14
MPC862/857T/857DSL Hardware Specifications
For More Information On This Product,
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MOTOROLA
Thermal Calculation and Measurement
If the board temperature is known, an estimate of the junction temperature in the environment can be made
using the following equation:
T
J
= T
B
+(R
θ
JB
x
P
D
)
where:
R
θ
JB
= junction-to-board thermal resistance (oC/W)
T
B
= board temperature (oC)
P
D
= 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 (
Ψ
JT
) 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:
T
J
= T
T
+(
Ψ
JT
x
P
D
)
where:
Ψ
JT
= thermal characterization parameter
T
T
= thermocouple temperature on top of package
P
D
= 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 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.
7.6
References
Semiconductor Equipment and Materials International
805 East Middlefield Rd.
Mountain View, CA 94043
(415) 964-5111
MIL-SPEC and EIA/JESD (JEDEC) Specifications
(Available from Global Engineering Documents)
800-854-7179 or
303-397-7956
JEDEC Specifications
http://www.jedec.org
F
Freescale Semiconductor, Inc.
n
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