MPC855 datasheet(11/76 Pages) MOTOROLA

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MOTOROLA

MPC860 Family Hardware Speciﬁcations

Estimation with Junction-to-Board Thermal Resistance

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 7-1.

Figure 7-1. Effect of Board Temperature Rise on Thermal Behavior

100

2 04 06 0

8 0

Board Temperture Rise Above Ambient Divided by Package

Board Temperature Rise Above Ambient Divided by Package Power

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