Electrical and Thermal Characteristics
MPC5121E/MPC5123 Data Sheet, Rev. 1
Freescale Semiconductor
23
3.1.6
Thermal Characteristics
3.1.6.1
Heat Dissipation
An estimation of the chip-junction temperature, TJ, can be obtained from the following equation:
TJ =TA+(RθJA × PD)
Eqn. 3
where:
TA = ambient temperature for the package (C)
RθJA = junction to ambient thermal resistance (C/W)
PD = power dissipation in package (W)
The junction to ambient thermal resistance is an industry standard value, which 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 PBGA, these values can be different by a factor of two. Which
value is correct 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.
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:
Table 10. Thermal Resistance Data
Rating
Board Layers
Sym
Value
Unit
SpecID
Junction to Ambient Natural
Convection1,2
1 Junction temperature is a function of die size, on-chip power dissipation, package thermal resistance,
mounting site (board) temperature, ambient temperature, air flow, power dissipation of other
components on the board, and board thermal resistance.
2 Per SEMI G38-87 and JEDEC JESD51-2 with the single layer board horizontal.
Single layer board (1s)
RθJA
30
°C/W
D6.1
Junction to Ambient Natural
3 Per JEDEC JESD51-6 with the board horizontal.
Four layer board (2s2p)
RθJMA
22
°C/W
D6.2
Junction to Ambient (@200
Single layer board (1s)
RθJMA
24
°C/W
D6.3
Junction to Ambient (@200
Four layer board (2s2p)
RθJMA
19
°C/W
D6.4
Junction to Board4
4 Thermal resistance between the die and the printed circuit board per JEDEC JESD51-8. Board
temperature is measured on the top surface of the board near the package.
—RθJB
14
°C/W
D6.5
Junction to Case5
5 Thermal resistance between the die and the case top surface as measured by the cold plate method
(MIL SPEC-883 Method 1012.1).
—RθJC
8°C/W
D6.6
Junction to Package Top6
6 Thermal characterization parameter indicating the temperature difference between package top and
the junction temperature per JEDEC JESD51-2. When Greek letters are not available, the thermal
characterization parameter is written as Psi-JT.
Natural Convection
Ψ
JT
2°C/W
D6.7
Preliminary