MPC852T datasheet(13/88 Pages) FREESCALE

13 / 88 page

MPC862/857T/857DSL PowerQUICC™ Family Hardware Specifications, Rev. 3

Freescale Semiconductor

Thermal Calculation and Measurement

7.2

Estimation with Junction-to-Case Thermal Resistance

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:

RθJA = RθJC + RθCA

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 3.

Figure 3. Effect of Board Temperature Rise on Thermal Behavior

100

020

Board Temperture Rise Above Ambient Divided by Package

Power

MPC852T Fiches technique(PDF) 13 Page - Freescale Semiconductor, Inc