PXR40 Datasheet, PDF (54/100 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

Electrical characteristics
5.2.1 General notes for specifications at maximum junction temperature
An estimation of the chip junction temperature, T_J, can be obtained from the equation:
T_J = T_A + (R_JA * P_D)
Eqn. 1
where:
T_A = ambient temperature for the package (^oC)
R_JA = junction to ambient thermal resistance (^oC/W)
P_D = power dissipation in the package (W)
The junction to ambient thermal resistance is an industry standard value that provides a quick and easy estimation of thermal
performance. Unfortunately, there are two values in common usage: the value determined on a single layer board and the value
obtained on a board with two planes. For packages such as the TEPBGA, these values can be different by a factor of two. Which
value is closer to the application depends on the power dissipated by other components on the board. The value obtained on a
single layer board is appropriate for the tightly packed printed circuit board. The value obtained on the board with the internal
planes is usually appropriate if the board has low power dissipation and the components are well separated.
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_JA = R_JC + R_CA
Eqn. 2
where:
R_JA = junction to ambient thermal resistance (^oC/W)
R_JC = junction to case thermal resistance (^oC/W)
R_CA = case to ambient thermal resistance (^oC/W)
R_JC is device related and cannot be influenced by the user. The user controls the thermal environment to change the case to
ambient thermal resistance, R_CA. For instance, the user can change the size of the heat sink, the air flow around the device, the
interface material, the mounting arrangement on printed circuit board, or change the thermal dissipation on the printed circuit
board surrounding the device.
To determine the junction temperature of the device in the application when heat sinks are not used, the Thermal
Characterization Parameter (_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 + (_JT x P_D)
Eqn. 3
where:
T_T = thermocouple temperature on top of the package (^oC)
_JT = thermal characterization parameter (^oC/W)
P_D = power dissipation in the package (W)
The thermal characterization parameter is measured per 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.
References:
Semiconductor Equipment and Materials International
3081 Zanker Road
PXR40 Microcontroller Data Sheet, Rev. 1
54
Freescale Semiconductor