Fundamentals of Thermal Management

Source: FUNDAMENTALS OF MICROSYSTEMS PACKAGING CHAPTER 6 FUNDAMENTALS OF THERMAL MANAGEMENT Prof. Avram Bar-Cohen University of Minnesota Dr. Abhay Watwe Intel Corporation Prof. Kankanhally N. Seetharamu University of Science, Malaysia ................................................................................................................. Design Environment IC Thermal Management Packaging Single Materials Chip Opto and RF Functions Discrete Passives Encapsulation IC Reliability IC Assembly Inspection PWB MEMS Board Manufacturing Assembly Test Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FUNDAMENTALS OF THERMAL MANAGEMENT ................................................................................................................. 6.1 What Is Thermal Management? 6.2 Why Thermal Management? 6.3 Cooling Requirements for Microsystems 6.4 Thermal Management Fundamentals 6.5 Thermal Management of IC and PWB Packages 6.6 Electronic Cooling Methods 6.7 Summary and Future Trends 6.8 Homework Problems 6.9 Suggested Reading CHAPTER OBJECTIVES Ⅲ Describe the need for thermal management of electronic components Ⅲ Introduce the fundamental heat transfer mechanisms of conduction, convection and radiation which play a major role in the cooling of electronic components Ⅲ Introduce the concept of thermal resistance and illustrate its usefulness in detecting the most critical thermal transfer path Ⅲ Provide simple equations and tabulate commonly used thermal properties to enable the reader to perform a first order analysis of heat transfer from an electronic package Ⅲ Describe various cooling methods typically used or considered CHAPTER INTRODUCTION All microelectronic devices require power for their use. A typical microprocessor of today uses about 35 watts, and in 2005 it is expected to use about 100 watts. This is an enormous power density that, if not properly managed, can result in microprocessor failure. This chapter addresses all thermal technologies required to understand and deal with the consequences of heat generation in electronic components. 213 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FUNDAMENTALS OF THERMAL MANAGEMENT 214 FUNDAMENTALS OF MICROSYSTEMS PACKAGING 6.1 WHAT IS THERMAL MANAGEMENT? The resistance to the flow of electrical current through the leads, poly-silicon layers, and transistors comprising a semiconductor device, results in significant internal heat generation within an operating microelectronic component. In the absence of cooling—that is, heat removal mechanisms—the temperature of such an operating component would rise at a constant rate until it reaches a value at which the electronic operation of the device ceases or the component loses its physical integrity. Placing the device in contact with a lower temperature solid or fluid, facilitates heat flow away from the component. Due to this cooling, the temperature rise is moderated as it asymptotically approaches an acceptable steady-state value. At steady-state, all the heat generated by the component is transferred to the surround- ing structure and/or fluid. Thus, the more intense the heat transfer mechanism—for example, high velocity air jets rather than natural convection, or boiling rather than low velocity liquid flow—the smaller the component temperature rise above the ambient temperature. Thermal conduction, convection, and radiation, as well as phase change processes, all play a role in electronics cooling. Successful thermal packaging relies on a judicious combination of materials and heat transfer mechanisms to stabilize the component temperature at an acceptable level. 6.2 WHY THERMAL MANAGEMENT? Despite the wide variety in size, power dissipation, and sensitivity to temperature, the thermal management of all microelectronic components is motivated by similar concerns and a common hierarchy of design considerations. The prevention of catastrophic failure—an immediate and total loss of electronic function and package integrity—is the primary and foremost aim of electronics thermal control. Catastrophic failure is often associated with a large temperature rise, which may lead to a drastic deterioration in semiconductor behavior, and/or fracture, delamination, melting, vaporization, and even combustion of the packaging materials. An understanding of the catastrophic vulnerability of the specified component(s) makes it possible to select the appropriate fluid, heat transfer mode, and inlet coolant temperature, and thus establish the required thermal control strategy early in the design process. Following selection of the thermal packaging strategy, attention can be turned to meet- ing the desired level of reliability and the associated target failure rates of each component and subassembly. Individual solid-state electronic devices are inherently reliable. How- ever, since a single microelectronic chip may include as many as 15 million transistors and 600 leads, and since many tens of such components may be used in a single system, achieving failure-free operation over the useful life of the product is a most formidable challenge. The minimization or elimination of thermally-induced failures often requires the reduction of the temperature rise above the ambient and minimization of temperature variations within the packaging structure(s). 6.2.1 Failure Rate Increases with Temperature The reliability of a system is defined by the probability that the system will meet the required specifications for a given period of time. As an individual electronic component contains no moving parts, it can often perform reliably for many years, especially when Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FUNDAMENTALS OF THERMAL MANAGEMENT Chapter 6 Fundamentals of Thermal Management 215 operating at or near room temperature. In practice, integrated circuits operate at substantially higher temperatures and, unfortunately, most electronic components are prone to failure from prolonged exposure to elevated temperatures. This accelerated failure rate results from mechanical creep in the bonding materials, parasitic chemical reactions, and dopant diffusion, to mention just a few possibilities. These and related failure modes establish a direct link between component reliability and operating temperature, which often takes the form illustrated in Figure 6.1. This figure is shown to reflect a near- exponential dependence of the thermal acceleration factor on component temperature. Thus, a rise in temperature from 75ЊC to 125ЊC can be expected to result in a five-fold increase in failure rate. Under some conditions, a 10ЊCto20ЊC increase in chip temperature can double the component failure rate. As a consequence, for many package cat- egories, temperature is the strongest contributor to the loss of reliability. In such systems, thermal management is critical to the success of the electronic system. In the final thermal design stages of an electronic system, the reliability, availability, and maintainability of the proposed thermal control alternatives must be evaluated and used to guide the final technology and equipment choice. It is the role of the packaging engineer to assure that the enhanced reliability of the components, resulting from lower operating temperature, are sufficient to compensate for the additional life-cycle cost and inherent failure rate of fans, pumps, and special interface materials. 6.2.2 Packaging Levels and Heat Removal To initiate the development of a thermal design for a specified electronic product, it is first necessary to define the relevant packaging level (see Figure 6.2). The commonly accepted categorization places the chip package, which houses and protects the chip, at 10 FIGURE 6.1 Effect of temperature on failure rate. 8 6 (Failure rate) (Failure 4 Thermal Acceleration Factor 2 0 0 40 80 120 160 Temperature oC Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. FUNDAMENTALS OF THERMAL MANAGEMENT 216 FUNDAMENTALS OF MICROSYSTEMS PACKAGING Wafer Chip First Level Package (Single Chip Module) (Multichip Module ) Second Level Package (PWB) Third Level Package (Motherboard or Backplane) FIGURE 6.2 Illustration of electronics packaging levels [16]. the bottom of the packaging hierarchy (level 1). The printed wiring board (PWB), which provides the means for chip-to-chip communication, constitutes level 2, while the backplane or ‘‘motherboard,’’ which interconnects the printed wiring boards, is termed level 3 packaging. The box, rack, or cabinet which houses the entire system is generally referred to as level 4. The primary thermal transport mechanisms and the commonly used heat removal techniques vary substantially from one packaging level to the next. Level 1 thermal packaging is primarily concerned with conducting heat from the chip to the package surface and then into the printed wiring board. At this packaging level, reduction of the

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