MPC8640DEC datasheet(118/140 Pages) FREESCALE

118 / 140 page

MPC8640 and MPC8640D Integrated Host Processor Hardware Specifications, Rev. 1

118

Freescale Semiconductor

Thermal

Shin-Etsu MicroSi, Inc.

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10028 S. 51st St.

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Internet: www.microsi.com

Thermagon Inc.

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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.

19.2.3 Heat Sink Selection Example

For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:

Tj = Ti + Tr + (RθJC + Rθint + Rθsa) × Pd

where:

Tj is the die-junction temperature

Ti 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 2. 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 (Ti)

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 0.2°C/W. For

example, assuming a Ti of 30°C, a Tr of 5°C, a package RθJC = 0.1, and a typical power consumption (Pd)

of 43.4 W, the following expression for Tj is obtained:

Die-junction temperature:

Tj = 30°C + 5°C + (0.1°C/W + 0.2°C/W + θsa) × 43.4 W

For this example, a Rθsavalue of 1.32 °C/W or less is required to maintain the die junction temperature

below the maximum value of Table 2.

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

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