66-MHz Electrical Characteristics
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
G-5
RθJC is device related and cannot be influenced. The user controls the thermal environment to change the
case to ambient thermal resistance, RθCA. For instance, the air flow can be changed around the device, add
a heat sink, change the mounting arrangement on printed circuit board, or change the thermal dissipation
on the printed circuit board surrounding the device. This description is most useful for ceramic packages
with heat sinks where about 90% of the heat flow is through the case to the heat sink to ambient. For most
packages, a better model is required.
The simplest thermal model of a package which has demonstrated reasonable accuracy (about 20 percent)
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 will be 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. It has been observed that the
thermal performance of most plastic packages and especially PBGA packages is strongly dependent on the
board. temperature.
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 x PD)
where:
TB = board temperature (°C)
RθJB = package junction to board resistance (°C/W)
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.
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. Consultation on the creation of the
complex model is available.
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 x PD)
where:
TT = thermocouple temperature on top of package (°C)
ΨJT = thermal characterization parameter
PD = power dissipation in package
The thermal characterization parameter is measured per JESD51-2 specification published by JEDEC
using a 40 gauge type-T thermocouple epoxied to the top center of the package case. The thermocouple