Thermal
(edge) of the package is approximately the same as the local air temperature near the device. Specifying
the local ambient conditions explicitly as the board temperature provides a more precise description of the
local ambient conditions that determine the temperature of the device.
At a known board temperature, the junction temperature is estimated using the following equation:
T = T + (R
× P )
D
J
A
θJA
where:
T = junction temperature (°C)
J
T = ambient temperature for the package (°C)
A
R
= junction-to-ambient thermal resistance (°C/W)
θJA
P = power dissipation in the package (W)
D
When the heat loss from the package case to the air can be ignored, acceptable predictions of junction
temperature can be made. The application board should be similar to the thermal test condition: the
component is soldered to a board with internal planes.
20.2.3 Experimental Determination of Junction Temperature
To determine the junction temperature of the device in the application after prototypes are available, use
the thermal characterization parameter (Ψ ) to determine the junction temperature and a measure of the
JT
temperature at the top center of the package case using the following equation:
T = T + (Ψ × P )
J
T
JT
D
where:
T = junction temperature (°C)
J
T = thermocouple temperature on top of package (°C)
T
Ψ
= junction-to-ambient thermal resistance (°C/W)
JT
P = power dissipation in the package (W)
D
The thermal characterization parameter is measured per the 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.
20.2.4 Heat Sinks and Junction-to-Case Thermal Resistance
Some application environments require a heat sink to provide the necessary thermal management of the
device. When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case
thermal resistance and a case-to-ambient thermal resistance:
R
= R
+ R
θJA
θJC θCA
MPC8349EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 13
Freescale Semiconductor
76
FREESCALE [ Freescale ]
