What is static electricity?

A. Static Electricity Phenomenon
1. The benefits and harms of static electricity:
Benefits: Electrostatic imaging, electrostatic spraying, and electrostatic dust removal have been widely used.
Harms: 1. The first Apollo manned spacecraft was killed in a fire and explosion due to electrostatic discharge (ESD).
2. 45% of the unqualified devices in Japanese IC production are caused by static electricity.
3. In 1988, the United States lost $5 billion due to ESD.
4. In the early 1990s, a company in Beijing tried to produce high-end digital multimeters. Because the IC did not pay attention to anti-static, most of its products were unqualified.
2. What is static electricity?

Generally speaking, plastics have a very high electrical resistance, so plastics can retain a static charge for a long time. Metals, on the other hand, have a very low electrical resistance, so grounded metals retain a static charge for a very short time. Static electricity is usually measured in volts. While 220 volts AC is dangerous, 100 kV of static electricity is quite common. The voltage in an object is determined by two factors: the charge of the object and the capacitance of the object. This can be expressed as a simple relationship, Q=CV, where Q is the charge, V is the voltage, and C is the capacitance of the object.
Given the charge of an object, the lower the capacitance, the higher the voltage, and vice versa. Plastics generally have a very low capacitance, so a small amount of charge can produce a high voltage. Conversely, metals have a very high capacitance, so a larger amount of charge produces a lower voltage. This is why static electricity is a more serious problem when working with plastics in real applications. High voltages can attract dust, cause electric shocks to operators, or cause changes in the properties of the object.

There are two main types of static electricity: bulk static electricity and surface static electricity. Bulk static electricity refers to the charge distributed throughout the interior of an object. Surface charge refers to the charge on the outermost surface of an object.
In practical applications, static electricity problems are basically related to surface static electricity. Although there is no way to neutralize body static electricity, body static electricity rarely causes problems. Generally, compared with surface static electricity, the impact of body static electricity is very small.

1. Static electricity is the excess or insufficient static charge on the surface of an object. It is a kind of electrical energy.
Static electricity is the result of the loss of balance between positive and negative charges in a local area. Static electricity is formed by the transfer of electrons or ions.
2. Static electricity is the relatively stationary charge carried by an object.
3. Electrostatic discharge (ESD) phenomenon:
1. Organic glass absorbs small pieces of paper after rubbing with cotton cloth.
2. When taking off a synthetic fiber top, a crackling sound is produced, and sparks can be seen at night (the air breakdown field strength is 30KV/cm).
4. The generation of static electricity:
contact, friction, rush flow, piezoelectricity, temperature difference, freezing, electrolysis
5. Characteristics of static electricity:
High potential: can reach tens of thousands to hundreds of thousands of volts, and often hundreds and thousands of volts during operation.
Low power: static current is mostly microampere (mA).
Short action time: microseconds
Greatly affected by the environment: especially humidity, humidity rises, static electricity decreases.
6. Unit of static charge:
Coulomb (C), millicoulomb (mC), microcoulomb (mC), picocoulomb (rrC)
7. Surface charge density:
C/㎡ s= (q=charge; s=surface area)
8. Distinguish the conductivity of materials from electrostatics:
Static conductor: less than 1×106Wcm (volume resistance) Example: metal
Static subconductor: greater than 1×106Wcm and less than 1×1010Wcm anti-static equipment
Static insulator: greater than 1×1010Wcm hard plastic
Note: Due to the characteristics of static electricity (high potential, small charge), it is different from the division of conductors and insulators in ordinary electrical engineering.
B. Physical process of static electricity generation
1. Electrostatic charging phenomenon:
When fur rubs against a bakelite stick, each of them carries charges of opposite polarity and the same amount. One loses electrons or holes and becomes charged, such as fur with positive charge, and the other gains electrons and becomes charged with electrons, such as bakelite stick - negative charge.
2. Electrostatic sequence:
air → human hands → asbestos → rabbit hair → glass → mica → human hair → nylon → wool → aluminum → silk → aluminum → paper → cotton → steel → wood → amber → wax → hard rubber → nickel, copper → brass, silver → gold, platinum → sulfur → rayon → polyester → celluloid → aurora → polyurethane → polyethylene → polypropylene → polyvinyl chloride → silicon dioxide → polytetrafluoroethylene
Among them, after any two substances are rubbed, the former is positively charged and the latter is negative. But it is not absolute, and it is related to the cleanliness of the material surface, environmental conditions, contact pressure, smoothness, surface size, separation speed, etc.
