Sun Microsystems SME5224AUPA-400 - Manual (Page 46)

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SME5224AUPA-400 400 MHz CPU, 4.0 MB E-Cache
UltraSPARC ™ -II CPU Module
July 1999 Sun Microsystems, Inc
T HERMAL S PECIFICATIONS
The maximum CPU operating frequency and I/O timing is reduced when the junction temperature (Tj) of the
CPU device is raised. Airflow must be directed to the CPU heatsink to keep the CPU device cool. Correct air-
flow maintains the junction temperature within its operating range. The airflow directed to the CPU is usually
sufficient to keep the surrounding devices on the topside of the module cool, including the SRAMs and clock
circuitry. The cooling of the backside SRAMs is less critical, but still requires airflow according to the specifi-
cations found in the section "Airflow Bottomside," on page 20.
The CPU temperature specification is provided in terms of its junction temperature. It is related to the case
temperature by the thermal resistance of the package and the power the CPU is dissipating.
The case temperature can be measured directly by a thermocouple probe, verifying that the CPU junction
temperature is correctly maintained over the entire operating range of the system. This includes both the
compute load and the environmental conditions for the system. If measuring the case temperature is prob-
lematic, then, measure the heatsink temperature and calculate the junction temperature. Both approaches for
calculating junction temperature are explained in this section. Irrespective of which method is used, accurate
measurement is required.
Two Step Approach to Thermal Design
Step One determines the ducted airflow requirements based on the CPU power dissipation, the thermal char-
acteristics of the CPU package, and the surrounding heatsink assembly.
See "Thermal Definitions and Specifications," on page 19 for the modules specifications. The specifications for
the heatsinks are found in the table "Heatsink-to-Air Thermal Resistance," page 20.
Step Two verifies the cooling effectiveness of the design, by measuring the heatsink or case temperature
and calculating the junction temperature. The junction temperature must not exceed the CPU specification.
In addition, the lower the junction temperature, the higher the system reliability. The CPU temperature
must be verified under a range of system compute loads and system environmental conditions, using one of
the temperature measuring methods described herein.