mpc880 Freescale Semiconductor, Inc, mpc880 Datasheet - Page 13

• Manufacturer Part Number: mpc880
• Description: Mpc885 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
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
Freescale Semiconductor
θJC
is device-related and cannot be influenced by the user. The user adjusts the thermal environment to
4.
R
R
R
θJA
θJC
θCA
Estimation with Junction-to-Case Thermal Resistance
Estimation with Junction-to-Board Thermal Resistance
= junction-to-case thermal resistance (ºC/W)
= junction-to-ambient thermal resistance (ºC/W)
= case-to-ambient thermal resistance (ºC/W)
R
θJA
= R
Figure 4. Effect of Board Temperature Rise on Thermal Behavior
θJC
MPC885/MPC880 PowerQUICC™ Hardware Specifications, Rev. 4
+ R
θCA
θCA
. For instance, the user can change the airflow around
Thermal Calculation and Measurement
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