Estimation with Junction-to-Board Thermal Resistance
where:
RθJA = junction-to-ambient thermal resistance (ºC/W)
RθJC = junction-to-case thermal resistance (ºC/W)
RθCA = case-to-ambient thermal resistance (ºC/W)
RθJC is device related and cannot be influenced by the user. The user adjusts the thermal
environment to affect the case-to-ambient thermal resistance, RθCA. For instance, the user
can change the air flow around the device, add a heat sink, change the mounting
arrangement on the printed circuit board, or change the thermal dissipation on the printed
circuit board surrounding the device. This thermal model is most useful for ceramic
packages with heat sinks where some 90% of the heat flows through the case and the heat
sink to the ambient environment. For most packages, a better model is required.
7.3 Estimation with Junction-to-Board Thermal
Resistance
A simple package thermal model which has demonstrated reasonable accuracy (about 20%)
is a two resistor model consisting of a junction-to-board and a junction-to-case thermal
resistance. The junction-to-case covers the situation where a heat sink is used or where a
substantial amount of heat is dissipated from the top of the package. The 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; see Figure 7-1.
100
9 0
8 0
7 0
6 0
5 0
4 0
3 0
2 0
1 0
0
0
2 0
4 0
6 0
8 0
Board Temperature Rise Above Ambient Divided by Package Power
Board Temperture Rise Above Ambient Divided by Package
Figure 7-1. Effect of Board Temperature Rise on Thermal Behavior
MOTOROLA
MPC860 Family Hardware Specifications
11
MOTOROLA [ MOTOROLA ]
