1． Electrostatic discharge
Electrostatic discharge (ESD) is a well-known electromagnetic compatibility problem that can cause electronic equipment to malfunction or damage it. When semiconductor devices are placed alone or installed in circuit modules, they can cause permanent damage to these devices even when they are not powered on. Electrostatic discharge sensitive components are called electrostatic discharge sensitive devices (ESDS).
If the voltage between two or more pins of a component exceeds the breakdown strength of the component's dielectric, the component will be damaged. This is the main reason for the failure of MOS devices. The thinner the oxide layer, the more sensitive the device is to electrostatic discharge. A fault usually manifests itself as a short circuit with a certain resistance to the power supply itself. For bipolar devices, damage generally occurs in areas of active semiconductors that have been metallized and separated by a thin oxide layer, and therefore a path of severe leakage will occur.
Another failure is caused when the temperature of the node exceeds the melting point of the semiconductor silicon (1415 ° C). The energy of an electrostatic discharge pulse can cause localized heating, so this mechanism fails. This failure can occur even if the voltage is below the dielectric breakdown voltage. A typical example is that the breakdown between the emitter and the base of an NPN transistor will cause the current gain to decrease sharply.
After the device is affected by electrostatic discharge, functional damage may not occur immediately. These potentially damaged components are often referred to as "cripples" and, when used, will be more sensitive to subsequent electrostatic discharges or conductive transients. It is important to pay close attention to the damage that occurs to components that are not easily detectable by the discharge voltage. The human body feels the electrostatic discharge voltage between 3000-5000V, however, the voltage when the component is damaged is only a few hundred volts. The harmful effects of electrostatic discharge began to be recognized in the 1970s. This is due to the development of new technologies that have made components more and more sensitive to damage caused by electrostatic discharge. Losses from electrostatic discharge can reach more than several million dollars each year. Therefore, many large component and equipment manufacturers have introduced professional technology to reduce the accumulation of static electricity in the production environment, thereby improving the product qualification rate and reliability. Users also understand the importance of preventing electrostatic discharge damage based on their own experience.
2． How to deal with electrostatic discharge
The first step in controlling the buildup of static electricity is to understand the mechanism of electrostatic charge generation. Electrostatic voltage is generated by the contact and separation of different kinds of substances. Although friction can accumulate more charge, friction is not necessary. This effect is known as triboelectric charging, and the voltage produced depends on the characteristics of the materials that are rubbing against each other. The friction electrification sequence table lists the degree of difficulty of charging various types of materials. For two substances in contact with each other, the electrons will change from the upper substance in the sequence list to the lower substance, which will cause the two substances to have positive and negative charges, respectively. The farther the materials in the sequence table are, the greater the amount of charge they carry.
The frictional electrification sequence of common substances is shown in the following table:
3． Practical problem solving
The solution to the problem includes: If electrostatic discharge sensitive components (ESDS) are exposed to the outside during production and maintenance, the accumulation of charge should be prevented near these components, and these components should be protected against electrostatic discharge during transportation and storage Method packaging. There are many ways to prevent static discharge. The best way is to meet the requirements and the lowest cost method, this method is different for different products and different occasions.
4． Electrostatic Discharge Protected Area (EPA)
The electrostatic discharge protection area (EPA), sometimes referred to as the safe operating area, is at the heart of any electrostatic discharge control measure. In this area, electrostatic discharge sensitive elements (ESDS) or circuit boards, or components containing these, can work safely because the amount of charge is controlled without generating damaging voltages. This area usually contains workbenches or workgroups, workstations, processing equipment such as automatic plug-in machines, or a production area. The scope of the EPA must be clearly marked, and it is best to set up a fence to prevent unauthorised people from entering. Materials with minimal static charge buildup should be used in the EPA area, and the charge can be discharged into the ground in a controlled manner.
5. Safety
Power tools and equipment are generally available within the EPA. In this environment, it is dangerous to connect a single object or device directly to the ground. It is for this reason that a resistance of not less than 1M should be connected in series at the connection of the wrist ground wire, the runner and the toe band. Some wristband grounding conductors have such a resistor on each end, so even if the wristband grounding conductor is connected to the live terminal of a powered-on product for repair, there is no danger. Wrist grounding wire tester is an instrument to check whether the resistance of the resistor is appropriate (if it is too high, it is impossible to achieve equipotential bonding; if it is too low, a safety hazard will occur). The wristband grounding wire should be equipped with a plug that is incompatible with other electrical sockets, which can be easily removed, and it can be easily removed in an emergency.
6. Practical work in electrostatic discharge protection zones
In an electrostatic discharge protection zone, charges and potentials cannot be kept within permissible ranges unless clear operating specifications are followed. Some examples of problems that can cause problems include bringing documents, plastic containers, cups, etc. in non-static antistatic plastic covers into electrostatic discharge protection areas, and using cleaners that can damage the electrostatic characteristics of floors or work surfaces. Relevant personnel should be adequately trained not only to learn the procedures to be followed, but also to understand the reasons why they must be followed. It is also useful to know the relevant parameters of components that may be damaged. A special person should be appointed to take care of the maintenance and maintenance of the electrostatic discharge protection area, and at the same time, check the implementation of the regulations. These checks should also be checked as part of the quality management system certification.
7. Transportation and storage
When transporting leaded components, conductive foam is often used. This can prevent higher potential differences between the component pins. For components in dual in-line packages, static dissipative tubes are often used during bulk transportation. For circuit board components, when they are located outside the ESD protection area, they should be transported in an electrostatic shielding bag or a conductive carrying case. Some packaging bags are made of conductive materials, which can ensure that all components are at the same potential under stable conditions, and at the same time dissipate electrostatic charges that accidentally run on the bag. This method cannot be used for circuit boards with batteries. In this case, a packaging bag with a static dissipative material lining and a conductive material outer layer should be used. These bags are more expensive but provide excellent protection for both powered and unpowered components. Similarly, conductive boxes with guides for fixed circuit boards inside cannot be used with power-on circuit boards with bare connectors on the edges.
8. Field repair
An electrostatic connection point should be set on the product to be repaired on site, so that the maintenance technician can connect the ground wire of the wristband before opening the cover of the device. Spare parts should be transported in static-shielded bags or cases unless they do not contain electrostatic discharge-sensitive components. If the module is working in an exposed state, connect a static-dissipative floor mat to the product's electrostatic bonding point and use it as a work surface.
9. Related standards
In 1987, the United Kingdom made its first attempt to document practices, with the result being BS5783. Rather than calling it a standard about what tests should be performed, it is better to call it a code of practice. The second phase of this work is to translate this standard into a specification in a European organization, whose number is CECC 000151, and its title is: "Basic Specification: Protection of Static Sensitive Components. Part 1: General Requirements." The standard was published in 1991 and renumbered as EN 1000151 in 1992. The other sections were published in 1993 (part two: requirements for low humidity conditions) and 1994 (part three: requirements for clean areas, and part four: requirements for high pressure environments). The content of these sections is beyond the scope of this article. The standard not only includes requirements for installation, maintenance, and inspection of the measures described in this article, but also details the detailed requirements of the ESD protection device itself, including test methods. The continuous development of technology and processes and the experience accumulated in the implementation of standards, as well as the widespread use of automated machinery and equipment, have led to the continuous improvement of these standards, including the rationalization of their structure and the separation of user guides from standardized versions. The revision work has been included in the international forum organized by the International Electrotechnical Commission. The newly developed standards will be published in the IEC 1340 series. There is no doubt that this is complementary to European standards.
