Application
Pressure
The pressure required to attach an interface material to a heat
sink or to a microprocessor. Application pressure is typically
measured in pounds per square inch (psi). Contact
Pressure
The pressure between the microprocessor and the heat sink. This
pressure is typically generated and maintained by the heat sink
clips which attach to a socket. Contact pressure is typically measured
in pounds per square inch (psi). Contact
Resistance
In thermal transfer, air equals resistance. Thus, contact resistance
is a theoretical measure of the volume of air voids along the interface
of any two surfaces. These microscopic voids are formed by surface
roughness, surface concavity or the interface material ineffectively
conforming to a component’s surface. This is illustrated
in the magnified section of Diagram 2 Dielectric
Strength
A measure of the voltage required to cause a breakdown of a specific
thickness of interface material. Dielectric strength is typically
expressed in units of volts/mil. Interface Gap
The gap which results between the microprocessor and the heat sink
due to the stack-up of flatness specification tolerances. Two nominally
flat surfaces will always produce an interface gap when placed
together Thermal
Conductivity
The ability of a material to conduct heat after the heat has entered
that material. Thermal conductivity values can be misleading when
used to evaluate thermal interface materials since actual performance
is affected by the contact resistance with both the heat sink and
the microprocessor. Thermal conductivity is typically expressed
in units of W/m-K. Thermal Impedance
A defined parameter which is calculated by dividing the temperature
difference across the interface by the power output of the microprocessor.
Thermal impedance values are quite valuable in thermal management
design since they inherently reflect the impact of contact resistances
on interface performance. Low thermal resistances indicate a system
which dissipates heat effectively. Thermal impedance is typically
expressed in units of °C-in2/W. to
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