of heat resistance from a hot mating surface through an interface material and into a cold surface.
Thermal conductivity is calculated off of the thermal impedance data, and refers to the material’s ability to conduct thermal energy (or heat). Thermal interface materials have high thermal conductivity in a means to protect surrounding materials and device components.
For more detailed information on types of materials and technical specifications relating to thermal interface materials, see below.
Alumina (Al2 O3 ): A relatively inexpensive ceramic in powder or sintered sheet form. Its thermal conductivity of 30 W/m-K and excellent dielectric properties make it useful in low to moderate power commercial applications.
Ambient Temperature: The temperature of the air surrounding a heat source.
Apparent Thermal Conductivity: This value differs from bulk thermal conductivity as apparent thermal conductivity also includes contact resistance when measured, as described in the Heat Transfer Fundamentals section of this guide. Also, see Thermal Conductivity.
Arcing: An electrical discharge between the edges of metal-semiconductor package and the metal heat sink on which it is mounted.
Binder: A polymer (i.e. silicone, urethanes, acrylic, epoxy etc.) used in thermal interface materials to provide desired mechanical, thermal and electrical properties and hold in a stable form the fillers whose primary purpose is the transfer of heat. Binders are also good electrical insulators.
Bondline Thickness: Average thickness between heat spreading device and components.
Boron Nitride (BN): A non-abrasive ceramic material that has higher thermal conductivity than alumina. Because it is an expensive raw material, it is usually used in high-performance interface materials.
Breakdown Voltage: The amount of voltage required to cause a dielectric failure through an insulator when tested under a set of specific conditions. This value does not imply that the insulator can be operated at those voltages.
Burr: A thin ragged fin left on the edge of a piece of metal (semiconductor package or heat sink) by a cutting or punching tool.
Calorie: A unit of energy equal to the quantity of heat required to raise the temperature of 1 gram of water by one degree Celsius.
Ceramic: A name given to oxides of metals. Ceramics are usually hard, heat and corrosion resistant and high dielectric strength powders that can be formed into shapes by fusion or sintering.
Chamfer: A bevel cut into the edge of heat sink mounting holes.
Coefficient of Thermal Expansion (CTE): A measure of a material’s change in volume in response to a change in temperature.
Compression Set: The permanent deformation of an elastomeric material caused by a compressive force.
Conduction: The transfer of heat energy through matter.
Convection: The transfer of heat that results from motion of a fluid (gas or liquid).
Corona: An electrical discharge within or on an insulator accompanied by ionization of the air within or contacting the surface of the insulator. Also called partial discharge. It is the main mode of insulation failure exposed to long term AC voltages.
Creep Distance: The distance that an insulator has to extend beyond the edge of a semiconductor package to prevent arcing.
Cure-In-Place: Any material that is dispensed as a liquid and cures in the application.
Cut-Through: A phenomenon that occurs when sharp edges or burrs on the metal-semiconductor package or heat sink cut through the thermal pads and reduce or eliminate their insulating strength.
Compression / Deflection: The change in thickness of an elastomeric interface material in response to a compressive load. Because these materials are incompressible, deflection is accompanied by a proportional increase in area.
Degreaser or Degreasing Solvent: The solvent used to clean flux and other organic residues off printed circuit boards after they are manufactured. Interface materials must be able to tolerate exposure to degreasing solvents without degrading performance.
Dielectric: A material that acts as an insulator.
Dielectric Constant: See Permittivity.
Dielectric Strength: The voltage gradient, expressed as kV/mm, that will cause a dielectric failure in an insulating material under very specific test conditions. Dielectric strength does not imply that the insulator can withstand those potential gradients for an extended period of time.
Durometer: An instrument for measuring the hardness of rubber. Measures the resistance to the penetration of an indentor point into the surface of the rubber.
Electronic Control Unit or Electronic Control module (ECU/ECM): Various electronic controllers, typically used in automotive applications. (i.e. steering and braking)
Electrical Insulator: A material having high electrical resistivity and high dielectric strength and therefore suitable for separating components at different potentials to prevent electrical contact between them.
