Thermals: CHO-THERM® – General Information
Critical Performance Characteristics
There are generally two objectives that must be satisfied in the interface between power semiconductor devices and their heat sinks:
- To Enhance the flow of heat from the device to the metal heat sink.
- To electrically isolate the device from the metal heat sink.
CHO-THERM® insulators perform both functions simultaneously, effectivly replacing the commonly used combination of ceramic insulators and silicone grease usually required to achieve these objectives in high performance applications.
Thermal Interface Impedance
The thermal performance of interface materials is generally characterized by the thermal impedance across the interface in C-in2/watt. The lower the value of thermal interface impedance, the better the thermal performance.
The thermal impedance of an interface depends greatly on a number of different parameters, including the flatness and smoothness of the mating surfaces forming the interface and the contact pressure between them, as well as the thickness of the interface material, its thermal conductivity and conformability.
Improvement In Thermal Impedance With Time
The thermal impedance characteristics of CHO-THERM® materials can be expected to improve during use due to stress relaxation of the elastomer and consequent addition filling of the microscopic viods in the interface. Improvement can be as much as 10-15% after the first few weeks of use
Voltage Breakdown Charcteristics
When using thermal interface pads to electrically isolate a component from a metal heat sink or chassis, the critical material property for the pad is its dielectric strength. Dielectric strength is a measurment of how well a material can prevent the voltage on the component case from acring through the material and allowing an electrical short cicuit between the components and the metal mounting surface. This property is commonly presented as the voltage breakdown shown in the Typical P, and is determined by electrical testing of multiple flat sheet samples in accordance with the test procedures detailed in ASTM D149. The higher the value of voltage breakdown, the better the material is at withstanding applied voltages.
The dielectric strength of a material can also be affected by many external factors including: insulator thickness, area of the contact surfaces, temperature, humidity, mechanical stress applied to the insulator, the presence of partial discharge, etc.
Contact Pressure & Mounting Torque
The optimum contact pressure range for CHO-THERM® materials is 300-500psi. Beyond this range, thermal parformance gains are negligible. To convert mounting torque into contact pressure, use the following equation:
P = (T) (N) / (0.2) (D) (A)
P =
Contact pressure
T =
Torque
N =
Number of Fasteners
(0.2) =
Average Friction Factor
D =
Diameter
A =
Contact Area
