Introduction

In a system environment, the Pentium® processor's temperature is a function of both the system and component thermal characteristics. The system level thermal constraints imposed on the package are local ambient temperature and thermal conductivity (i.e., airflow over the device). The Pentium® processor thermal characteristics depend on the package (size and material), the type of interconnection to the printed circuit board (PCB), the presence of a heat sink, and the thermal conductivity and the power density of the PCB.

All of these parameters are aggravated by the continued push of technology to increase the operating speeds and the packaging density. As operating frequencies increase and packaging size decreases the power density increases and the heat sink size and airflow become more constrained. The result is an increased importance on system design to ensure that thermal design requirements are met for each component in the system.

In addition to heat sinks and fans, there are other solutions for cooling integrated circuit devices. A few of these solutions are: fan mounted on heat sink, heat pipes, thermoelectric (peltier) cooling, liquid cooling, etc. While these alternatives are capable of dissipating additional heat, they have disadvantages in terms of system cost, complexity, reliability, and efficiency. These techniques are more expensive than a passive heat sink and fan. The introduction of active devices can also decrease reliability. Finally, the power efficiency of some of these techniques is poor, and gets worse as the amount of power being dissipated increases. Despite these disadvantages, each of these solutions may be the right one for particular system implementations.

However, for the purpose of this application note, Intel has focused its efforts on describing solutions using passive heat sinks and fans.

The goal of this document is to provide thermal performance information for the Pentium® processor and recommendations for meeting the thermal requirements imposed on systems. This application note attempts to provide an understanding of the thermal characteristics of the Pentium® processor and some examples of how the thermal requirements can be met.

Thermal management of an electronic system encompasses all of the thermal processes and technologies that must be employed to remove and transfer heat from individual components to the system's thermal sink in a controlled manner.

The objective of thermal management is to ensure that the temperature of all components is maintained within functional and absolute maximum limits. The functional temperature limit is the range within which the electrical circuits can be expected to meet their specified performance requirements. Operation outside the functional limit can degrade system performance or cause logic errors. The absolute maximum temperature limit is the highest temperature that a portion of the component may be safely exposed. Temperatures exceeding the limit can cause physical destruction or may result in irreversible changes in operating characteristics. Higher temperatures result in earlier failure of the devices in the system. For every 10°C rise above the operating range means a halving of the mean time between failures.

The Pentium® processor's power dissipation for 60 and 66 MHz is shown in Table 1.

To ensure functionality and reliability of the Pentium® processor, maximum device junction temperature must remain below 100°C. Considering the power dissipation levels and typical ambient environments of 40°C to 45°C, the Pentium® processor's junction temperatures cannot be maintained below 100° C without additional thermal enhancement to dissipate the heat generated by this level of power consumption.

The thermal characterization data described in Table 2 illustrates that both a heat sink and airflow are needed. The size of heat sink and the amount of airflow are interrelated and can be traded off against each other. For example, an increase in heat sink size decreases the amount of airflow required. In a typical system, heat sink size is limited by board layout, spacing, and component placement. Airflow is limited by the size and number of fans along with their placement in relation to the components and the airflow channels. In addition, acoustic noise constraints may limit the size or types of fans limiting the airflow.

To develop a reliable thermal solution, all of the above variables must be considered. Thermal characterization and simulation should be carried out at the entire system level accounting for the thermal requirements of each component.

Component power dissipation results in a rise in temperature relative to the temperature of a reference point. The amount of rise in temperature depends on the net thermal resistance between the junction and the reference point. Thermal resistance is the key factor in determining the power handling capability of any electronic package.
Thermal resistance from junction to case (THETA_JC), and from junction to ambient (THETA_JA) are the two most often specified thermal parameters for integrated circuit packages.

Ambient temperature is the temperature of the undistributed ambient air surrounding the package. Denoted TA, ambient temperature is usually measured at a specified distance away from the package. In the laboratory test environment, ambient temperature is measured 12 inches upstream from the package under investigation. In a system environment, ambient temperature is the temperature of the air upstream to the package and in its close vicinity.

