Thermal Metrology
Thermal and Mechanical Design Guidelines
27
3
Thermal Metrology
This chapter discusses guidelines for testing thermal solutions, including measuring
processor temperatures. In all cases, the thermal engineer must measure power
dissipation and temperature to validate a thermal solution. To define the performance
of a thermal solution the “thermal characterization parameter”,
Ψ
(“psi”) will be used.
3.1
Characterizing Cooling Performance
Requirements
The idea of a “thermal characterization parameter”,
Ψ
(“psi”), is a convenient way to
characterize the performance needed for the thermal solution and to compare thermal
solutions in identical situations (same heat source and local ambient conditions). The
thermal characterization parameter is calculated using total package power.
Note:
Heat transfer is a three-dimensional phenomenon that can rarely be accurately and
easily modeled by a single resistance parameter like
Ψ
.
The case-to-local ambient thermal characterization parameter value (
Ψ
CA
) is used as a
measure of the thermal performance of the overall thermal solution that is attached to
the processor package. It is defined by the following equation, and measured in units
of °C/W:
Ψ
CA
= (T
C
– T
A
) / P
D
(Equation
1)
Where:
Ψ
CA
= Case-to-local ambient thermal characterization parameter (°C/W)
T
C
= Processor case temperature (°C)
T
A
= Local ambient temperature in chassis at processor (°C)
P
D
= Processor total power dissipation (W) (assumes all power dissipates
through the IHS)
The case-to-local ambient thermal characterization parameter of the processor,
Ψ
CA
, is
comprised of
Ψ
CS
, the thermal interface material thermal characterization parameter,
and of
Ψ
SA
, the sink-to-local ambient thermal characterization parameter:
Ψ
CA
=
Ψ
CS
+
Ψ
SA
(Equation 2)
Where:
Ψ
CS
= Thermal characterization parameter of the thermal interface material
(°C/W)
Ψ
SA
= Thermal characterization parameter from heatsink-to-local ambient
(°C/W)
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