THERMAL SPECIFICATIONS
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
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.
18 | Sun Microsystems, Inc | July 1999 |
