4.34 Consider the thin integrated circuit (chip) of Problem
3.150. Instead of attaching the heat sink to the chip surface, an engineer suggests that sufficient cooling might be achieved by mounting the top of the chip onto a large copper (k = 400 W/m · K) surface that is located nearby. The metallurgical joint between the chip and the substrate provides a contact resistance of
Rt”,c = 5 X 10-6 m2 · K/W, and the maximum allowable chip temperature is 85 C. If the large substrate temper- ature is T2 = 25 C at locations far from the chip, what is the maximum allowable chip power dissipation qc?
4.35 An electronic device, in the form of a disk 20 mm in diameter, dissipates 100 W when mounted flush on a large aluminum alloy (2024) block whose tempera- ture is maintained at 27 C. The mounting arrangement is such that a contact resistance of Rt”,c = 5 X 10-5 m2 · K/W exists at the interface between the device and the block.
4.36 The elemental unit of an air heater consists of a long circular rod of diameter D, which is encapsulated by a finned sleeve and in which thermal energy is generated by ohmic heating. The N fins of thickness t and length L are integrally fabricated with the square sleeve of width
w. Under steady-state operating conditions, the rate of thermal energy generation corresponds to the rate of heat transfer to airflow over the sleeve.
