Datasheet

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5
Freescale Semiconductor 47
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
The following section provides a heat sink selection example using one of the commercially available heat
sinks.
9.8.3 Heat Sink Selection Example
For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:
T
j
= T
i
+ T
r
+ (R
θJC
+ R
θint
+ R
θsa
) × P
d
where:
T
j
is the die-junction temperature
T
i
is the inlet cabinet ambient temperature
T
r
is the air temperature rise within the computer cabinet
R
θJC
is the junction-to-case thermal resistance
R
θint
is the adhesive or interface material thermal resistance
R
θsa
is the heat sink base-to-ambient thermal resistance
P
d
is the power dissipated by the device
During operation, the die-junction temperatures (T
j
) should be maintained less than the value specified in
Table 4. 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 (T
i
)
may range from 30° to 40°C. The air temperature rise within a cabinet (T
r
) may be in the range of 5° to
10°C. The thermal resistance of the thermal interface material (R
θint
) is typically about 1.5°C/W. For
example, assuming a T
i
of 30°C, a T
r
of 5°C, an HCTE package R
θJC
= 0.1, and a typical power
consumption (P
d
) of 18.7 W, the following expression for T
j
is obtained:
Die-junction temperature: T
j
= 30°C + 5°C + (0.1°C/W + 1.5°C/W + R
θsa
) × 18.7 W
For this example, a R
θsa
value of 2.1°C/W or less is required to maintain the die junction temperature below
the maximum value of Table 4.
Though the die-junction-to-ambient and the heat-sink-to-ambient thermal resistances are a common
figure-of-merit used for comparing the thermal performance of various microelectronic packaging
technologies, one should exercise caution when only using this metric in determining thermal management
because no single parameter can adequately describe three-dimensional heat flow. The final die-junction
operating temperature is not only a function of the component-level thermal resistance, but the
system-level design and its operating conditions. In addition to the component's power consumption, a
number of factors affect the final operating die-junction temperature—airflow, board population (local