Guide

Intel
®
Celeron
®
D Processor for Embedded Applications Thermal Design Guide 23
Characterizing Cooling Performance Requirements
3.0 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 (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 Equation 1 and measured in units of °C/W:
Equation 1. Case-to-Local Ambient Thermal Characterization Parameter Value (1)
Where:
Ψ
CA
= Case-to-local ambient thermal characterization parameter (°C/W)
T
C
= Processor case temperature (°C)
T
A
= Local ambient temperature (T
LA
)in chassis at processor (°C)
P
D
= Processor total power distribution (W). Assumes all power dissipates through the IHS
The case-to-local ambient thermal characterization parameter of the processor, Ψ
CA
, is composed
of Ψ
CS
, the thermal interface material thermal characterization parameter, and of Ψ
SA
, the
sink-to-local ambient thermal characterization parameter in Equation 2:
Equation 2. Case-to-Local Ambient Thermal Characterization Parameter Value (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)
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.
ψ
CA
T
C
T
A
–()P
D
⁄=
ψ
CA
ψ
CS
ψ
SA
+=