Design Considerations
The junction-to-ambient thermal resistance is an industry-standard value that provides a
quick and easy estimation of thermal performance. Unfortunately, there are two values in
common usage: the value determined on a single-layer board and the value obtained on a
board with two planes. For packages such as the PBGA, these values can be different by
a factor of two. Which value is closer to the application depends on the power dissipated
by other components on the board. The value obtained on a single layer board is
appropriate for the tightly packed printed circuit board. The value obtained on the board
with the internal planes is usually appropriate if the board has low-power dissipation and
the components are well separated.
When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-
case thermal resistance and a case-to-ambient thermal resistance:
RΘJA = RΘJC + RΘCA
Where,
RΘJA = Package junction-to-ambient thermal resistance (°C/W)
RΘJC = Package junction-to-case thermal resistance (°C/W)
RΘCA = Package case-to-ambient thermal resistance (°C/W)
RΘJC is device related and cannot be adjusted. You control the thermal environment to
change the case to ambient thermal resistance, RΘCA. For instance, you can change the
size of the heat sink, the air flow around the device, the interface material, the mounting
arrangement on printed circuit board, or change the thermal dissipation on the printed
circuit board surrounding the device.
To determine the junction temperature of the device in the application when heat sinks
are not used, the thermal characterization parameter (YJT) can be used to determine the
junction temperature with a measurement of the temperature at the top center of the
package case using the following equation:
TJ = TT + (ΨJT x PD)
Where,
TT = Thermocouple temperature on top of package (°C/W)
ΨJT = hermal characterization parameter (°C/W)
PD = Power dissipation in package (W)
The thermal characterization parameter is measured per JESD51–2 specification 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
MC56F8455x Advance Information Data Sheet, Rev. 2, 06/2012.
Freescale Semiconductor, Inc.
59
Preliminary
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