
56
MPC8241 Integrated Processor Hardware Specifications
MOTOROLA
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
System Design Information
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 is important to minimize the change in
thermal performance that is caused by removing part of the thermal interface to the heat sink. Considering
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
where 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.
7.8 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.