
18
SME5224AUPA-360
360 MHz CPU, 4.0 MB E-Cache
UltraSPARC-II CPU Module
July 1999
Sun Microsystems, Inc
THERMAL SPECIFICATIONS
The maximum CPU operating frequency and I/O timing is reduced when the junction temperature (Tj) of the
CPU device is raised. Airow must be directed to the CPU heatsink to keep the CPU device cool. Correct air-
ow maintains the junction temperature within its operating range. The airow directed to the CPU is usually
sufcient 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 airow according to the speci-
The CPU temperature specication 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 airow requirements based on the CPU power dissipation, the thermal char-
acteristics of the CPU package, and the surrounding heatsink assembly.
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 tem-
perature measuring methods described herein.