Sun Microsystems SME5224AUPA-400 - Manual (Page 20)

20
SME5224AUPA-400 400 MHz CPU, 4.0 MB E-Cache
UltraSPARC ™ -II CPU Module
July 1999 Sun Microsystems, Inc
Temperature Estimating and Measuring Methods
The following methods can be used to estimate air cooling requirements and calculate junction temperature
based on thermo-couple temperature measurements.
Airflow Cooling Measurement Method
The relationship between air temperature and junction temperature is described in the following thermal
equation:
Tj = Ta + [Pd (
θ
jc +
θ
cs +
θ
sa)]
Note: Testing is done with the worst-case power draw, software loading, and ambient air temperature.
Determination of the ambient air temperature (Ta) and the “free-stream” air velocity is required in order to
apply the airflow method. The table "Heatsink-to-Air Thermal Resistance," illustrates the thermal resistance
between the heatsink and air (
θ
sa).
Note that the airflow velocity can be measured using a velocity meter. Alternatively it may be determined by
knowing the performance of the fan that is supplying the airflow. Calculating the airflow velocity is difficult.
It is subject to the interpretation of the term “free-stream.”
Note: The Airflow Cooling Estimate method is an estimate. Use it solely when an approximate value
suffices. Accuracy can only be assured using the Case Temperature measuring method or the
Heatsink Temperature measuring method. Apply these methods to insure a reliable performance.
"Heatsink-to-Air Thermal Resistance," specifies the thermal resistance of the heatsink as a function of the air
velocity.
Air Velocity Specifications
These specifications are recommended for a typical configuration:
Airflow Topside
150 LFM @ 30
°
C up to 2,000 feet, altitude, maximum
300 LFM @ 40
°
C up to 10,000 feet, altitude, maximum
Airflow Bottomside
100 LFM @ 30
°
C up to 2,000 feet, altitude, maximum
150 LFM @ 40
°
C up to 10,000 feet, altitude, maximum
Heatsink-to-Air Thermal Resistance
Air Velocity
(ft/min) [1]
1. Ducted airflow through the heatsinks.
150 200 300 400 500 650 800 1000
θ
SA
(
°
C/W) [2]
2. Airflow direction parallel to the shorter axis of the pin-fin heatsink (1.9"L x 3.6"W x 1.1"H)
1.21 1.05 0.91 0.84 0.78 0.72 0.67 0.64