MPC855T datasheet(10/80 Pages) FREESCALE

10 / 80 page

MPC860 Family Hardware Specifications, Rev. 7

Freescale Semiconductor

Thermal Calculation and Measurement

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 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,

especially PBGA packages, is strongly dependent on the board temperature; see Figure 1.

Figure 1. 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:

TJ = TB + (RθJB × PD)

where:

RθJB = junction-to-board thermal resistance (ºC/W)

TB = board temperature (ºC)

PD = 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 by attaching the thermal balls to the ground plane.

Board Temperature Rise Above Ambient Divided by Package Power

MPC855T Arkusz danych(PDF) 10 Page - Freescale Semiconductor, Inc