Filler: A fine, dispersible ceramic or metallic powder (i.e. boron nitride, alumina, graphite, silver flake, etc.) whose thermal conductivity is at least twenty times greater than that of the binder.
Flow Rate: The volume, mass or weight of a fluid passing through a device of any type, per unit of time, expressed in gallons- or liters-per-hour.
Flux: An organic compound used to enhance the wetting and adhesion of metal solder to the copper surfaces on printed circuit boards.
Footprint: The area of the base of an electronic device which comes in contact with a thermal interface material.
Hard Tooling: A die-cutting tool manufactured from a machined metal block. The cost is high, therefore it is normally used when long runs are anticipated.
Hardness: A measure of the ability of a material to withstand penetration by a hard pointed object. Regarding thermal interface materials, this property is usually inversely proportional to the ability of a material to conform to uneven surfaces.
Hardness Shore A (Shore D, Shore 00): An instrument reading on a scale of 0 to 100 measuring the hardness of a material. There are three scales: Shore 00, A and D. Shore 00 is used for soft rubbers like gels, Shore A is used for hard rubbers and Shore D for inelastic plastics.
Heat (Q): A form of energy generated by the motion of atoms or molecules. Heat energy is expressed in units of joules.
Heat Capacity: The measure of a material’s ability to store heat.
Heat Flow: The rate at which heat is flowing per unit time expressed as Watts.
Heat Flux (Q/A): The rate of heat flow per unit surface area expressed as Watts/cm2 .
Heat Transfer: The movement of heat from one body to another (solid, liquid, gas, or a combination) by means of conduction, convection, or radiation.
Interface: A boundary that exists between any two contacting surfaces. There are five types of interfaces that can exist between the different forms of matter: gas-liquid, liquid-liquid, gas-solid, liquid-solid, and solid-solid.
Junction: The junction is the active part of a semiconductor, usually silicon, where the current flow causes heat to be generated.
MBLT: Minimum bond line thickness. When two opposing substrates obtain closest possible distance under pressure.
Micro-inch: This unit of measure, a millionth of an inch, is used to describe the roughness of a surface and is the average distance between the peaks and valleys on the surface.
Mil: A unit of length equal to onethousandth of an inch.
PCM: Abbreviation of phase change material.
Permanent Set: Permanent Set is defined as the amount of residual displacement in a rubber part after the distorting load has been removed.
Permeability: A measure of a material’s ability to align its magnetic domains in response to an applied magnetic field.
Permittivity: A measure of a dielectric material’s ability to polarize in response to an applied electric field, and transmit the electric field through the material.
Polyimide: An organic polymer with exceptional electrical insulation and high-temperature capabilities. In film form, it is used on everything from printed circuit boards to space suits.
Power Supply: A self-contained unit which converts AC current to DC for use in electronic devices.
Pressure Sensitive Adhesive (PSA): An adhesive that is tacky at normal temperatures and requires only slight pressure to form a permanent bond. A PSA requires no further cure to maintain the bond.
PSH: Class of polymer solder hybrid. A synergistic blend of eutectic solder and specialty polymers. They provide a highly reliable thermal interface material with a resin carrier and filler content that both melt to obtain minimum bond line thickness.
Radiation: A heat transfer process whereby heat is given off through electromagnetic radiation, usually infrared rays.
Reinforcement: A woven glass mesh or polymer film that is used as a support in thermal interface materials.
Relaxation: Stress Relaxation is a gradual increase in deformation of an elastomer under constant load over time, accompanied by a corresponding reduction in stress level.
Rheology: The science of the deformation and flow of materials.
Semiconductor: An electronic material that can be an insulator under one condition and switch to a conductor under a different condition
Shear-Thinning: A characteristic of a fluid whereby the fluid’s viscosity decreases with increased shear stress. Materials the exhibit shear-thinning are also described as pseudoplastic. Filled polymer resins commonly exhibit this behavior. (Example: toothpaste is shear-thinning. It does not flow when left alone, but when squeezed with increased force, it flows more readily.)