Case temperature, denoted TC, is measured at the center of the top surface of the package, typically the hottest point on the package case. Special care is required when measuring the case temperature to ensure an accurate temperature measurement. Thermocouples are often used to measure TC. Before any temperature measurements, the thermocouples have to be calibrated. When measuring the temperature of a surface which is at a different temperature from the surrounding ambient air, errors could be introduced in the measurements. The measurement errors could be due to having a poor thermal contact between the thermocouple junction and the surface, heat loss by radiation or by conduction through thermocouple leads. To minimize the measurement errors, it is recommended to use the following approach:

• Use 36 gauge or finer diameter K, T, or J type thermocouples. The laboratory testing was done using a thermocouple made by Omega (part number: 5TC-TTK-36-36).
• Attach the thermocouple bead or junction to the center of the package top surface using high thermal conductivity cements. The laboratory testing was done by using Omega Bond (part number: OB-100).
• The thermocouple should be attached at a 90° angle as shown in Figure 1. When a heat sink is attached a hole (no larger than 0.15'') should be drilled through the heat sink to allow probing the center of the package as shown in Figure 1.
• If the case temperature is measured with a heat sink attached to the package, drill a hole through the heat sink to route the thermocouple wire out.

Figure 1. Thermocouple Attachment

Junction Temperature Junction temperature, denoted TJ, is the average temperature of the die within the package.
The junction temperature for a given junction-to-ambient thermal resistance, power dissipation, and ambient temperature is given by the following formula:

T_J = P_D * THETA_JA + T_A

T_J = P_D * (THETA_JC + THETA_CS + THETA_SA) + T_A

THETA_CS is the thermal resistance from the component (case) to the heat sink.

Thermal Resistance

Thermal resistance (Figure 2) values for junction-to-ambient, THETA_JA, and junction-to-case, THETA_JC, are used as measures of IC package thermal performance. QJC is a measure of the package's internal thermal resistance along the major heat flow path from silicon die to package exterior. This value is strongly dependent on the material, thermal conductivity, and geometry of the package. THETA_JC values also depend on the location of the reference point (in this case center of the package top surface), the external cooling configurations and the heat flow paths from the package to the ambient. For example, if a heat sink is attached to the package top surface or more heat is pulled into the board through the pins, the THETA_JC values measured with reference to the center of the package top surface will change. THETA_JA values include not only internal thermal resistance, but also the radiative and convective thermal resistance from the package exterior to ambient air. THETA_JA values depend on the material, thermal conductivity, and geometry of the package and also on ambient conditions such as airflow rates and coolant physical properties.

In order to obtain thermal resistance values, junction temperature is measured using the temperature sensitive parameter (TSP) method. With this method, special design thermal test structures are used which are approximately the same size as the Pentium® processor die. The test structure consists of resistors and diodes. Resistors are used to simulate the Pentium® processor power dissipation and thereby heat up the package. Diodes, which are located at the center of the thermal test die, are used to measure the die temperature. The measurements are carried out in a wind tunnel environment. The air flow rate and the ambient temperature are measure 12 inches away from the package in the upstream air.

The parameters are defined by the following relationships:

THETA_JA = (T_J - T_A) / P_D

THETA_JC = (T_J - T_C) / P_D

THETA_JA = THETA_JC + THETA_CA

where:
THETA_JA = junction-to-ambient thermal resistance (°C/W)
THETA_JC = junction-to-case thermal resistance (°C/W)
THETA_CA = case-to-ambient thermal resistance (°C/W)
T_J = average die (junction) temperature (°C)
T_C = case temperature at a pre-defined location (°C)
T_A = ambient temperature (°C)
P_D = device power dissipation (W)

Table 2 lists the junction-to-case and case-to-ambient thermal resistance's for the Pentium® processor (with and without a heat sink).

NOTE:Heat Sink: 2.05 sq. in. omni-directional pin, aluminum heat sink with 0.050 in. pin width, 0.143 in pin-to-pin center spacing and 0.150 in. base thickness. A thin layer of thermal grease with an average thickness of .003''to .005'' was used as the interface material between the heat sinks and the package.

At this point the application note turns from describing the characteristics that define thermal performance to describing how designers should use these characteristics to assess thermal requirements of PC system designs. The Pentium® processor specifies a maximum case temperature, TC, of 85° C. This case temperature limit along with the Pentium® processor's power and thermal resistance characteristics can be used to determine the ambient temperature required to keep the Pentium® processor operating within its specified limits. Using these parameters in the following equations:

T_A = T_C- (P * (THETA_JA- THETA_JC))

T_A = 85°C - (16W * (12.8°C/W -1.2°C/W))

T_A = -100.6°C

The maximum ambient temperature required in a Pentium® processor system without any additional thermal enhancement is -100.6° C at 66 MHz. Obviously, this ambient temperature is impractical and unachievable in a PC system. In order to be able to maintain the case temperature at 85°C in a typical system ambient with air temperature of 40 - 45° C, the thermal resistance between the case and the ambient must be reduced.

