Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 10

Electrostatic discharge (ESD) can damage or destroy many types of modern electronic components, or modules or assemblies containing electrostatic discharge–susceptible (ESDS) components.

In electronics manufacturing, sensitivity to ESD became a general concern after the adoption of metal‐oxide‐semiconductor (MOS) transistor technology exacerbated by decreasing internal physical size of semiconductor component features and the rise of integrated circuits (ICs). The first Electrical Overstress/Electrostatic Discharge Symposium in the USA was organized in 1979 (Reliability Analysis Center 1979). The 1980 Symposium (Reliability Analysis Center 1980) shows papers on topics as diverse as theory and practice, device failure analysis studies, failure mechanisms and modeling, design of device protective networks, and implementing ESD controls, facility evaluation and effective training. Standards and technical handbooks also emerged around this time. The standards gave requirements for an ESD control program, while the technical handbooks gave technical data and tutorial material useful for educating the user and developing the ESD control program. ElectroStatic Attraction (ESA) of contaminant particles is a problem for manufacturers of semiconductor devices and displays. For operating electronic systems, ESD provides a source of electromagnetic interference (EMI) that can result in system crash, malfunction or data corruption.

Thus, the issues of ESD in electronic components and systems give two areas of interest. Issues of ESD control during electronic component, assembly and system manufacture are largely concerned in preventing damage in the unpowered non‐operational state and ensuring that product reaches the customer in good condition without compromise to appearance or reliability. This area can itself be further subdivided into

 Electrostatic and ESD issues affecting product yield and quality during semiconductor wafer scale fabrication

 ESD issues affecting product yield and quality during component, assembly and system manufacture and assembly, sometimes known as “factory issues”

 Design of semiconductor devices to withstand ESD up to target levels

ESD interference and damage during working electronic system operation is generally viewed as part of ElectroMagnetic Compatibility (EMC) and the responsibility of a different community. In some areas (e.g. Europe) electronic products are subject to ESD immunity test as part of their evaluation for fitness to be placed on the market (and qualification for CE marking) (Williams 2007).

Nevertheless, there is some overlap and often confusion between these areas. EMI caused by ESD in the manufacturing process can cause interference to product testers and lead to rejection of product and hence reduction of yield. In ESD test in EMC, ESD applied to exposed circuit connectors can lead to component or system hardware failure and may lead to requirements for ESD robustness of components that connect to the outside world.

This book is largely concerned with the development and maintenance of an ESD control program for protection of ESD susceptible components and assemblies during electronic system and assembly manufacture. This is the so‐called “factory issues” area of ESD control. It is intended that the book can be used as a handbook or practical guide for personnel working in ESD control in electronics or other companies that handle unprotected ESD susceptible parts. At the same time, sufficient background information and technical explanation is given to enable the user to understand the principles and practice of effective ESD control.

Personnel working in this field can have a wide variety of technical background and do not necessarily have strong electronics or electrical understanding. Many will not have had opportunity to attend courses on ESD control other than basic ESD awareness. It is surprising that at the time of writing very few University electronics related courses offer any modules on ESD control. Conversely there are as yet few industry courses available that deal with the subject in any depth other than basic ESD awareness. Worldwide, there are still only a very few courses and qualifications available to those who wish to obtain a good grounding in the field.

So, I have attempted to present the subject with a minimum of theory to make it accessible to those who do not have a strong relevant theoretical background. This is balanced by sufficient description of background theory for understanding the material presented, with references and a bibliography of further reading for those who wish to go into the subject in greater depth. The intention is to reveal and clarify the principles behind an area often considered a mysterious “black art.” In many ways, I have tried to write the book I would have liked to have found when I started learning about ESD control in the electronics industry.

A widespread current approach to development of an ESD control program is to comply with the requirements of an ESD control standard such as ANSI/ESD S20.20 or IEC 61340‐5‐1. It is often thought that this is sufficient to ensure that product ESD damage is brought under control. While this can be successful, if applied with insufficient knowledge it can lead to a program that is not well optimized or fails to address all the ESD threats (Smallwood et al. 2014; Lin et al. 2014). With knowledge and understanding an optimized and effective ESD control program may be achieved and maintained, often with lower costs. Nevertheless, compliance with an ESD control standard is advantageous and can help demonstrate, especially to customers, the seriousness with which ESD control is treated in the facility. Development of an ESD control program in compliance with the most widely used and respected ESD control standards 61340‐5‐1 and S20.20 is therefore discussed in some length. Properly specified ESD control programs compliant with these standards are held to be adequate to protect ESD sensitive devices with withstand voltages down to 100 V Human Body Model (HBM), while also addressing basic ESD risks due to charged metal objects and charged devices.