Silicon: A non-metallic element occurring extensively in the earth’s crust in silica and silicates. Silicon is the basis for the junction found in most semiconductor devices.
Solder: A mixture of metals that is used to connect electronic devices to the copper patterns on a printed circuit board.
Solvent Resistance: The ability of thermal management products to resist swelling when exposed to organic solvents such as degreasing solvents, hydraulic fluids, coolants and jet fuel.
Specific Gravity: The ratio of the density of a substance to the density of water. The specific gravity of water is 1 at standard condition temperature and pressure.
Specific Heat: The amount of heat per unit mass required to raise the temperature by one degree Celsius. (See Heat Capacity.)
Steel Mill Die: A die cutting tool of moderate cost, cast from steel. It is used for high speed cutting.
Steel Rule Die: A low cost die cutting tool manufactured by shaping sharpened steel foil to the desired shape and fixing in a plywood and steel rule metal. It is used for short runs.
Surface Finish: A measure of the roughness of a surfaces, usually expressed in units of micro-inches.
Swelling: A phenomenon that results when an elastomer is exposed to a degreasing solvent and the elastomer absorbs the solvent. The volume of the elastomer increases and its physical strength is greatly reduced. In this swollen state, the elastomer can be easily damaged and should not be subjected to any mechanical stress until the elastomer has been dried.
Tear Strength: A measure of the ability of a material to withstand tearing/ ripping stresses. It is usually measured in pounds force per inch of thickness.
Temperature: A measure of the average kinetic energy of a material. The standard unit of temperature is a Kelvin (K). Temperature determines the direction of heat flow between any two systems in thermal contact. Heat will always flow from the area of higher temperature (T source) to one of lower temperature (T sink).
Temperature Gradient (∆T): The difference in temperatures in the direction of the heat flow between two points in a system.
Tensile Strength: A measure of the ability of a material to withstand a tension (pulling apart) force. It is usually measured in MPa or psi of material cross section.
Thermal Conductivity (K): A quantitative measure of the ability of a material to conduct heat expressed in units of W/m-K.
Thermal Contact Resistance (Ri): The resistance to the flow of heat caused by interstitial air trapped in the irregularities of between contacting solid surfaces. Units are K-cm2 /W.
Thermogravimetric Analysis: Chemical analysis by the measurement of weight changes of a system or compound as a function of increasing temperature.
Thermal Impedance (θ): Thermal impedance is the sum of the thermal resistance of an interface material and the thermal resistances at the interfaces in contact with the material. K-in2 /Watt.
Thermal Interface Materials (TIMs): Materials that are inserted between two contacting solid surfaces and aid heat flow by eliminating gaps between the irregular surfaces. Interstitial air is replaced by material that is significantly more conductive than air.
Thermal Resistivity: The quantitative measure of a material’s resistance to the conduction of heat. (It is the inverse of thermal conductivity.)
Thermocouple: A thermoelectric device consisting of two dissimilar metallic wires fused into a bead which generates a voltage proportional to the temperature of the bead.
Thixotropy: A characteristic of a fluid whereby the fluid’s viscosity decreases as a function of time at a fixed shear rate. Viscosity tends to re-build with time as the shear stress is reduced. (Example: gels and colloids are often thixotropic. The longer they are shaken in a can, the more readily they flow)
Tolerance: The permissible variations in the dimensions or other characteristic of a part or substance.
Torque: A turning or twisting that is equal to the value of the force (f) multiplied by the rotational distance over which it is applied (usually measured in ft-lbs.).
Viscoelastic material: A material whose response to a deforming load combines both viscous (does not recover its original shape/ size when load removed) and elastic (will recover size/shape when load removed) qualities. The common name for such a material is “plastic.”
Volume Resistivity: A measure of a material’s inherent electrical resistance expressed as ohm-cm.
Watt: An SI unit of power equal to one joule per second.