Chassis and System Thermal Considerations for Intel® LGA775-Based Processors


This document is written for professional system integrators building PCs that use industry-accepted motherboards, chassis, and peripherals. It provides information and recommendations for thermal management in desktop systems using boxed Intel® desktop processors. (The term "boxed processors" refers to processors packaged for use by system integrators.)

The reader should have a general knowledge of and experience with desktop PC operation, integration, and thermal management. Integrators who follow the recommendations presented can provide their customers with more reliable PCs and will see fewer customers returning with thermal management issues.

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Thermal management

Systems using boxed processors require thermal management. The thermal management refers to two major elements: a heatsink properly mounted to the processor, and effective airflow through the system chassis. The ultimate goal of thermal management is to keep the processor at or below its maximum operating temperature.

Proper thermal management is achieved when heat is transferred from the processor to the system air, which is then vented out of the system. Desktop boxed processors are shipped with a high-quality fan heatsink that effectively transfer processor heat to the system air. It is the responsibility of the system integrator to ensure adequate system airflow.

This document makes recommendations for achieving good system airflow and provides suggestions for improving the effectiveness of a system's thermal management solution.

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Fan heatsink

Boxed processors are shipped in several processor packages:

  • Flip Chip Pin Grid Array 2 (FC-PGA2)
  • Flip Chip Land Grid Array 4 (FC-LGA4)

All boxed Intel processors for desktop systems are shipped with a fan heatsink. Follow the directions in the boxed processor installation notes when using these items. Thermal interface material provides effective heat transfer from the processor to the fan heatsink. Thermal interface material is already applied to the fan heatsink base.

Boxed processors also include an attached fan cable. The fan cable provides power to the fan by connecting to a motherboard-mounted power header. Some boxed processor fan heatsinks provide fan speed information to the motherboard. (Only motherboards with hardware monitoring circuitry can use the fan speed signal.)

Boxed processors use high-quality ball-bearing fans that provide a good local air stream. This local air stream transfers heat from the heatsink to the air inside the system. However, moving heat to the system air is only half the task. Sufficient system airflow is required in order to exhaust the air. Without a steady stream of air through the system, the fan heatsink will recirculate warm air, and therefore may not adequately cool the processor.

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System airflow

System airflow is determined by:

  • Chassis design
  • Chassis size
  • Location of chassis air intake and exhaust vents
  • Power supply fan capacity and venting
  • Location of the processor slot(s)
  • Placement of add-in cards and cables

System integrators must ensure airflow through the system to allow the fan heatsink to work effectively. Proper attention to airflow when selecting subassemblies and building PCs is important for good thermal management and reliable system operation.

Integrators use three basic chassis form factors for desktop systems: ATX or microATX.

In systems using ATX components, airflow is usually from front to back. Air enters the chassis from vents at the front and is drawn through the chassis by the power supply fan and rear chassis fan. The power supply fan exhausts the air through the back of the chassis. Figure 1 shows the airflow.

Intel recommends the use of ATX and microATX form factor motherboards and chassis for boxed processors. The ATX and microATX form factors provide consistency of airflow to the processor and simplify desktop system assembly and upgrade.

ATX thermal management components are different than Baby AT components. In an ATX, the processor is located close to the power supply, rather than close to the front panel of the chassis. Power supplies that blow air out of the chassis provide proper airflow for active fan heatsinks. The boxed processor's active fan heatsink cools the processor more effectively when combined with an exhausting power supply fan. Consequently, the airflow in systems using the boxed processor should flow from the front of the chassis directly across the motherboard and processor and out through the power supply exhaust vents. Boxed processors with chassis that conform to the ATX Specification Revision 2.01 or later are highly recommended. For more information on the ATX form factor, please visit the ATX Web site. A list of ATX chassis manufacturers can also be found on the ATX web site.

ATX tower chassis optimized for the
boxed processor with an Active Fan Heatsink

One difference between microATX chassis and ATX chassis is that the power supply location and type may vary. Thermal management improvements that apply to ATX chassis will also apply to microATX. For more information about the microATX form factor, please visit the microATX Web site. A list of microATX chassis manufacturers can also be found on the microATX web site.

The following list provides guidelines to be used when integrating a system. Specific reference to ATX or microATX components is made where necessary.

