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CHAPTER 3: ELECTRICAL SPECIFICATIONS
θjc is the junction-to-case thermal resistance
θint is the adhesive or interface material thermal resistance
θsa is the heat sink base-to-ambient thermal resistance
P d is the power dissipated by the device
During operation, the die-junction temperatures (T j ) should be maintained less than the
value specified in Table 36. The temperature of the air cooling the component greatly
depends upon the ambient inlet air temperature and the air temperature rise within the
electronic cabinet. An electronic cabinet inlet-air temperature (T a ) may range from 30° to
40°C. The air temperature rise within a cabinet (T r ) may be in the range of 5° to 10°C. The
thermal resistance of the thermal interface material (θint ) is typically about 1.5°C/W. For
example, assuming a T a of 30°C, a T r of 5°C, a CBGA package θjc = 0.1, and a maximum
power consumption (P d ) of 7.5 W, the following expression for T j is obtained:
Die-junction temperature: T j = 30°C + 5°C + (0.1°C/W + 1.5°C/W + θsa ) x 7.5 W
For this example, a θsa value of 10.4°C/W or less is required to maintain the die junction
temperature below the maximum value of Table 36.
Though the die junction-to-ambient and the heat sink-to-ambient thermal resistances are
a common figure-of-merit used for comparing the thermal performance of various
microelectronic packaging technologies, one should exercise caution when only using
this metric in determining thermal management because no single parameter can
adequately describe three-dimensional heat flow. The final die-junction operating
temperature is not only a function of the component-level thermal resistance, but the
system-level design and its operating conditions. In addition to the component's power
consumption, a number of factors affect the final operating die-junction
temperature—airflow, board population (local heat flux of adjacent components), heat
sink efficiency, heat sink attach, heat sink placement, next-level interconnect technology,
system air temperature rise, altitude, etc.
Due to the complexity and the many variations of system-level boundary conditions for
today's microelectronic equipment, the combined effects of the heat transfer
mechanisms (radiation, convection,and conduction) may vary widely. For these reasons,
we recommend using conjugate heat transfer models for the board, as well as
system-level designs.
C3EN
FREESCALE [ Freescale ]
