Thermal Design of Electronic Equipment

Ralph Remsburg

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9780849300820
• eBook ISBN: 9781351835916
• Topic: Technology & Engineering
• Subtopic: Industrial Design
• Index: Technology & Engineering

In a field where change and growth is inevitable, new electronic packaging problems continually arise. Smaller, more powerful devices are prone to overheating, causing intermittent system failures, corrupted signals, lower MTBF, and outright system failure. Since convection cooling is the heat transfer path most engineers take to deal with thermal problems, it is appropriate to gain as much understanding about the underlying mechanisms of fluid motion as possible. Thermal Design of Electronic Equipment is the only book that specifically targets the formulas used by electronic packaging and thermal engineers. It presents heat transfer equations dealing with polyalphaolephin (PAO), silicone oils, perfluorocarbons, and silicate ester-based liquids. Instead of relying on theoretical expressions and text explanations, the author presents empirical formulas and practical techniques that allow you to quickly solve nearly any thermal engineering problem in electronic packaging.

1

Introduction to Thermal Design of Electronic Equipment

1.1 Introduction to the Modes of Heat Transfer in Electronic Equipment

Electronic devices produce heat as a by-product of normal operation. When electrical current flows through a semiconductor or a passive device, a portion of the power is dissipated as heat energy. Besides the damage that excess heat can cause, it also increases the movement of free electrons in a semiconductor, which can cause an increase in signal noise. The primary focus of this book is to examine various ways to reduce the temperature of a semiconductor, or group of semiconductors. If we do not allow the heat to dissipate, the device junction temperature will exceed the maximum safe operating temperature specified by the manufacturer. When a device exceeds the specified temperature, semiconductor performance, life, and reliability are tremendously reduced, as shown in Figure 1.1. The basic objective, then, is to hold the junction temperature below the maximum temperature specified by the semiconductor manufacturer.

Nature transfers heat in three ways, convection, conduction, and radiation. We will explore these in greater detail in subsequent chapters, but a simple definition of each is appropriate at this stage.

1.1.1 Convection

Convection is a combination of the bulk transportation and mixing of macroscopic parts of hot and cold fluid elements, heat conduction within the coolant media, and energy storage. Convection can be due to the expansion of the coolant media in contact with the device. This is called free convection, or natural convection. Convection can also be due to other forces, such as a fan or pump forcing the coolant media into motion. The basic relationship of convection from a hot object to a fluid coolant presumes a linear dependence on the temperature rise along the surface of the solid, known as Newtonian cooling. Therefore:

q_c = h_cA_s(T_s − T_m)

FIGURE 1.1 Component failure rates with temperature for Programmable Array Logic (PAL), 256K Dynamic Random Access Memory (DRAM), and Microprocessors. Data from MIL-HDBK-217.

where:

qc	= convective heat flow rate from the surface (W)
As	= surface area for heat transfer (m^2)
Ts	= surface temperature (°C)
Tm	= coolant media temperature (°C)
hc	= coefficient of convective heat transfer (W/m^2)

This equation is often rearranged to solve for ΔT, by which:

$$\mathrm{\Delta }\mathit{T}\mathrm{=}\frac{{\displaystyle q_c}}{{\displaystyle {\mathit{h}}_{\mathit{c}}{\mathit{A}}_{\mathit{s}}}}$$

1.1.2 Conduction

Conduction is the transfer of heat from an area of high energy (temperature) to an area of lower relative energy. Conduction occurs by the energy of motion between adjacent molecules and, to varying degrees, by the movement of free electrons and the vibration of the atomic lattice structure. In the...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Preface
• Author
• Nomenclature and Symbology
• Table of Contents
• Chapter 1 Introduction to Thermal Design of Electronic Equipment
• Chapter 2 Formulas for Conductive Heat Transfer
• Chapter 3 Fluid Dynamics for Electronic Equipment
• Chapter 4 Convection Heat Transfer in Electronic Equipment
• Chapter 5 Radiation Heat Transfer in Electronic Equipment
• Chapter 6 Heat Transfer with Phase Change
• Chapter 7 Combined Modes of Heat Transfer for Electronic Equipment
• Appendix
• Index