Thermal Calculation and Measurement
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 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
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:
T = T + (R
× P )
D
J
B
θJB
where:
R
= junction-to-board thermal resistance (ºC/W)
θJB
T = board temperature ºC
B
P = power dissipation in package
D
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.
7.5 Experimental Determination
To determine the junction temperature of the device in the application after prototypes are available, the thermal
characterization parameter (Ψ ) can be used to determine the junction temperature with a measurement of the
JT
temperature at the top center of the package case using the following equation:
T = T + (Ψ × P )
J
T
JT
D
where:
Ψ
= thermal characterization parameter
JT
T = thermocouple temperature on top of package
T
P = power dissipation in package
D
The thermal characterization parameter is measured per the JESD51-2 specification published by JEDEC using a 40
gauge type T thermocouple epoxied to the top center of the package case. The thermocouple should be positioned
so that the thermocouple junction rests on the package. A small amount of epoxy is placed over the thermocouple
junction and over about 1 mm of wire extending from the junction. The thermocouple wire is placed flat against the
package case to avoid measurement errors caused by the cooling effects of the thermocouple wire.
MPC875/MPC870 Hardware Specifications, Rev. 3.0
12
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
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