
118 Heat Transfer: Thermal Management of Electronics
Example 6.2
Consider a 40 mm × 40 mm × 2 mm package that is exposed to laminar forced
convection airflow on its case. The junction-to-case, junction-to-board, and
board-to-air thermal resistances of this package are 4°C/W, 10°C/W, and 25°C/W,
respectively, and it dissipates 10 W. What is the minimum air velocity to keep the
junction temperature of this package below 105°C if the ambient air temperature
is 25°C? Neglect radiation heat transfer rate.
Solution
The air velocity affects case-to-air and therefore junction-to-air thermal resistances
of this package. To keep its junction temperature ...