
66-MHz Electrical Characteristics
MPC561/MPC563 Reference Manual, Rev. 1.2
G-4
Freescale Semiconductor
An estimation of the chip junction temperature, TJ, in °C can be obtained from the equation:
TJ = TA + (RθJA x PD)
where:
TA = ambient temperature (°C)
RθJA = package junction to ambient resistance (°C/W)
PD = power dissipation in package
The junction to ambient thermal resistance is an industry standard value which provides a quick and easy
estimation of thermal performance. Unfortunately, the answer is only an estimate; test cases have
demonstrated that errors of a factor of two are possible. As a result, more detailed thermal characterization
is supplied.
Historically, the thermal resistance has frequently been expressed as the sum of a junction to case thermal
resistance and a case to ambient thermal resistance:
RθJA = RθJC + RθCA
where:
RθJA = junction to ambient thermal resistance (°C/W)
RθJC = junction to case thermal resistance (°C/W)
RθJA = case to ambient thermal resistance (°C/W)
BGA Package Thermal Resistance,
Junction to Board
RθJB
21.2 3,6
°C/W
BGA Package Thermal Resistance,
Junction to Case (top)
RθJT
7.03,7
°C/W
BGA Package Thermal Resistance,
Junction to Package Top, Natural Convection
Ψ
JT
1.68
°C/W
1 Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting site
(board) temperature, ambient temperature, air flow, power dissipation of other components on the board, and
the board thermal resistance.
2 Per SEMI G38-87 and JESD51-2 with the board horizontal.
3 These values are the mean + 3 standard deviations of characterized data.
4 Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting site
(board) temperature, ambient temperature, air flow, power dissipation of other components on the board, and
the board thermal resistance.
5 Per JESD51-6 with the board horizontal.
6 Thermal resistance between the die and the printed circuit board (Four layer (2s2p) board, natural convection).
7 Indicates the thermal resistance between the die and the case top surface as measured by the cold plate
method (MIL SPEC-883 Method 1012.1) with the cold plate temperature used for the case
temperature.
8 Thermal characterization parameter indicating the temperature difference between package top and the
junction temperature per EIA/JESD51-2.
Table G-2. Thermal Characteristics (continued)
Characteristic
Symbol
Value
Unit