MPC852T datasheet(9/80 Pages) FREESCALE

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MPC852T Hardware Specifications, Rev. 3.1

Freescale Semiconductor

Thermal Calculation and Measurement

The junction-to-ambient thermal resistance is an industry standard value that provides a quick and easy estimation

of thermal performance. However, the answer is only an estimate; test cases have demonstrated that errors of a factor

of two (in the quantity TJ-TA) are possible.

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:

R JA = R JC + R CA

where:

R JA = junction-to-ambient thermal resistance (ºC/W)

R JC = junction-to-case thermal resistance (ºC/W)

R CA = case-to-ambient thermal resistance (ºC/W)

R JC is device-related and cannot be influenced by the user. The user adjusts the thermal environment to affect the

case-to-ambient thermal resistance, R CA. 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 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. 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:

TJ = TB +(R JB x PD)

where:

R JB = 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 vias attaching the thermal balls to the ground plane.

7.4 Estimation Using Simulation

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.

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