System Design Information
Dow-Corning Corporation
Dow-Corning Electronic Materials
2200 W. Salzburg Rd.
800-248-2481
Midland, MI 48686-0997
Internet: www.dow.com
Shin-Etsu MicroSi, Inc.
10028 S. 51st St.
Phoenix, AZ 85044
888-642-7674
888-246-9050
Internet: www.microsi.com
Thermagon Inc.
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 = T + T + (R
+ R
+ R ) × P
θint θsa d
j
a
r
θJC
where:
T is the die-junction temperature
j
T is the inlet cabinet ambient temperature
a
T is the air temperature rise within the computer cabinet
r
R
R
R
is the junction-to-case thermal resistance
θJC
θint
θsa
is the adhesive or interface material thermal resistance
is the heat sink base-to-ambient thermal resistance
P is the power dissipated by the device
d
During operation, the die-junction temperatures (T ) should be maintained less than the value specified in Table 4.
j
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 ) may range from 30°
a
to 40°C. The air temperature rise within a cabinet (T ) may be in the range of 5° to 10°C. The thermal resistance of
r
the thermal interface material (R ) is typically about 1.5°C/W. For example, assuming a T of 30°C, a T of 5°C,
θint
a
r
a CBGA package R
obtained:
= 0.1, and a typical power consumption (P ) of 15.0 W, the following expression for T is
θJC
d
j
Die-junction temperature: T = 30°C + 5°C + (0.1°C/W + 1.5°C/W + R ) × 15 W
j
θsa
For this example, a R value of 3.1°C/W or less is required to maintain the die-junction temperature below the
θsa
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
MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1
Freescale Semiconductor
57
FREESCALE [ Freescale ]
