MPC8245 Integrated Processor Hardware Specifications, Rev. 10
Freescale Semiconductor
55
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 (
Ψ
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)
Ψ
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.