MPC755 RISC Microprocessor Hardware Specifications, Rev. 8
48
Freescale Semiconductor
System Design Information
Shin-Etsu MicroSi, Inc.
888-642-7674
10028 S. 51st St.
Phoenix, AZ 85044
Internet: www.microsi.com
Thermagon Inc.
888-246-9050
4707 Detroit Ave.
Cleveland, OH 44102
Internet: www.thermagon.com
8.8.3
Heat Sink Selection Example
This section provides a heat sink selection example using one of the commercially-available heat sinks.
For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:
Tj = Ta + Tr + (θjc + θint + θsa) × Pd
where:
Tj is the die-junction temperature
Ta is the inlet cabinet ambient temperature
Tr is the air temperature rise within the computer cabinet
θ
jc is the junction-to-case thermal resistance
θ
int is the adhesive or interface material thermal resistance
θ
sa is the heat sink base-to-ambient thermal resistance
Pd is the power dissipated by the device
During operation the die-junction temperatures (Tj) should be maintained less than the value specified in
Table 3. The temperature of air cooling the component greatly depends on the ambient inlet air temperature
and the air temperature rise within the electronic cabinet. An electronic cabinet inlet-air temperature (Ta)
may range from 30° to 40°C. The air temperature rise within a cabinet (Tr) may be in the range of 5° to
10°C. The thermal resistance of the thermal interface material (
θ
int) is typically about 1°C/W. Assuming
a Ta of 30°C, a Tr of 5°C, a CBGA package Rθjc < 0.1, and a power consumption (Pd) of 5.0 W, the
following expression for Tj is obtained:
Die-junction temperature:
Tj = 30°C + 5°C + (0.1°C/W + 1.0°C/W + θsa) × 5.0 W
For a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (
θ
sa) versus airflow
Assuming an air velocity of 0.5 m/s, we have an effective Rsa of 7°C/W, thus
Tj = 30°C + 5°C + (0.1°C/W + 1.0°C/W + 7°C/W) × 5.0 W,
resulting in a die-junction temperature of approximately 76°C which is well within the maximum
operating temperature of the component.
Other heat sinks offered by Aavid Thermalloy, Alpha Novatech, The Bergquist Company, IERC, Chip
Coolers, and Wakefield Engineering offer different heat sink-to-ambient thermal resistances, and may or
may not need airflow.