Intel Xeon Processor with 800 MHz System Bus Thermal/Mechanical Design Guide

Intel® Xeon™ Processor with 800 MHz System Bus Thermal/Mechanical Design Guidelines 21
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 8. 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).
P
D
= 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 9. 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 2-6 illustrates the combination of the different thermal characterization parameters.