3. Solid electrification:
related to the electron work function (work function), temperature, and charge carrier concentration.
Forms: 1. Fracture electrification
The charged particles of the material are separated due to mechanical rupture, so that both sides carry equal and opposite charges.
2. Piezoelectric effect: quartz has a potential difference of 0.5V, such as: electric shoes, gas lighters
3. Thermoelectric effect: When the piezoelectric crystal is heated, one end is positively charged and the other end is negatively charged; when cooled, the polarity is reversed
4. Peeling electrification: When objects that are closely combined with each other are peeled off, it will cause charge separation, causing both sides of the separated objects to be charged, such as when stacked plastic bags are separated, the secondary current insulation film.
4. Powder electrification:
The electrostatic particles caused by the collision between powder and the wall, powder and powder, contact separation, friction, and fragmentation are smaller, the stronger the interaction, the larger the collision area, and the stronger the electrification. 5. Liquid electrification:
1. Liquid double electric layer
The interface between the liquid phase and the solid phase can also form a double electric layer: the ionizable impurities and other ions in the liquid (petroleum) are attracted to the metal wall. Positive and negative ions cannot have the same affinity for metals. Those with greater affinity are attracted and adsorbed by the metal surface, while ions of opposite sign with equal charge in the liquid remain in the liquid phase and gather near the interface. A double electric layer is formed at the interface. The inner layer is the ions close to the solid surface, called the fixed layer, while the outer layer ions are movable, called the active layer or diffusion layer.
2. Flowing charge
When the liquid flows, the charged particles in the flowing layer flow with the liquid to form a flowing current, and the opposite charged particles remain in the pipeline. If the pipeline is grounded, they flow into the earth, and the flowing charge forms a flowing current. In steady state, the charged particles are constantly carried away by the flowing liquid, and the fixed layer charge is neutralized through the grounded pipeline. This phenomenon is a bit like electrolysis. Different affinities, diffusion and liquid flow replace the electric field effect in electrolysis. Factors affecting the size and polarity of the charge are: 1. Liquid type and characteristics; 2. Pipeline material and surface smoothness; 3. Flow rate, temperature, water content, air, mixture and impurity particles.
6. Human static electricity
The human body is one of the main sources of static electricity
The human body's activities convert part of the mechanical energy into electrical energy
Daily heat consumption: 4.18×107J = 10KWh
Instantaneous discharge power reaches 90 kilowatts
Walking charges about 800V
When standing up from a PVC chair, the human body is charged 18KV
The resistance of the human body to the ground is mainly determined by shoes, socks, and the ground
Humidity has a great impact: Sitting on a plastic chair
10~20% 18KV
65~90% 1.5KV
Human body equivalent capacitance: 100PF
Resistance: 1KW (~1.5KW)
Safe electrostatic voltage: 100V (national standard SJ/T 10630-95)
C. Electrostatic damage
1. Soft damage - performance degradation, cumulative effect accounts for 90%, implicit
Hard damage - one-time permanent failure, accounting for 10%
2. Classification of device sensitivity:
Class I 0~1999V
Class II 2000V~3999V
Class III 4000V~15999V
Non-sensitive 16000V or above
3. Electrostatic sensitivity of some devices:
As can be seen from the above table, although the range of electrostatic sensitivity of each device is large, its lower limit is generally only tens of volts to hundreds of volts, which is lower than the static voltage of operators, work surfaces, and tools in electronic industrial production, so ESD damage may occur.
Single boards equipped with electrostatic sensitive devices are also susceptible to electrostatic damage. Circuit design, layout, and processing should all be paid attention to.
IV. Electrostatic damage model:
Human body charged model: The human body may generate different values ​​of static electricity in daily activities and production operations, with voltages ranging from ten volts to tens of thousands of volts. HBM is based on the fact that an operator with static electricity contacts the pins of a device during work, discharges the static charge stored in the human body to the ground through the device, causing the device to fail. Therefore, it is called the human body charged model, and its equivalent circuit is shown in the figure.
E High-voltage DC power supply (0~5kV)
Rb Human body equivalent resistance (1.5k 1%)
Cb Human body equivalent capacitance (100PF 10%)
Rs Charging current limiting resistor (1~10M)
DUT Test device
K1 High-voltage relay
In addition, there are "device charged model" and "electric field induction model".
V. Damage example:
1. Motorola, MOS large-scale integrated circuit - CPU, aging for 11 weeks. At the beginning, the rejection rate was 40×10－n when the conductive box was not used for placement. However, it dropped to 15×10－n after four weeks when the nickel-plated box was used for placement. This experiment was followed for more than 7 weeks, with an average of 18×10－n.