The most common way to improve the package thermal performance is to increase the surface area of the device by attaching a large piece of metal (a heat sink) to the ceramic package. The heat sink is usually made of Aluminum and is chosen for its price/thermal-performance ratio. There are materials that offer higher conductivity such as copper, but cost becomes prohibitive. To maximize the flow of heat for a given junction temperature rise over the ambient temperature, the thermal resistance from heat sink to air can be reduced by a) maximizing the surface area, and b) maximizing the air flow across the surface area (maximizing air flow through heat sink fins in most cases).

The testing was done in a wind tunnel in the configuration (in Figure 3) where the heat sink was mounted on a real Pentium® processor ceramic package with a thermal die mounted inside to generate the 16 Watts of power. The package is then mounted in a socket which is soldered to a 2-layer PCB that brings power to the die.

Based on these tests, three specific heat sink and airflow combinations have been identified that properly dissipate the Pentium® processor's 16 Watts and maintains a case temperature below 85°C. The three heat sinks are shown in Figure 4.

Heat sink B: H = 1.2''for 200 LFMs of Air

Heat sink C: H = 0.5''for 500 LFMs of Air

Assumption: Air Temp = 45°C

Figure 4. Recommended Combinations

In addition, testing has been done to provide more general guidelines which allow deviating from the above conditions. These guidelines allow systems to derive various combinations of heat sink size and airflow that ensure the Pentium® processor thermal specifications are met. For example, by increasing the heat sink x-y dimensions and extending it over the package footprint, the heat sink height can be reduced while maintaining the same thermal performance as the taller heat sink with the same footprint as that of the package. The first three charts (Figures 5, 6, 7) show the thermal resistance as a function of heat sink size and airflow. The last three charts (Figures 8, 9, 10) show the power dissipation achievable with a given heat sink size and airflow. The power dissipation calculations assume T_C = 85°C, T_A = 45°C, and THETA_JC = 0.6°C/W.

P_max = (T_C- T_A) / (THETA_JA- THETA_JC) = 40 / (THETA_JA- 0.6)

A key assumption in all of these calculations is that a perfect thermal connection can be achieved between the case and the heat sink. One can extrapolate the heat sink solutions by adding the additional thermal resistance of any chosen heat sink attach process.

Figure 6. Thermal Resistance

Figure 7. Thermal Resistance

Figure 8. Power Dissipation

Figure 9. Power Dissipation

Figure 10. Power Dissipation

Airflow

To improve the effectiveness of heat sinks it is important to manage the air flow so as to maximize the amount of air that flows over the device or heat sink's surface area. In the system, the air flow around the processor can be increased by providing an additional fan or increasing the output of existing fan. If this is not possible, baffling the airflow to direct it across the device may help. This means the addition of sheet metal or objects to guide the air to the target device. Often the addition of simple baffles can eliminate the need for an extra fan. In addition, the order in which air passes over devices can impact the amount of heat dissipated.

Fans are often needed to assist in moving the air inside a chassis. A typical variable speed fan capable of up to 100 CFM of air can be found for approximately $10.

A fan may be placed at the top of a heat sink to produce direct air impingement on the heat sink for efficient heat removal. A key issue with fans is their reliability. Although many fans are rated for approximately 50,000 hours of operation, operating conditions such as operating temperature, pressure drop across the fan and the particles in the air can significantly reduce the fans useful life.

As the complexity of today's microprocessors continues to increase so do the power dissipation requirements. Care must be taken to ensure the additional heat resulting from the power is properly dissipated. As documented, the heat can be dissipated using passive heat sinks, fans and/or active cooling devices.

The simplest and probably most cost effective method is to use a heat sink and a fan. The size of the heat sink and the output of the fan can be varied to balance the tradeoffs between size and space constraints versus noise. For example, if space is available a 1.2'' high heat sink can be used with only 200 LFM to cool the 66-MHz Pentium® processor and the 16W it dissipates. Another example in which space is restricted shows a 0.5'' high heat sink can be used if approximately 500 LFM of airflow is provided.

These are not the only valid solutions, but both provide adequate cooling to maintain the Pentium® processor case temperature at or below the 85° C specified limit. By maintaining this specification, the system can guarantee proper functionality and reliability of the Pentium® processor.

This applies to:

Intel® Pentium® Processor