MPC8245 datasheet(55/68 Pages) FREESCALE

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MPC8245 Integrated Processor Hardware Specifications, Rev. 10

Freescale Semiconductor

System Design

RθJC is device-related and cannot be influenced by the user. The user controls the thermal environment to

change the case-to-ambient thermal resistance, RθCA. For instance, the user can change the size of the heat

sink, the airflow around the device, the interface material, the mounting arrangement on the printed-circuit

board, or the thermal dissipation on the printed-circuit board surrounding the device.

To determine the junction temperature of the device in the application without a heat sink, the thermal

characterization parameter (

JT) measures the temperature at the top center of the package case using the

following equation:

TJ = TT + (ΨJT × PD)

where:

TT = thermocouple temperature atop the package (°C)

JT = thermal characterization parameter (°C/W)

PD = power dissipation in package (W)

The thermal characterization parameter is measured per JESD51-2 specification using a 40-gauge type T

thermocouple epoxied to the top center of the package case. The thermocouple should be positioned so

that the thermocouple junction rests on the package. A small amount of epoxy is placed over the

thermocouple junction and over about 1 mm of wire extending from the junction. The thermocouple wire

is placed flat against the package case to avoid measurement errors caused by cooling effects of the

thermocouple wire.

When a heat sink is used, the junction temperature is determined from a thermocouple inserted at the

interface between the case of the package and the interface material. A clearance slot or hole is normally

required in the heat sink. Minimizing the size of the clearance minimizes the change in thermal

performance caused by removing part of the thermal interface to the heat sink. Because of the experimental

difficulties with this technique, many engineers measure the heat sink temperature and then back-calculate

the case temperature using a separate measurement of the thermal resistance of the interface. From this

case temperature, the junction temperature is determined from the junction-to-case thermal resistance.

In many cases, it is appropriate to simulate the system environment using a computational fluid dynamics

thermal simulation tool. In such a tool, the simplest thermal model of a package that has demonstrated

reasonable accuracy (about 20%) is a two-resistor model consisting of a junction-to-board and a

junction-to-case thermal resistance. The junction-to-case covers the situation where a heat sink is used or

a substantial amount of heat is dissipated from the top of the package. The junction-to-board thermal

resistance describes the thermal performance when most of the heat is conducted to the printed-circuit

board.

7.9

References

Semiconductor Equipment and Materials International

805 East Middlefield Rd.

Mountain View, CA 94043

(415) 964-5111

MIL-SPEC and EIA/JESD (JEDEC) specifications are available from Global Engineering Documents at

800-854-7179 or 303-397-7956.

JEDEC specifications are available on the web at http://www.jedec.org.

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