PXD20 Datasheet, PDF (68/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

Electrical characteristics
Table 14. Thermal characteristics for 416-pin TEPBGA¹ (continued)
Symbol
C
Parameter
Conditions
Value Unit SpecID
_JT
CC D Junction to Package Top Natural Convec-
—
tion⁵
2 °C/W D3.21
1 _{Thermal characteristics are targets based on simulation that are subject to change per device characterization.}
2 _{Junction-to-Ambient Thermal Resistance determined per JEDEC JESD51-3 and JESD51-6. Thermal test board meets}
JEDEC specification for this package.
3 _{Junction-to-Board thermal resistance determined per JEDEC JESD51-8. Thermal test board meets JEDEC specification for}
the specified package.
4 _{Junction-to-Case at the top of the package determined using MIL-STD 883 Method 1012.1. The cold plate temperature is used}
for the case temperature. Reported value includes the thermal resistance of the interface layer.
5 _{Thermal characterization parameter indicating the temperature difference between the package top and the junction}
temperature per JEDEC JESD51-2. When Greek letters are not available, the thermal characterization parameter is written
as Psi-JT.
4.5.1 General notes for specifications at maximum junction temperature
An estimate of the chip junction temperature, T_J, can be obtained from the equation:
T_J = T_A + (R_JA * P_D)
Eqn. 1
where:
T_A= ambient temperature for the package (^oC)
R_JA= junction to ambient thermal resistance (^oC/W)
P_D= power dissipation in the package (W)
The thermal resistance values used are based on the JEDEC JESD51 series of standards to provide consistent values for
estimations and comparisons. The difference between the values determined for the single-layer (1s) board compared to a
four-layer board that has two signal layers, a power and a ground plane (2s2p), demonstrate that the effective thermal resistance
is not a constant. The thermal resistance depends on the:
• Construction of the application board (number of planes)
• Effective size of the board which cools the component
• 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/cm2
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.
PXD20 Microcontroller Data Sheet, Rev. 2
68
Preliminary—Subject to Change Without Notice
Freescale Semiconductor