MPC7445 datasheet(57/64 Pages) FREESCALE

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MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1

Freescale Semiconductor

System Design Information

Dow-Corning Corporation

800-248-2481

Dow-Corning Electronic Materials

2200 W. Salzburg Rd.

Midland, MI 48686-0997

Internet: www.dow.com

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:

Tj = Ta + Tr + (RθJC + Rθint + Rθsa) × 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

RθJC is the junction-to-case thermal resistance

Rθint is the adhesive or interface material thermal resistance

Rθsa 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 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 (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 (Rθint) is typically about 1.5°C/W. For example, assuming a Ta of 30°C, a Tr of 5°C,

a CBGA package RθJC = 0.1, and a typical power consumption (Pd) of 15.0 W, the following expression for Tj is

obtained:

Die-junction temperature:

Tj = 30°C + 5°C + (0.1°C/W + 1.5°C/W + Rθsa) × 15 W

For this example, a Rθsa value of 3.1°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,

number

factors affect

the

final

operating

die-junction

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