Intel Pentium 4 Processor on 90 nm Process Thermal and Mechanical Design Guidelines

Thermal Requirements

18 Intel

Pentium

4 on 90 nm Process Thermal Design Guide

to TDP instead of maximum power. Thermal Monitor can protect the processor in rare excursions

of workload above TDP. Implementation options and recommendations are described in Section

3.4.

3.2.2.3 Omni Directional Airflow

Intel recommends that the heatsink exhaust air in all directions parallel to the motherboard, thus,

allowing airflow in the direction of the memory, chipset, and voltage regulator components.

Airflow speed may be difficult to determine; however, it is suggested that the low fan set point

flow rate be greater than 150 lfm at board level upstream from the fore mentioned components.

Using the exhaust air from the heatsink may provide a cost effective option for system thermal

designers in lieu of additional hardware or fans. Of course, the efficiency of the shared airflow is

dependant on many board and system variables (such as, board layout, air velocity profile, air

speed, air temperature, chassis configuration, flow obstructions, and other tangible and intangible

variables).

Figure 2. Heatsink Exhaust Providing Platform Subsystem Cooling

Tri

USB

LAN &

2x USB

Serial

Parallel

GMCH

SIO

FWH

ICH

USB

LAN

Heatsink Airflow Exhaust

3.2.3 Characterizing Cooling Performance Requirements

The idea of a “thermal characterization parameter” Ψ (psi) is a convenient way to characterize the

performance needed for the thermal solution and to compare thermal solutions in identical

situations (same heating source and local ambient conditions). A thermal characterization

parameter is convenient in that it is calculated using total package power, whereas actual thermal

resistance, Θ (theta), is calculated using actual power dissipated between two points. Measuring

actual power dissipated into the heatsink is difficult since some of the power is dissipated via heat

transfer into the socket and board. Be aware, however, of the limitations of lumped parameters

such as Ψ in a real design. Heat transfer is a three-dimensional phenomenon that can rarely be

accurately and easily modeled by lump values.