Intel Pentium 4 Processor In the 423-pin Package Thermal Design Guidelines

Pentium® 4 processor in the 423-pin package Thermal Design Guidelines

5 DESIGNING FOR THERMAL PERFORMANCE

In designing for thermal performance, the goal is to keep the processor within the operational thermal specifications.

Failure to do so will shorten the life of the processor and potentially cause erratic system behavior. The thermal

design is required to ensure these operational thermal specifications are maintained. The heat generated by

components within the chassis must be removed to provide an adequate operating environment for both the

processor and other system components. Moving air through the chassis transports the heat generated by the

processor and other system components out of the system, while bringing in air from the external ambient

environment.

5.1 Airflow Management

It is important to manage the amount of air that flows within the system, as well as how it flows, to maximize the

amount of cool air that flows over the processor. System airflow can be increased by adding one or more fans to the

system, or by increasing the output (increasing the speed or size) of an existing system fan(s). Managing the airflow

direction using baffles or ducts can also increase local airflow. Heating effects from chipset, voltage regulators, add-

in boards, memory, and disk drives greatly reduce the cooling efficiency of this air, as does re-circulation of warm

interior air through the system fan. Care must be taken to minimize the heating effects of other system components,

and to eliminate warm air re-circulation.

If no air path exists across the processor, the warm air from the processor will not be removed from the system,

resulting in localized heating ("hot spots") around the processor. Heat sink fins passive thermal solution designs

should be aligned with the direction of airflow. If the airflow is horizontal the fins should be oriented horizontally.

Similarly, for a vertical airflow, the heat sink fins should be oriented vertically.

Figure 2 shows two examples of air exchange through a PC style chassis. The system on the left is an example of

good air exchange incorporating both the power supply fan as well as an additional system fan. The system on the

right shows a poorly vented system using only the power supply fan to move the air, resulting in inadequate airflow.

Drive Bays Drive Bays

Vents

ents

Power

Supply

Power

Supply

Fan Fan

Good CPU Airflow

Poor CPU Airflow

IO Cards

Good Venting = Good Air Exchange Poor Venting = Poor Air Exchange

FAN

Proc.

IO Cards

000967

Figure 2. Example of Air Exchange through a PC Chassis

5.2 Bypass

Bypass is the distance around the heat sink where air may travel without passing through the fins of the heat sink. A

heat sink will have infinite bypass if it is sitting in free space. A heat sink which has a duct or other devices

surrounding it which are 0.2” (5.1mm) away from the outer edges of the heat sink has a bypass of 0.2” (5.1mm). A

smaller bypass forces more air to pass through the fins of the heat sink, rather than around the heat sink. This is

especially important as the heat sink fin density increases. The higher the fin density, the more resistance the heat