Thermal Management for the Boxed Intel® Pentium® Processor

Introduction

Systems using Intel® Pentium® processors all require thermal management. This document assumes a general knowledge of and experience with system operation, integration, and thermal management. Integrators who follow the recommendations presented can provide their customers with more reliable systems and will see fewer customers returning with thermal management issues. (The term "boxed Intel® Pentium® processors" refers to processors packaged for use by system integrators.)

Thermal management in boxed Intel® Pentium processor-based systems can affect both the performance (Intel® Thermal Monitor feature) and noise level (variable speed fan) of the system.

The Intel® Pentium® processor uses the Intel® Thermal Monitor feature (see Intel® Pentium® processor datasheet for details) to protect the processor during times where the silicon would otherwise operate above specification. The feature is intended to help prevent long-term reliability damage to the processor and provide protection for unusual circumstances like higher than normal internal chassis temperatures (and inlet air temperature, defined as the air temperature entering the processor fan heatsink) or failure of a system thermal management component (such as a system fan). In its active state, the Intel Thermal Monitor feature scales back processor power consumption, if the factory programmed thermal design temperature is exceeded (See Table 2 or the Intel® Pentium® processor datasheet for complete thermal specifications.). While the Intel Thermal Monitor feature is active, the system's performance may drop below its normal peak performance level. It is critical that systems be designed to maintain low enough internal chassis and processor inlet air temperatures to prevent the Pentium processor from entering a Intel Thermal Monitor active state. In a properly thermal managed and designed system, the Intel Thermal Monitor feature should never become active. It is recommended that the internal chassis temperature for boxed Intel Pentium processor-based systems remain below the lower set point of 38C for nominal operating environments, as shown in Table 1.

In addition to the Intel Thermal Monitor feature, the boxed Intel® Pentium® processor fan heatsink uses a newly designed, high quality variable speed fan which allows the processor to remain within its operating thermal specifications by running at different speeds over a short range of internal chassis temperatures and processor power consumption levels.

As processor power has increased, required thermal solutions have generated more noise. Intel has added an option to the boxed processor that allows system integrators to have a quieter system in the most common usage.

Previous generation boxed Intel® fan heatsinks contain built in circuitry to control fan speed. They have a thermistor in the fan hub which measures the chassis ambient air temperature. The fan circuitry then adjusts the fan speed to properly cool the processor at the slowest speed allowable. If the chassis ambient temperature is cool then the processor will run slower and quieter. If the ambient temperature is hot, then the fan will run faster.

This fan was designed to work in a variety of operating conditions so it had to be designed in such a way that it would cool the processor when running at its maximum power at any given ambient temperature (up to 38C). In normal operating environments the processor rarely reaches its maximum power rating.

Under most conditions the fan is spinning faster and louder than necessary. (The fan heatsink is required to work this way so that it will properly cool the CPU in all specified operating environments.)

Intel has been aware of customer concerns over increasing fan noise. Intel has now designed a new fan speed control technology to take advantage of the fact that the processor is not always running at its maximum power. This was done by basing the fan speed control on actual CPU temperature and power usage.

The speed of the new fan heatsink is controlled by the additional 4th wire of the fan cable. (The new technology is sometimes referred to as “4-wire fan speed control.”)

The additional 4th wire sends a signal from the motherboard to the fan heatsink to control its speed. There is a thermal diode in the processor which measures actual CPU temperature. The processor sends information to the motherboard about its specific thermal requirements and the actual processor temperature. The motherboard then uses this information to optimally control the speed of the processor fan.

Figure 1 shows the current fan speed curve (red) of a 3-wire, fan heatsink-thermistor based fan speed control. The additional curves in blue represent fan operations at lower CPU temperature and power consumption levels based on the 4-wire fan speed control fan heatsink.

NOTE:
The boxed processor fan heatsink circuitry will not allow the fan to spin faster than necessary in order to meet worse case power usage at any given ambient temperature. The thermistor control is always active to limit maximum fan speed.
The "Max Temp" in Figure 1 represents the upper set point or worse case ambient temperature of 38C. The "Min Temp" represents the lower set point or the slowest possible fan speed at an ambient temperature of 30C. (Also see Table 1.)

The acoustic benefits of the 4-wire based fan speed control may vary depending on the specific motherboard implementation. (The acoustic benefits are reliant on the motherboard implementation of fan speed control.)

Intel has recently developed a new revolutionary motherboard based fan speed control include with the latest Intel based motherboards called Intel® Quiet System Technology (Intel® QST). This new technology uses a PID controller that can measure the rate of change of the processor temperature, thus predicting when the processor will reach its maximum temperature. If implemented correctly by the motherboard manufacture, the control algorithm will operate the processor fan at minimum speed under most operating conditions. Since Intel® QST can predict when the processor will reach its maximum temperature, it will delay increasing the fan speed until just the right moment in order to keep the processor from exceeding its maximum temperature. Consult your motherboard manufacture to see which motherboards they offer with support for Intel® QST.

A 4-wire fan does not guarantee a quieter system. If the processor is being used in a hot environment and under heavy loads the fan will have to run fast enough to properly cool the processor. The internal chassis temperature is required to be maintained at 38°C (or lower).

Table 1. Boxed processor variable fan heatsink set points

Thermal management

Proper "thermal management" depends on 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. Boxed Intel Pentium processors are shipped with a high-quality variable speed 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.

Fan heatsink

The fan heatsink included with the boxed Intel Pentium processor must be securely attached to the processor. Thermal interface material (preapplied attached to the bottom of the heatsink) provides effective heat transfer from the processor to the fan heatsink. The fan cable provides power to the fan by connecting to a motherboard-mounted power header and also allows the transfer of information to and from the fan with the motherboard. (Only motherboards with hardware monitoring circuitry can use the fan speed signal. Additional circuitry is required for motherboard based fan speed control.) Be sure to follow the installation procedures documented in the Integration Overview for the boxed Intel® Pentium® Processor.

The fan is a high-quality ball bearing fan that provides a good local air stream. This 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 re-circulate warm air, and therefore may not cool the processor adequately.

Thermal interface material replacement for fan heatsink

Chassis recommendations

Systems based on the Intel® Pentium® processor must use a chassis that comply with the ATX specification (revision 2.2 or later) or microATX specification (revision 1.0 or later), depending on the motherboard form factor. Intel recommends system integrators using ATX form factor motherboards to choose a chassis that complies with the ATX specification (revision 2.2 or later). Likewise, system integrators using microATX form factor motherboards should choose a chassis that complies with the microATX specification (1.0 or later).

The chassis must also support a lower internal ambient temperature than many standard ATX and microATX desktop chassis. The internal chassis temperature for systems based on Pentium processors should not exceed 38°C when the chassis is used in a maximum expected room temperature of 35°C. Most chassis designed for the Pentium processor use extra internal chassis fans to improve airflow and many include ducting to bring cool air directly to the processor fan heatsink. A 38°C internal chassis temperature is best acieved by using a Thermally Advantaged Chassis (TAC) version 1.1.

It is recommended to use a Thermally Advantaged Chassis on the tested chassis list to ensure proper chassis airflow, electrical support (ATX12V or SFX12V power supply), and compatibility with boxed Intel Pentium® processors. Chassis that pass this thermal testing provide system integrators with a starting place for determining which chassis to evaluate. It is strongly recommended that system integrators perform thermal testing on the chassis selected for each configuration of Pentium® processor-based systems, even when using a chassis on the tested chassis list.

Intel® Pentium® processor thermal specifications

The Intel® Pentium® processor datasheet (also listed in Table 2) lists the power dissipation of Intel Pentium processors at various processor numbers. System integrators can perform thermal testing using thermocouples to determine the temperature of the processor's integrated heat spreader (see Intel® Pentium® processor Thermal Design Guidelines, for details) or the temperature of the air entering the boxed processor fan heatsink inlet.

A simple evaluation of the temperature of the air entering the fan heatsink can provide confidence in the system's thermal management. For boxed Intel Pentium processors, the testing point is at the center of the fan hub, approximately 0.3 inches above the fan. Evaluation of test data makes it possible to determine if a system has sufficient thermal management for the boxed processor. Systems based on the Intel Pentium processor should have a maximum expected temperature of 38°C in the maximum expected external ambient (which is typically 35°C). This is best achieved by using a Thermally Advantaged Chassis.

Table 2. Boxed Intel® Pentium® processor thermal specifications

Notes:

1. These specifications are from the Intel® Pentium® processor datasheet.