2. After screening and storage, the CMOS circuits produced by a domestic factory were found to have a 5% failure rate in random inspections, and the failure rate mode was an increase in input leakage current. After analysis, it was found to be related to ESD. The CMOS circuits produced by the factory were placed in ordinary plastic boxes before and after testing, and the static charge on the plastic was transferred to the CMOS circuit. During the test, when the device touched the human body or the grounded metal on the desktop, it would cause discharge, resulting in ESD damage and failure. Later, a series of ESD measures were adopted and the anti-static box was changed, and the phenomenon disappeared.
Electrostatic insulator, resistivity>1012, hard plastic, organic glass. D. Anti-static method
1. Anti-static area design principles:
① Suppress the accumulation of static charge and the generation of static voltage. For example, equipment, instruments, and tooling do not use plastic, organic glass, and ordinary plastic bags.
② Safely, quickly and effectively eliminate the static charge that has been generated, use a roped anti-static wrist, anti-static chair, car, and box.
③ Ensure that the static voltage is less than 100V.
2. Ground
Anti-static ground, (anti-static terrazzo, anti-static floor) 105~1010, lay a ground wire network.
3. Workstation
The table, work chair, and stool surface should be made of ESD protection materials.
4. Human body
Wear anti-static clothing, shoes, wrists, etc. (1M).
5. Grounding
a. The anti-static work area must have a safe and reliable anti-static grounding device with a ground resistance of less than 4. The anti-static ground wire must not be connected to the power supply neutral line, and must not be shared with the lightning protection ground wire. When using a three-phase five-wire power supply, its ground wire can be used as an anti-static ground wire.
b. The work table, floor mat, chair, stool and other ESD protection measures that conduct static electricity should be connected to the ground wire through a current limiting resistor. The wrist strap should be connected to the ground wire through the grounding point on the top of the workbench. The workbenches cannot be connected to each other in series.
c. The grounding system of the anti-static work area, including the current limiting resistor and the connection terminal, should be reliably connected and have a certain current carrying capacity. The resistance value of the current limiting resistor should be selected to ensure that the leakage current does not exceed 5mA, and the lower limit is 1M Ω.
6. Humidity
When it is less than 60%, an anti-static operating system must be built.
7. Ionizer
In situations where static charge cannot be effectively discharged, an ionizer can be used to prevent and neutralize the charge accumulation on components and other objects through positive and negative ions in the air. The ionization capacity is greater than 250V/s.
8. Humidification
The humidifier can make humid air flow to prevent the accumulation of static charge. This method is not suitable for places where harmful effects are produced after humidification.
9. Packaging
Electrostatic sensitive devices should be protected;
the packaging of electrostatic sensitive devices must use anti-static storage boxes and anti-static plastic bags.
10. Transportation and storage
SSD must be placed in anti-static containers (boxes, bags) and transported with anti-static transportation tools (cars).
The warehouse meets the requirements of the anti-static operating system. SSD must be placed in an anti-static container and kept away from static electricity, electromagnetic fields or radiation fields during storage and transportation.
11. SSD components should be handled and placed in categories. The electrostatic sensitive symbols
The symbols comply with the provisions of GJB1649.
E. Measurement methods and timeliness of anti-static materials
1. Friction charging method: cotton cloth 120 times/min 20 times.
2. Resistivity measurement method, less than 1010W is better.
3. Attenuation constant RC
Red plastic bags and red foam have a timeliness of half a year, and cannot be exposed to the sun. Black anti-static boxes have a timeliness of 5 years. Plastic tubes for ICs also have timeliness.
F. Problems existing on the production system site
Friction method test
1. 3 production lines
Repair welding line 1 (pilot line) The static voltage is as high as 880V
Repair welding line 3 The static voltage is as high as 580V (belt line)
Cable pallet line 180V
2. Component packaging plastic bags reach 2000V, containing various ICs
3. Parts box
3300V, containing various ICs.
4. Automatic line and tooling
White plastic pressure bar for user board test line
5. The shell of the software writer is ordinary plastic, which can carry hundreds of volts of static electricity after friction.
G. Static electricity testing instruments
1. Non-contact static electricity field measuring instrument
2. Surface resistance measuring instrument
H. Work to be done
1. Improve the anti-static process specifications in all links of production
2. Classification and anti-static regulations of electrostatic sensitive devices
3. Improve the anti-static design requirements of electrostatic sensitive devices in circuits
4. Quality assurance of anti-static materials and equipment
5. Training