MPC875 Freescale Semiconductor, Inc, MPC875 Datasheet - Page 13

• Manufacturer Part Number: MPC875
• Description: Mpc875 Powerquicc Integrated Communications Processor
• Manufacturer: Freescale Semiconductor, Inc

7.2
Historically, thermal resistance has frequently been expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance:
where:
R
affect the case-to-ambient thermal resistance, R
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
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
thermal resistance 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. If the board temperature is known, an estimate of the junction temperature in the
environment can be made using the following equation:
where:
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
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.
Freescale Semiconductor
θJC
is device-related and cannot be influenced by the user. The user adjusts the thermal environment to
R
R
R
R
T
P
D
B
θJA
θJC
θCA
θJB
Estimation with Junction-to-Case Thermal Resistance
Estimation with Junction-to-Board Thermal Resistance
Estimation Using Simulation
= board temperature (°C)
= power dissipation in package
= junction-to-case thermal resistance (°C/W)
= junction-to-board thermal resistance (°C/W)
= junction-to-ambient thermal resistance (°C/W)
= case-to-ambient thermal resistance (°C/W)
R
T
θJA
J
= T
= R
B
+ (R
θJC
MPC875/MPC870 PowerQUICC™ Hardware Specifications, Rev. 4
θJB
+ R
θCA
× P
D
)
θCA
. For instance, the user can change the airflow around
Thermal Calculation and Measurement
13