ESD susceptible components become ever more sensitive to ESD damage as time goes on, due to component technology developments. The need for development of ESD control programs through knowledge and understanding rather than rote application of standard techniques will grow as a greater number of more ESD sensitive components are handled in electronic manufacturing, assembly, and maintenance processes in the future. In parallel with the development of ESD control techniques and standards, a massive research effort has supported on‐chip ESD protection networks aimed at reducing device ESD susceptibility, with target withstand voltages of 2 kV HBM, 200 V Machine Model (MM) and 500 V Charged Device Model (CDM) (Industry Council 2011). In the early 2000s The Industry Council on ESD Target Levels was formed with members from IC manufacturing and electronics assembly companies, and independent consultants in the industry. In the face of increasing difficulty in achieving the target ESD withstand levels, and the belief that modern electronic manufacturing companies have ESD control programs routinely achieving the standard protection levels, they recommended reduction of on‐chip target protection levels to 1 kV HBM, 30 V MM and 250 V CDM (Industry Council 2011, 2010a,b). At the same time, many discrete components and ICs exist that for various reasons do not have on‐chip ESD protection or otherwise have lower ESD withstand voltage than these levels. It seems likely that this will be the first of many reductions in target level driven by technology changes and the assumption that industry can handle lower ESD withstand components.

While this book is primarily intended to support the industry factory practitioner, I hope that this book will encourage and enable Universities and Further Education organizations to offer courses and modules on ESD control for personnel who wish to make a career in electronics production and related fields.

This book does not attempt to address electrostatics and ESD control in semiconductor wafers and device manufacture, or device design for ESD protection. The former may be still as yet inadequately covered by the very few books available on the subject but is better discussed in a book more focused on this technology area such as Welker (2006). The topic of device design is best covered by specialist books such as Amerasekera and Duvvury (2002) and Wang (2002).

ESD immunity of operating electronic systems is left to be treated in other books as part of EMC issues, except for some discussion confined to areas of overlap with the ESD factory issues topic. This field is more concerned with the design of electronic systems for immunity to ESD than it is with ESD control (Ind. Co. White Paper 3, Johnson and Graham 1993; Montrose 2000; Williams 2007).

While electrostatic control is used in other industries such as explosives and flammable materials handling (the latter known in Europe as “ATEX”), these are typically governed by other standards or regulations. They are only mentioned in this book to draw attention to possible confusion areas and help avoid mistakes, for example in equipment specification and sourcing.

While this book could be read “cover to cover” it is probably more likely that the reader will “dip into” specific chapters as the need arises to learn about different topics while working in ESD control. The book has been written with this in mind. For those who wish to go deeper into the subject, lists of references are provided with each chapter.

Every specialist subject has its own set of specialist terms or uses specific terms in specific ways. Chapter 1 defines and introduces the reader to the commonly used terminology in ESD control. Whilst this chapter forms a general introduction to the key concepts and terminology in the field, it is also likely to be used to revise or clarify the meaning of terms during reading of other chapters. This is why the definitions and terminology has been provided together in one chapter rather than being defined and explained as required during the remainder of the book. Chapter 2 then explains in more detail the principles that underlie ESD control work.

Chapter 3 discusses ESD susceptible devices, and how ESD susceptibility of a component is measured. The range of ESD susceptibility of components, and current trends in ESD susceptibility are reviewed. The topic of failure analysis as it is applied to ESD failed components is outlined. Some case studies of ESD failures from the literature are briefly described.

Chapter 4 describes the “seven habits of a highly effective ESD program.” This is a way of explaining the essential activities of an effective ESD control program, that the author has used in ESD training work for many years. If these activities are effectively and habitually implemented, it is likely that an ESD control program will be, and remain, effective. If any one of them is neglected, it is likely the effectiveness of the ESD control program will eventually suffer.

Most basic ESD control techniques and standards mainly address manual handling of ESD susceptible devices, components, and assemblies. Chapter 5 extends the discussion to ESD control in automated systems, processes and handling, which form a major part of modern electronic manufacture.

Chapter 6 explains the approach and requirements given by the IEC 61340‐5‐1 and ANSI/ESD S20.20 ESD control standards at the time of writing. These standards are continually updated as time goes on, and so the reader is advised check for current versions available at the time of reading.

Chapter 7 gives an overview of the equipment and furniture commonly used in ESD control and commonly specified for use in an electrostatic discharge protected area (EPA) to control common ESD risks. The chapter explains how these often work together as part of a system and must be specified with that in mind.

ESD protective packaging is one of the most misunderstood areas of ESD control. ESD packaging is now available in an extraordinary range of forms from bags to boxes and bubble wrap to tape and reel packaging for automated processes. The principles and practice of ESD protective packaging are explained in Chapter 8. This is a deep and constantly developing subject in itself, and this chapter can barely do more than give an introduction to it.

The thorny question of how to evaluate an ESD control program is addressed in Chapter 9 with a goal of compliance with a standard as well as effective control of ESD risks and possible customer perceptions.

Whilst evaluating an existing ESD control program provides challenges, developing an ESD control program from scratch provides others. Chapter 10 gives an approach to this.

ESD control product qualification and compliance verification is an essential part of an ESD control program. Standard test methods have been developed and specified to go with compliance with ESD control standards. These are explained in Chapter 11. The ESD control program may also need to use control measures and equipment that are not currently specified in the standards. Some examples of test methods that may be used with these are also given in this chapter.

ESD Training has long been recognized as essential in maintaining effective ESD control. Chapter 12 discusses this in more detail. It describes some demonstrations and techniques which the author has used to help trainees understand static electricity, ESD and static control in practice.

Finally, Chapter 13 attempts to look at where ESD control may go in the near future.