Thermal Testing with Thermocouples and Thermal Meters on Intel Boxed Processor-Based Desktop PCs

• Introduction 
• Thermal Management 
• Fan Heatsink 
• Using Thermocouples and Thermal Meters 
  • Fan Inlet Temperature Measurement 
  • Heatsink Temperature Measurement
• Scaling Temperatures Measured at Room Temperature 
• Thermally Stressful Applications 
• Fahrenheit to Celsius Conversion Chart

Introduction

This document is for professional system integrators building PCs that use industry-accepted motherboards, chassis, and peripherals. It provides information and recommendations for thermal evaluation on systems using desktop Intel® boxed processors. Thermal testing should be performed on new system configurations built with boxed processors. Two evaluation methods that apply to several desktop boxed processors are discussed in detail. Each boxed processor has a thermal management application note. The application note and this document will enable a system integrator to evaluate the thermal management of a system configuration. A third document, System Thermal Management for Boxed Intel Processor-Based Desktop PCs, has information about building systems with quality thermal management and improving thermal management in systems that have insufficient airflow.

The temperature specifications for Intel processors are documented in the processor datasheets that are available from the developer site. A thermal metrology is also documented for each processor. This document does not discuss a method to measure the processor thermal specification, but focuses on two low cost evaluation methods that can provide system integrators with confidence in their system's thermal management.

The reader should have a general knowledge of and experience with PC operation, integration, and thermal management. Integrators who follow the recommendations presented can provide their customers with more reliable PCs and may see fewer customers returning with thermal management issues. (The term "boxed processors" refers to processors packaged for use by system integrators.)

Thermal Management

Systems using boxed processors require thermal management. 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 and then vented out of the system. Boxed processors include 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 provides procedures for determining the effectiveness of a system's thermal management solution. The temperature specifications for Intel processors are documented in the processor datasheets that are available from the Developer site. A thermal metrology is also documented for each processor. The metrology describes how to verify that the processor never exceeds its maximum specified temperature. Most thermal metrologies for current processors require drilling heatsinks, soldering wires to baseboards, and the purchase of outside equipment. This document details two methods for evaluating a system's thermal management. These evaluations do not validate that the processor specification will never be exceeded, but provide confidence in the system's ability to provide the proper internal environment for the boxed processor.

Before testing the system, it is important to evaluate the conditions in which the system will operate. An important factor to consider is the maximum ambient temperature in which the system will operate. Many system manufacturers specify the maximum ambient temperature as 35°C (95°F). In areas without air conditioning 40°C (104°F) may be more appropriate. System integrators should choose a value that is appropriate and specify that value as the maximum operating temperature to the customer.

Fan Heatsink

Boxed processors include a fan heatsink and fan power cable. When using these items, follow the directions in the installation notes included in the boxed processor box.

Fans used on boxed processors are 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 away from the heatsink to the system air is only half the task. Sufficient system airflow is also needed to vent warm air out of the system. Without steady airflow through the system, the fan heatsink will re-circulate warm air, which does not adequately cool the processor.

A common feature on all desktop boxed processors is the fan inlet. On each heatsink, the fan hub has a label or hologram in the center, and openings that allow air to flow into the fan center and out the sides of the fan heatsink. The temperature of air entering the fan inlet is a critical factor in cooling the boxed processor. Measuring the temperature of air entering the fan inlet can determine if the boxed processor fan heatsink is able keep the processor temperature within its operating range. To determine the recommended maximum ambient air temperature for a specific processor, see the thermal management document for that product.

The boxed processor fan heatsinks have metal bases that allow for quick heat transfer. The temperature of the heatsink base at a specified location can also be used to determine if the boxed processor fan heatsink is able to keep the processor within specification (on select boxed processors only). See the thermal management application note to determine if this is an appropriate testing method.

Using Thermocouples and Thermal Meters

Thermocouples provide accurate temperature measurements if properly placed and carefully routed. Thermocouples and thermal meters can be purchased through electronics supply catalogs. Some Digital Multi-Meters (DMMs) provide a thermocouple input and are able to function as a thermal meter (see Figure 1).

Figure 1. Digital Multi-Meter with Thermocouple Attachment (Left)
Thermal Meters with Thermocouple (Right)

The system thermal management for the desktop boxed processor can be evaluated by taking a temperature measurement of air entering the fan inlet, or by measuring the temperature of the heatsink base at a specific location. Refer to the boxed processor thermal management document for the specific measurement location, fan inlet temperature values, and heatsink temperatures for each processor speed.

A simple way to test these temperatures is to place a thermocouple at the desired location and read the temperature values from a thermal meter after the system has been running a thermally stressful application, which will simulate the worst case system load. For fan inlet measurements, it is recommended that a single thermocouple be placed exactly 0.3 inches above the center of the fan inlet. For more reliable averaged inlet temperature values, place four or five thermocouples equally spaced above the fan inlet. For heatsink base temperature measurements, secure a single thermocouple to the test point with thermally conductive adhesive. (Securing the thermocouple at a second point inside the chassis keeps the test point from experiencing stresses as the thermocouple is moved around during test setup.)

Make sure the thermocouples do not make contact with any other heat dissipating components in the system (such as the hard drive or graphics card). The thermocouple tip must be the same temperature as the desired test point. Any undesired heating of the thermocouple wire could cause the tip to be warmer than the test point. For air temperature measurements, it is most important to route the thermocouple out of the system without any contact to heat sources. For temperature measurements of metal surfaces, it is most important to have good contact with the metal surface and have sufficient adhesion and bracing (to prevent the thermocouple from dislodging during the evaluation).

Fan Inlet Temperature Measurement

In addition to the thermocouple and thermal meter (see Figure 2), the equipment required for testing includes:

• A thermometer (to measure room temperature)
• A thermally stressful program to run on the host system (see Thermally Stressful Applications)
• Tape or adhesive (for attaching the thermocouple to the fan shroud)

The procedure for thermal testing is as follows:

Note: If you are testing a system with a variable-speed system fan, you must run the test at the maximum operating room temperature you have specified for the system.

1. To ensure maximum power consumption during the test, you must disable the system's automatic power-down modes or "green features." These features are controlled either within the system BIOS or by operating system drivers.
   

   Make sure the processor is cool enough before applying the thermocouple to the fan shroud. If the system is powered on at the start of the test, wait at least 15 minutes after turning off power and removing the chassis cover.

2. Thread the thermocouple(s) through a small opening in the chassis (like a vent) and attach it to the fan shroud so that the tip of the thermocouple is located exactly 0.3 inches above the center of the fan hub. If more than one thermocouple is used, verify that the tips of the thermocouples are 0.3 inches above the center of the fan hub and that they are evenly distributed above the fan hub within the hub circumference. (Be sure that the material used to attach the thermocouple(s) will retain it at high temperatures and is not flammable.) Figure 3 shows the correct location.
   
   
   Figure 2. Boxed Processor with Thermocouple in Place

3. Be sure to connect the fan power cable as described in the boxed processor installation notes.
4. If you disconnected cables or removed boards to install the processor, replace them now. Quickly power up and power down the system to make sure the fan rotates. Check the fan heatsink power connections if the fan is not rotating.
5. Attach the chassis cover and replace cover screws.
6. Power up the PC. If the system has been assembled properly, and the processor is properly installed and seated, the system boots into the intended operating system (OS).
7. Run the thermally stressful application. (See the Thermally Stressful Applications section for more information.)
8. Allow the program to run for two hours. This allows the entire system to heat up and the internal temperature to stabilize. Record the temperature reported by the thermal meter once every five minutes for the last twenty minutes. This yields four temperature measurements. Record the room temperature at the end of the two-hour period.

Make sure the processor is cool enough before applying the thermocouple to the fan shroud. After recording the room temperature, power the system down. Remove the chassis cover. Allow the system to cool at least 15 minutes.

If using more than one thermocouple, average the measurements of the thermocouples for each time measurement. Use the highest of the four measurements taken from the thermal meter during the final twenty minutes for the temperature calculation below. If the evaluation was done at the maximum specified system ambient temperature (in a thermal chamber), then no temperature scaling is required. If the evaluation was done at room temperature, then some temperature scaling is required. The Scaling Measured Temperatures section describes this process.

Heatsink Temperature Measurement

In addition to the thermocouple and thermal meter (see Figure 2), the equipment required for testing includes:

• A thermometer (to measure room temperature)
• A thermally stressful program to run on the host system (see Thermally Stressful Applications)
• Thermally conductive adhesive (for attaching the thermocouple to the test point)
• Tape (for securing the thermocouple to the processor or heatsink)

The procedure for thermal testing is as follows:

Note: If you are testing a system with a variable-speed system fan, you must run the test at the maximum operating room temperature you have specified for the system.

1. To ensure maximum power consumption during the test, you must disable the system's automatic power-down modes or "green features." These features are controlled either within the system BIOS or by operating system drivers.
   

   Make sure the processor is cool enough before applying the thermocouple to the fan shroud. If the system is powered on at the start of the test, wait at least 15 minutes after turning off power and removing the chassis cover.

2. Thread the thermocouple through a small opening in the chassis (like a vent) and attach it using thermally conductive adhesive to the heatsink test location (documented in the processor thermal management document) so that the tip of the thermocouple is held firmly against the metal. It is important to brace the thermocouple wire to reduce stress on the attach point and prevent the thermocouple from becoming detached during the evaluation. Figure 4 shows a thermocouple attached to a heatsink base.
   
   
   Figure 3. Boxed Processor with Thermocouple Attached to the Heatsink Base

3. Be sure to connect the fan power cable as directed in the boxed processor installation notes.
4. If you disconnected cables or removed boards to install the processor, replace them now. Quickly power up and power down the system to make sure the fan rotates. Check the fan heatsink power connections if the fan is not rotating.
5. Attach the chassis cover and replace cover screws.
6. Power up the PC. If the system has been assembled properly, and the processor is properly installed and seated, the system boots into the intended operating system (OS).
7. Run the thermally stressful application. (See the Thermally Stressful Applications section for more information.)
8. Allow the program to run for two hours. This allows the entire system to heat up and the internal temperature to stabilize. Record the temperature reported by the thermal meter once every five minutes for the last 20 minutes. This yields four temperature measurements. Record the room temperature at the end of the two-hour period.

Make sure the processor is cool enough before applying the thermocouple to the fan shroud. After recording the room temperature, power the system down. Remove the chassis cover. Allow the system to cool at least 15 minutes.

Use the highest of the four measurements taken from the thermal meter during the final twenty minutes for the temperature calculation below. If the evaluation was done at the maximum specified system ambient temperature (in a thermal chamber), then no temperature scaling is required. If the evaluation was done at room temperature, then some temperature scaling is required. The Scaling Measured Temperatures section describes this process.

Scaling Temperatures Measured at Room Temperature

Once the maximum operating ambient temperature has been selected for the system, the scaling of the measured temperature can determine if the thermal management of the system under the worst case environment is sufficient. A common value for system temperature specification is 35°C (95°F). A value for systems where air conditioning is not available and ambient temperatures are high is 40°C (104°F). Choose a value that is right for your customer. Write this value on line A below.

Write the room temperature recorded after testing on line B below. Subtract line B from line A and write t he result on line C. This difference compensates for the fact that the test was likely conducted in a room that is cooler than the system's maximum operating temperature (see Fahrenheit to Celsius Conversion Chart).

A. _ _ _ _ _ (Maximum room temperature, typically 35°C or 40°C)

B. — ______ Room temperature °C at end of test

C. _ _ _ _ _ Scaling value (Copy to line E)

Write the highest temperature recorded from the thermal meter on line D below. Copy the number from line C to line E below. Add line D and line E and write the sum on line F. This number represents the highest temperature that would be measured when the system is used at its specified maximum operating room temperature running a similarly thermally stressful application. Use this value to determine that the system's thermal management is sufficient (see the thermal management document for the processor being tested).

D. _ _ _ _ _ Maximum temperature from thermal testing

E. + _______ Maximum operating temperature adjustment from line C above

F. _ _ _ _ _ Scaled temperature measurement using similarly stressful application

The final stage in evaluating a system's thermal management is to determine if the value on line F is acceptable or if the modifications should be made to the system to provide better cooling of the system components. See System Thermal Management for more information.

Summary
If the value on line F is found to be higher than acceptable (see the thermal management document for the processor being tested), there are two options:

1. Improve system airflow and retest the system.
2. Choose a lower maximum operating room temperature for the system. Be sure to consider the customer and the system's typical environment.

After implementing either option, you must re-evaluate the thermal management to verify the solution.

Thermally Stressful Applications

Some commercially available software applications will cause the processor to heat up and dissipate more power through the heatsink and into the system. These thermally stressful applications can be used during thermal testing to help ensure that typical processing loads are accounted for in the thermal management of the system. Software programs will uniquely affect each microprocessor architecture (with respect to power dissipation). Some cursory verification of the system under test can determine which software program provides the highest temperature in the system. Using this application to test can provide assurance that applications being run on the platform will not cause the system to exceed the desired temperature operating range.

Future applications may demand more processor power and therefore generate more heat in the system. Adding additional thermal headroom for future applications can provide additional confidence in the thermal management of a desktop PC. This would mean verifying that the temperature measured during the thermal test was below the target specification by a certain value. A typical value to allow for thermal headroom may be 5°C or 10°C.