Intel 5400 Series - Manual (Page 36)

Thermal/Mechanical Reference Design
36 Quad-Core Intel® Xeon® Processor 5400 Series TMDG
Assume the datasheet TDP is 85 W and the case temperature specification is 68 °C.
Assume as well that the system airflow has been designed such that the local processor
ambient temperature is 45°C. Then the following could be calculated using equation
(2-3) from above:
Equation 2-5.Ψ
CA
= (T
CASE
– T
LA
) / TDP = (68 – 45) / 85 = 0.27 °C/W
To determine the required heatsink performance, a heatsink solution provider would
need to determine Ψ
CS
performance for the selected TIM and mechanical load
configuration. If the heatsink solution was designed to work with a TIM material
performing at Ψ
CS
≤ 0.05 °C/W, solving for equation (2-4) from above, the performance
of the heatsink would be:
Equation 2-6.Ψ
SA
= Ψ
CA
− Ψ
CS
= 0.27 − 0.05 = 0.22 °C/W
If the local processor ambient temperature is assumed to be 40°C, the same
calculation can be carried out to determine the new case-to-ambient thermal
resistance:
Equation 2-7.Ψ
CA
= (T
CASE
– T
LA
) / TDP = (68 – 40) / 85 = 0.33 °C/W
It is evident from the above calculations that, a reduction in the local processor
ambient temperature has a significant positive effect on the case-to-ambient thermal
resistance requirement.
2.4.3 Chassis Thermal Design Considerations
2.4.3.1 Chassis Thermal Design Capabilities and Improvements
One of the critical parameters in thermal design is the local ambient temperature
assumption of the processor. Keeping the external chassis temperature fixed, internal
chassis temperature rise is the only component that can affect the processor local
ambient temperature. Every degree gained at the local ambient temperature directly
translates into a degree relief in the processor case temperature.
Given the thermal targets for the processor, it is extremely important to optimize the
chassis design to minimize the air temperature rise upstream to the processor (T
rise
),
hence minimizing the processor local ambient temperature.
The heat generated by components within the chassis must be removed to provide an
adequate operating environment for both the processor and other system components.
Moving air through the chassis brings in air from the external ambient environment and
transports the heat generated by the processor and other system components out of
the system. The number, size and relative position of fans, vents and other heat
generating components determine the chassis thermal performance, and the resulting
ambient temperature around the processor. The size and type (passive or active) of the
thermal solution and the amount of system airflow can be traded off against each other
to meet specific system design constraints. Additional constraints are board layout,
spacing, component placement, and structural considerations that limit the thermal
solution size.
In addition to passive heatsinks, fan heatsinks and system fans, other solutions exist
for cooling integrated circuit devices. For example, ducted blowers, heat pipes and
liquid cooling are all capable of dissipating additional heat. Due to their varying
attributes, each of these solutions may be appropriate for a particular system
implementation.