Sun Microsystems SME5224AUPA-400 manual Temperature Estimating and Measuring Methods, Air Velocity

UltraSPARC™-II CPU Module

SME5224AUPA-400400 MHz CPU, 4.0 MB E-Cache

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 (θjc + θcs + θsa)]

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 (θsa).

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.

Heatsink-to-Air Thermal Resistance

Air Velocity
(ft/min) [1]	150	200	300	400	500	650	800	1000
θSA (°C/W) [2]	1.21	1.05	0.91	0.84	0.78	0.72	0.67	0.64

1.Ducted airflow through the heatsinks.

2.Airflow direction parallel to the shorter axis of the pin-fin heatsink (1.9"L x 3.6"W x 1.1"H)

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

20 Sun Microsystems, Inc July 1999