Estimation Using Simulation
If the board temperature is known, an estimate of the junction temperature in the
environment can be made using the following equation:
TJ = TB + (RθJB × PD)
where:
RθJB = junction-to-board thermal resistance (ºC/W)
TB = board temperature (ºC)
PD = power dissipation in package
If the board temperature is known and the heat loss from the package case to the air can be
ignored, acceptable predictions of junction temperature can be made. For this method to
work, the board and board mounting must be similar to the test board used to determine the
junction-to-board thermal resistance, namely a 2s2p (board with a power and a ground
plane) and vias attaching the thermal balls to the ground plane.
7.4 Estimation Using Simulation
When the board temperature is not known, a thermal simulation of the application is
needed. The simple two resistor model can be used with the thermal simulation of the
application [2], or a more accurate and complex model of the package can be used in the
thermal simulation.
7.5 Experimental Determination
To determine the junction temperature of the device in the application after prototypes are
available, the thermal characterization parameter (ΨJT) can be used to determine the
junction temperature with a measurement of the temperature at the top center of the
package case using the following equation:
TJ = TT + (ΨJT × PD)
where:
ΨJT = thermal characterization parameter
TT = thermocouple temperature on top of package
PD = power dissipation in package
The thermal characterization parameter is measured per JEDEC 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.
12
MPC860 Family Hardware Specifications
MOTOROLA
MOTOROLA [ MOTOROLA ]
