Intel X5365 Design Guide - Page 35

Processor Thermal Characterization Parameter Relationships, Equation 2-3.

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Thermal/Mechanical Reference Design 2.3.2 Processor Thermal Characterization Parameter Relationships 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 conditions (heating source, local ambient conditions). A thermal characterization parameter is convenient in that it is calculated using total package power, whereas actual thermal resistance, θ (theta), is calculated using actual power dissipated between two points. Measuring actual power dissipated into the heatsink is difficult, since some of the power is dissipated via heat transfer into the socket and board. Be aware, however, of the limitations of lumped parameters such as Ψ when it comes to a real design. Heat transfer is a three-dimensional phenomenon that can rarely be accurately and easily modeled by lump values. The case-to-local ambient thermal characterization parameter value (ΨCA) 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: Equation 2-3.ΨCA = (TCASE - TLA) / TDP Where: ΨCA = TCASE = TLA = TDP = Case-to-local ambient thermal characterization parameter (°C/W). Processor case temperature (°C). Local ambient temperature in chassis at processor (°C). TDP dissipation (W) (assumes all power dissipates through the integrated heat spreader (IHS)). The case-to-local ambient thermal characterization parameter of the processor, ΨCA, is comprised of ΨCS, the TIM thermal characterization parameter, and of ΨSA, the sink-tolocal ambient thermal characterization parameter: Equation 2-4.ΨCA = ΨCS + ΨSA Where: ΨCS = ΨSA = Thermal characterization parameter of the TIM (°C/W). Thermal characterization parameter from heatsink-to-local ambient (°C/W). ΨCS is strongly dependent on the thermal conductivity and thickness of the TIM between the heatsink and IHS. ΨSA is a measure of the thermal characterization parameter from the bottom of the heatsink to the local ambient air. ΨSA is dependent on the heatsink material, thermal conductivity, and geometry. It is also strongly dependent on the air velocity through the fins of the heatsink. Figure 2-15 illustrates the combination of the different thermal characterization parameters. Quad-Core Intel® Xeon® Processor 5300 Series Thermal/Mechanical Design Guidelines (TMDG) 35

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Quad-Core Intel® Xeon® Processor 5300 Series Thermal/Mechanical Design Guidelines (TMDG)
35
Thermal/Mechanical Reference Design
2.3.2
Processor Thermal Characterization Parameter
Relationships
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 conditions (heating source, local ambient conditions). A thermal
characterization parameter is convenient in that it is calculated using total package
power, whereas actual thermal resistance,
θ
(theta), is calculated using actual power
dissipated between two points. Measuring actual power dissipated into the heatsink is
difficult, since some of the power is dissipated via heat transfer into the socket and
board. Be aware, however, of the limitations of lumped parameters such as
Ψ
when it
comes to a real design. Heat transfer is a three-dimensional phenomenon that can
rarely be accurately and easily modeled by lump values.
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:
Equation 2-3.
Ψ
CA
= (T
CASE
- T
LA
) / TDP
Where:
Ψ
CA
=
Case-to-local ambient thermal characterization parameter (°C/W).
T
CASE
=
Processor case temperature (°C).
T
LA
=
Local ambient temperature in chassis at processor (°C).
TDP
=
TDP dissipation (W) (assumes all power dissipates through the
integrated heat spreader (IHS)).
The case-to-local ambient thermal characterization parameter of the processor,
Ψ
CA
, is
comprised of
Ψ
CS
, the TIM thermal characterization parameter, and of
Ψ
SA
, the sink-to-
local ambient thermal characterization parameter:
Equation 2-4.
Ψ
CA
=
Ψ
CS
+
Ψ
SA
Where:
Ψ
CS
=
Thermal characterization parameter of the TIM (°C/W).
Ψ
SA
=
Thermal characterization parameter from heatsink-to-local ambient
(°C/W).
Ψ
CS
is strongly dependent on the thermal conductivity and thickness of the TIM
between the heatsink and IHS.
Ψ
SA
is a measure of the thermal characterization parameter from the bottom of the
heatsink to the local ambient air.
Ψ
SA
is dependent on the heatsink material, thermal
conductivity, and geometry. It is also strongly dependent on the air velocity through
the fins of the heatsink.
Figure 2-15
illustrates the combination of the different thermal characterization
parameters.