1. The generation and harm of static electricity
Static electricity is the product of electrons moving inside or between materials (including polarization and conduction). Two different materials are in contact with each other, and less than a certain distance between them, such as 10-25 cm, electrons in the two materials exchange through the interface due to the tunneling effect. When the exchange reaches equilibrium, a certain potential difference will be generated between the materials, and an equal amount of positive and negative charges appear on both sides of the interface. If the two materials after the contact are separated, the two materials will be charged with the same amount of positive and negative charges, which is the basic principle of static electricity generation.
There are three main ways to generate static electricity: triboelectric charging, conduction charging, and induction charging.
Frictional electrification: Because objects of different materials are separated after contact, because different nucleus has different binding abilities to electrons, when two different materials contact or rub, the peripheral electrons will be transferred to the side with strong binding ability, resulting in a material band. Positive, another material is negatively charged.
Conductive charging: Because in the conductor, electrons can move freely on his surface. When in contact with a charged body, electrons are transferred from the charge, resulting in a charge balance between the two, thereby forming an electrostatic phenomenon.
Inductive charging: refers to the induction of the adjacent electric field. For the conductor, the electron moves freely on the surface of the conductive material. If the conductor is placed in another electrostatic field, the opposite charges will attract each other, and the positive and negative ions will occur. In the transfer, the conductor is charged due to the electrostatic field induction causing the positive and negative charges to be unbalanced.
It can be seen from the basic principles and methods of static electricity generation that in many processes of general electronic product manufacturing, static electricity may be generated in many processes. Electrostatic manufacturing processes may cause static electricity to be generated by operators, work surfaces, tools, components, and packaging. As long as static electricity is present, there must be an ESD (Electro-Static Discharge) process, mainly in the case of instantaneous discharge current to the circuit. The noise generated by the induction, as well as the discharge current, causes the reference ground potential, such as the ground potential of the product ground and the signal ground, to fluctuate, causing interference to the normal operation of the circuit.
Electrostatic hazards have some different characteristics from general lightning protection or electromagnetic interference.
Concealment: The general electrostatic discharge is not perceived by the human body, but the components are damaged without knowing it.
Potential and Cumulative: Some components exhibit only a drop in performance parameters after being damaged by electrostatic discharge, but have not yet failed. In the case of continued use, failure may occur, so static electricity is potentially harmful to the device.
Randomness: Electrostatic discharge damage of electronic components may occur at any point, from any step of the process, from manufacturing to manufacturing to maintenance, in contact with any charged human body (or object), and is highly random.
Complexity: Some electrostatic damage is also difficult to distinguish from damage caused by other causes, causing people to mistake the electrostatic damage as other failures, thus making a wrong judgment.
For electronic product assembly, the danger of static electricity seriously affects the quality, yield and reliability of the product. Systematic anti-static measures must be taken for the clean room for electronic product assembly to minimize the electrostatic hazard during production. .
2, static protection
Good static protection usually requires the following three basic principles:
(1) reduce or prevent the opportunity for static charge accumulation;
(2) Build a safe electrostatic discharge path;
(3) Introduce necessary and effective static monitoring equipment for system static monitoring.
2.1 Good grounding system
To reduce or prevent the possibility of static charge accumulation and the foundation of a safe electrostatic discharge path, it is necessary to construct a good electrostatic grounding system. Electrostatic grounding refers to passing an electrostatic object or an object that may generate static electricity (non-insulator) through the electrical conductor. Form an electrical circuit with the earth so that it is at the same potential as the earth. Its purpose is to accelerate the flow and leakage of static electricity, so that the static charge of the charged material is effectively and smoothly released to avoid static electricity accumulation.
2.2 Environmental Control
The generation and size of static electricity are closely related to the ambient humidity and the concentration of ions in the air. The electrostatic potential is inversely proportional to the humidity. At the same time, in a place where the air is purer than in a normal place (such as a clean room), static electricity is more likely to occur due to the low ion concentration in the air. The same action can produce electrostatic voltages that differ by more than an order of magnitude at different humidity levels. However, it is worth noting that the higher the humidity, the better. If the humidity is too high, the equipment will be prone to dew condensation, so the humidity should be controlled within a reasonable range, such as 30% to 75%.
3, static monitoring
High humidity can reduce the static generated to the human body, but it is still enough to damage sensitive components. The correct concept is that high humidity can suppress the amount of static electricity generated, while low humidity is the opposite. Therefore, in order to meet the strict requirements of electrostatic control for some products, in addition to the traditional electrostatic protection measures, it is necessary to monitor and record the static electricity generation. . According to the actual situation, the anti-static access control system and the static online real-time monitoring system can be used to effectively monitor and record the static electricity.
3.1 anti-static access control system
In order to be more effective against static electricity, source control must be performed from an electrostatic source. To this end, an anti-static access control system is introduced into key areas. The access control system can detect whether the anti-static measures or equipment of each worker entering the anti-static area is qualified. System function modules include: identification of identity and permissions, detection of anti-static wrist straps and anti-static shoes, level control boards, etc. To further ensure the effectiveness of anti-static access control, for some production enterprises that require cleanliness of the working environment, the access control system can be linked with the air-leaf dust removal system leading to the work area, that is, the access control signal is integrated into the air shower door control system. In this mode, the first level of access to the work area will ensure that the anti-static equipment is qualified and effective.
3.2 anti-static real-time online monitoring system
Under normal circumstances, the general electronics manufacturing industry generally uses an electrostatic tester to test the employee's anti-static wrist. In order to ensure compliance with ISO9001 requirements, draw a circle or draw other symbols on a form to confirm that the test is detected and the test is qualified. . However, if the employee's anti-static wrist strap fails during operation, or if a certain loop of the entire grounding system is suddenly abnormally disconnected or is not effectively connected, it is difficult to find that the anti-static system has failed.
For example, an electronic factory introduces a real-time online monitoring module into all grounding systems, and utilizes the integrity characteristics of the grounding system loop. As long as which link is open or the connection resistance is too high (greater than 10 Ω), the red on the control panel The light flashes immediately and a buzzer alarm can be added if necessary. The biggest feature of this system is real-time monitoring. The work of static electricity protection is no longer a virtual matter, and there is no need to make some cumbersome and formal paper records.
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