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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:
T
= T
+ (R
J
B
where:
T
= board temperature (°C)
B
R
= package junction to board resistance (°C/W)
θJB
P
= power dissipation in package (
D
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 (Ψ
junction temperature with a measurement of the temperature at the top center of the
package case using the following equation:
T
= T
+ (Ψ
J
T
where:
T
= thermocouple temperature on top of package (°C)
T
Ψ
= thermal characterization parameter
JT
P
= power dissipation in package
D
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
one 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.
MOTOROLA
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
x P
)
θJB
D
x P
)
JT
D
Appendix K. Electrical Characteristics
Ω
)
) can be used to determine the
JT
Thermal Characteristics
K-5
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