Datasheet
• Quality of the thermal and electrical connections to the planes
• Power dissipated by adjacent components
Connect all the ground and power balls to the respective planes with one via per ball.
Using fewer vias to connect the package to the planes reduces the thermal performance.
Thinner planes also reduce the thermal performance. When the clearance between the
vias leave the planes virtually disconnected, the thermal performance is also greatly
reduced.
As a general rule, the value obtained on a single-layer board is within the normal range
for the tightly packed printed circuit board. The value obtained on a board with the
internal planes is usually within the normal range if the application board has:
• One oz. (35 micron nominal thickness) internal planes
• Components are well separated
• Overall power dissipation on the board is less than 0.02 W/cm
2
The thermal performance of any component depends on the power dissipation of the
surrounding components. In addition, the ambient temperature varies widely within the
application. For many natural convection and especially closed box applications, the
board temperature at the perimeter (edge) of the package is approximately the same as the
local air temperature near the device. Specifying the local ambient conditions explicitly
as the board temperature provides a more precise description of the local ambient
conditions that determine the temperature of the device.
At a known board temperature, the junction temperature is estimated using the following
equation:
where:
T
B
= board temperature for the package perimeter (°C)
R
ΘJB
= junction-to-board thermal resistance (°C/W) per JESD51-8
P
D
= power dissipation in the package (W)
When the heat loss from the package case to the air does not factor into the calculation,
the junction temperature is predictable if the application board is similar to the thermal
test condition, with the component soldered to a board with internal planes.
The thermal resistance is expressed as the sum of a junction-to-case thermal resistance
plus a case-to-ambient thermal resistance:
Package information
MPC5777C Microcontroller Data Sheet Data Sheet, Rev. 13, 08/2018.
NXP Semiconductors 85