This document is written for professional system integrators building PCs from industry accepted motherboards, chassis, and peripherals. It provides information and recommendations for thermal management in desktop systems using boxed Intel® Pentium® III, Pentium® II processors, and Celeron® processors. (The term "boxed processors" refers to processors packaged for use by system integrators.)
It is assumed that the reader has a general knowledge of and experience with desktop PC operation, integration, and thermal management. Integrators who follow the recommendations presented here can provide their customers with more reliable PCs and will see fewer customers returning with problems.
Systems using boxed processors all require thermal management. The term "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, which can 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.
Boxed processors are shipped in several processor packages:
- the Single Edge Contact Cartridge (S.E.C.C.)
- the Single Edge Contact Cartridge 2 (S.E.C.C.2)
- the Single Edge Processor Package (S.E.P.P.)
- and the Plastic Pin Grid Array (PPGA)
All boxed processors for desktop systems are shipped with a fan heatsink and fan power cable. These items should be used following the directions contained within the boxed processor installation notes included in the processor box. Thermal interface material (already applied) provides effective heat transfer from the processor to the fan heatsink. S.E.C.C., S.E.C.C.2, and S.E.P.P. boxed processors ship with an attached fan heatsink with the thermal interface material included between the processor and the fan heatsink. Current PPGA boxed processors ship with an unattached fan heatsink that includes thermal interface material on the fan heatsink base and a fan cable incorporated into the fan. 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 also needed 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 cool the processor adequately.
System airflow is determined by the following:
- 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, microATX, and the older Baby AT form factor.
In systems using Baby AT 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. The power supply fan exhausts the air through the back of the chassis. Figure 1 and Figure 2 show the airflow through Baby AT systems.
Figure 1. System Airflow Through Baby AT Desktop Chassis (Top View)
Figure 2. System Airflow Through Baby AT Tower Chassis (Side View)
Intel recommends the use of ATX and microATX form factor motherboards and chassis for boxed processors. The ATX and microATX form factors simplify assembly and upgrading of desktop systems, while improving the consistency of airflow to the processor.
With regard to thermal management, ATX components differ from Baby AT components in that the processor is located close to the power supply, rather than 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. Because of this, the airflow in systems using the boxed processor should flow from the front of the chassis, directly across the motherboard and processor, and out of the power supply exhaust vents. Figure 3 shows proper airflow through an ATX system to achieve the most effective cooling for a boxed processor with an active fan heatsink. For boxed processors, chassis that conform to the ATX Specification Revision 2.01 or later are highly recommended. For more information on the ATX form factor, and a list of ATX chassis manufacturers, please visit the ATX* Web site.
Figure 3. System Airflow Through ATX Tower Chassis Optimized For the Boxed Processor With an Active Fan Heatsink
One of the ways microATX chassis differ from 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 on the microATX form factor, and a list of microATX chassis manufacturers, please visit the microATX* web site.
The following is a list of guidelines to be used when integrating a system. Specific mention of Baby AT, 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 cosmetic vents only. 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; most air enters near the power supply and then immediately exits through the power supply or nearby vents. Very little air flows over the processor and other components. In ATX and microATX chassis, I/O shields must be present. Otherwise, the I/O opening may provide for excessive venting.
- Vents must be properly located
Systems must have properly located intake and exhaust vents. The best location for vents will allow air to enter the chassis and flow on a path through the system that is over various components and directly over the processor. Specific location of vents depends upon the type of chassis. For most desktop Baby AT systems, the processor is located near the front, and thus intake vents on the front panel work best. For Baby AT tower systems, vents on the bottom of the front panel work best. For ATX and microATX systems, vents should be located both in the bottom front and bottom rear of the chassis. Also for ATX and microATX systems, I/O shields must be present to allow the chassis to vent air as designed. Lack of an I/O shield may disrupt proper airflow or circulation within the chassis.
- Power Supply Airflow Direction
It is important to choose a power supply that has 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
Most, if not all, air flows through the power supply unit, which can be a significant restriction if not 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. It is important to make sure that floppy and hard drive cables 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.
Differences in motherboards, power supplies, and chassis all affect the operating temperature of processors. Thermal testing is highly recommended when choosing a new supplier for motherboards or chassis, or when starting to use new products. 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 of a reference 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 have extra add-in cards, graphics solutions with higher power requirements, warmer running hard drives, etc.
Thermal testing should be done on each chassis-power supply-motherboard configuration using the components that dissipate the most power. Variations in processor speed, graphics solutions, etc. do not require additional thermal testing if testing is done with the highest power-dissipating configuration.
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: