Thermal Calculation and Measurement
7.3 Estimation with Junction-to-Board Thermal Resistance
A simple package thermal model that 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 3. Effect of Board Temperature Rise on Thermal Behavior
If the board temperature is known, an estimate of the junction temperature in the environment can be made
using the following equation:
T
J
= T
B
+ (R
θJB
×
P
D
)
where:
R
θJB
= junction-to-board thermal resistance (ºC/W)
T
B
= board temperature ºC
P
D
= power dissipation in package
If the board temperature is known and the heat loss from the package case to the air can be ignored,
acceptable predictions of junction temperature can be made. For this method to work, the board and board
mounting must be similar to the test board used to determine the junction-to-board thermal resistance,
namely a 2s2p (board with a power and a ground plane) and vias attaching the thermal balls to the ground
plane.
7.4 Estimation Using Simulation
When the board temperature is not known, a thermal simulation of the application is needed. The simple
two resistor model can be used with the thermal simulation of the application [2], or a more accurate and
complex model of the package can be used in the thermal simulation.
MPC885/MPC880 Hardware Specifications, Rev. 3
Freescale Semiconductor
13
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