  • Chassis vents must be functional and not excessive in quantity: Integrators should be careful not to select chassis that contain only cosmetic vents. Cosmetic vents are designed to look like they allow air into the chassis, but no air (or little air) actually enters. Chassis with excessive air vents should also be avoided. For example, if a Baby AT chassis has large air vents on all sides, then most air enters near the power supply and immediately exits through the power supply or nearby vents. Consequently, very little air flows over the processor and other components. In ATX and microATX chassis, I/O shields must be present. Without shields the I/O opening may allow excessive venting.
  • Vents must be properly located: Systems must have properly located intake and exhaust vents. The best location for vents is one that will allow air to enter the chassis and flow on a path through the system over various components and directly over the processor. Specific location of vents depends upon the type of chassis. In most desktop Baby AT systems, the processor is located near the front, so intake vents on the front panel work best. In Baby AT tower systems, vents on the bottom of the front panel work best. In ATX and microATX systems, vents should be located both at the bottom front and bottom rear of the chassis. Also, in ATX and microATX systems, I/O shields must be installed in order for the chassis to correctly vent air. Lack of an I/O shield may disrupt proper airflow or circulation within the chassis.
  • Power supply airflow direction: The power supply must have a fan that draws air in the proper direction. For most ATX and microATX systems, power supplies that act as an exhaust fan drawing air out of the system work most efficiently with active fan heatsinks. For most Baby AT systems, the power supply fan acts as an exhaust fan, venting system air outside the chassis. Some power supplies have markings noting airflow direction. Ensure the proper power supply is used based upon the system form factor.
  • Power supply fan strength: PC power supplies contain a fan. Depending upon the type of power supply, the fan either draws air into or out of the chassis. If intake and exhaust vents are properly located, the power supply fan can draw enough air for most systems. For some chassis where the processor is running too warm, changing to a power supply with a stronger fan can greatly improve the airflow.
  • Power supply venting: Since almost all air flows through the power supply unit, it must be well vented. Choose a power supply unit with large vents. Wire finger guards for the power supply fan offer much less airflow resistance than openings stamped into the sheet metal casing of the power supply unit. Make sure that floppy and hard drive cables do not block the power supply air vents inside the chassis.
  • System fan - should it be used? Some chassis may contain a system fan (in addition to the power supply fan) to facilitate airflow. A system fan is typically used with passive heatsinks. With fan heatsinks, a system fan can have mixed results. In some situations, a system fan improves system cooling. However, sometimes a system fan recirculates warm air within the chassis, thereby reducing the thermal performance of the fan heatsink. When using processors with fan heatsinks, rather than adding a system fan, it is generally a better solution to change to a power supply with a more powerful fan. Thermal testing both with a system fan and without the fan will reveal which configuration is best for a specific chassis.
  • System fan airflow direction: When using a system fan, ensure that it draws air in the same direction as the overall system airflow. For example, a system fan in a Baby AT system might act as an intake fan, pulling in additional air from the front chassis vents.
  • Protect against hot spots: A system may have a strong airflow, but still contain "hot spots." Hot spots are areas within the chassis that are significantly warmer than the rest of the chassis air. Such areas can be created by improper positioning of the exhaust fan, adapter cards, cables, or chassis brackets and subassemblies blocking the airflow within the system. To avoid hot spots, place exhaust fans as needed, reposition full-length adapter cards or use half-length cards, reroute and tie cables, and ensure space is provided around and over the processor.

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Thermal testing

Differences in motherboards, power supplies, and chassis affect the operating temperature of processors. Thermal testing is highly recommended when starting to use new products and when choosing a new motherboard or chassis supplier. Thermal testing can show integrators if a specific chassis-power supply-motherboard configuration provides adequate airflow for boxed processors.

Testing using the proper thermal measurement tools can validate proper thermal management or demonstrate the need for improved thermal management. Verifying the thermal solution for a specific system allows integrators to minimize test time while incorporating the increased thermal demands of possible future end-user upgrades. Testing a representative system and an "upgraded" system provides confidence that a system's thermal management will be acceptable over the lifetime of the system. Upgraded systems may include extra add-in cards, graphic solutions with higher power requirements, or warmer running hard drives.

Thermal testing should be done on each chassis-power supply-motherboard configuration using the components that dissipate the most power. Variations in processor speed, graphic solutions, etc. do not require additional thermal testing if testing is done with the highest power-dissipating configuration.

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All desktop systems based on boxed Intel processors require thermal management. Boxed processors provide high quality fan heatsinks that provide an excellent local air stream. It is the responsibility of the integrator to ensure proper system thermal management by selecting chassis, motherboards, and power supplies that provide adequate system airflow through the system. Some specific chassis characteristics that affect system airflow include power supply fan size and strength, chassis venting, and additional system fans. Thermal testing should be done on each chassis-power supply-motherboard combination to verify the thermal management solution and ensure that the boxed processor is operating below its maximum operating temperature.

This applies to:

Intel® Pentium® 4 Processor Extreme Edition
Intel® Pentium® 4 Processors
Intel® Pentium® D Processor
Intel® Pentium® Processor Extreme Edition

Solution ID: CS-031421
Last Modified: 21-Oct-2014
Date Created: 22-Feb-2010
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