MPC7450EC datasheet(47/52 Pages) MOTOROLA

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MPC7450 RISC Microprocessor Hardware Speciﬁcations

System Design Information

Thermagon Inc.

888-246-9050

3256 West 25th Street

Cleveland, OH 44109-1668

Internet: www.thermagon.com

Loctite Corporation

860-571-5100

1001 Trout Brook Crossing

Rocky Hill, CT 06067-3910

Internet: www.loctite.com

The following section provides a heat sink selection example using one of the commercially available heat

sinks.

1.9.9.3

Heat Sink Selection Example

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

jc is the junction-to-case thermal resistance

int is the adhesive or interface material thermal resistance

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 4. The temperature of the air cooling the component greatly depends upon 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 (

int) is typically about

1.5°C/W. For example, assuming a Ta of 30°C, a Tr of 5°C, a CBGA package θjc = 0.1, and a typical power

consumption (Pd) of 13.0 W, the following expression for Tj is obtained:

Die-junction temperature:

Tj = 30°C + 5°C + (0.1°C/W + 1.5°C/W + θsa) × 13.0 W

For this example, a

sa value of 3.7°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 heat ﬂux of adjacent

components), heat sink efﬁciency, heat sink attach, heat sink placement, next-level interconnect technology,

system air temperature rise, altitude, etc.

Due to the complexity and the many variations of system-level boundary conditions for today's

microelectronic equipment, the combined effects of the heat transfer mechanisms (radiation, convection,

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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