ESD static hazards and protective measures
I. Terms and definitions
1. Static electricity: a static charge that is excessive or insufficient on the surface of the object
2. Electrostatic field: an electric field formed by static electricity around it
3. Electrostatic discharge: Two objects with different electrostatic potentials cause the transfer of electrostatic charges between two objects due to direct contact or electrostatic field induction. After the energy of the electrostatic electric field reaches a certain level, the phenomenon that the medium is broken down and discharged is electrostatic discharge.
4. Static sensitivity: the electrostatic discharge voltage that the component can withstand
5. Electrostatic sensitive devices: devices sensitive to electrostatic discharge
6. Grounding: Electrically connected to objects that can supply or accept a large amount of electric charge, such as earth, boat, etc.
7. Neutralization: Using static charge to make static electricity disappear
8. Anti-static work area: equipped with various anti-static equipment and equipment, can limit the electrostatic potential, has clear regional boundaries and specially marked work sites suitable for electrostatic protection operation
Second, the generation of static electricity:
1. Friction: In daily life, any two objects of different materials can be separated after contact, and static electricity can be generated. The most common method of generating static electricity is to generate electricity by friction. The better the insulation of the material, the easier it is to use triboelectricity. In addition, the separation of any two different substances can also generate static electricity after contact.
2. Induction: For conductive materials, because electrons can flow freely on its surface, such as placing it in an electric field, due to the same-sex repellent, the opposite sex attracts, and the positive and negative electrons will shift.
3. Conduction: For conductive materials, charge transfer occurs due to the free flow of electrons on its surface, such as contact with charged objects.
Third, the impact of static electricity on the electronics industry
The circuit of the integrated circuit components is reduced, the withstand voltage is reduced, and the line area is reduced, so that the electrostatic shock resistance of the device is weakened, and the electrostatic electric field (Static Electric Field) and the electrostatic current (ESDcurrent) become the deadly killers of these high-density components. At the same time, the widespread use of a large number of high-insulation materials such as plastic products has led to an increase in the chance of generating static electricity. Static electricity can be generated in daily life such as walking, air flow, and handling. It is generally believed that only CMOS-based chips are sensitive to static electricity. In fact, highly integrated component circuits are sensitive.
A. The effect of static electricity on electronic components
A) Electrostatic adsorption of dust, changing the impedance between the lines, affecting the function and life of the product.
B) The assembly is inoperable (completely destroyed) due to electric field or current damage to the insulation or conductor of the component.
C) Due to the heat generated by the instantaneous electric field or current, the component is injured, still working, and the life is damaged.
B, the characteristics of electrostatic damage:
1. Concealed human body can not directly sense static electricity, unless electrostatic discharge occurs, but electrostatic discharge occurs, the human body may not have the feeling of electric shock. This is because the human body perceives an electrostatic discharge voltage of 2-3 kV.
2. Latentity Some electronic components are not significantly degraded by electrostatic damage, but multiple accumulations of discharges can cause internal damage to the device, which increases the sensitivity of the device to static electricity. There are no cures for the problems that have arisen.
3. Under what circumstances will random electronic components be subjected to electrostatic damage? It can be said that all processes from the time of production of a component until it is damaged are threatened by static electricity, and the generation of such static electricity is also random. Since the generation and discharge of static electricity are instantaneous, it is difficult to predict and protect.
4. Complex electrostatic discharge damage sub-board work, due to the fine and small structural features of electronic products, it is time-consuming, labor-intensive, and costly. The more complicated technology requires the use of precision instruments such as scanning electron microscopy, even if there is some electrostatic damage. It is difficult to distinguish from the damage caused by other causes, causing people to mistake the electrostatic damage as other failures. This is often caused by early failure or unclear failure before the electrostatic discharge damage is not fully understood, thus unconsciously concealing. The real reason for failure.
5. Severity ESD problems seem to affect only the users of finished products, but actually affect manufacturers at all levels, such as: warranty, maintenance and company reputation.
Fourth, three types of ESD
1. The human body type means that the friction between the body and the clothes generates a triboelectric charge when the human body moves. When people hold ESD-sensitive devices without first placing charge on the ground, the triboelectric charge will move to the ESD-sensitive device and cause damage.
2. The charged type of microelectronics refers to both ESD-sensitive devices, especially for plastic parts. When automated production, triboelectric charges are generated, and these triboelectric charges are quickly discharged through a low-resistance line to a highly conductive, solid ground. The surface, thus causing damage; or causing damage to the metal portion of the ESD-sensitive device by induction.
3. The field type is surrounded by a strong electric field, which may come from plastic materials or human clothes, where electron conversion occurs across the oxide layer. If the potential difference exceeds the dielectric constant of the oxide layer, an arc is generated on the side to destroy the oxide layer, and as a result, it is short-circuited.
Five, static protection
Ground
Grounding is to directly discharge the connection of static electricity to a ground. This is the most direct and effective way to prevent static electricity. For conductors, grounding is usually used, such as artificially wearing an anti-static wrist strap and grounding the work surface.
Grounding is implemented by the following methods:
1) The human body is grounded through the wrist strap.
2) The human body is grounded through anti-static shoes (or shoelaces) and an anti-static floor.
3) The work surface is grounded.
4) Test instrument, tool holder, soldering iron grounding.
5) Anti-static floor, grounding pad.
6) Anti-static transfer vehicles, boxes and racks should be grounded as much as possible.
7) Ground the anti-static chair.
2. Electrostatic shielding
Electrostatic sensitive components are exposed to static electricity during storage or transportation. The electrostatic shielding method can weaken the influence of external static electricity on electronic components. The most common method is to use electrostatic shielding bags and anti-static turnover boxes as protection. In addition, the anti-static clothing has a certain shielding effect on the clothes of the human body.
3. Ion neutralization
Insulators tend to generate static electricity, and the static electricity of the insulator is eliminated. The grounding method is ineffective. The commonly used method is ion neutralization (partially using shielding), that is, using an ion fan in the working environment to provide an equipotential operation. area.
Therefore, in anti-static materials and anti-static facilities, they are derived from these three methods, which can be divided into anti-static meters, grounding system anti-static products, shielding anti-static packaging, transportation and storage of anti-static materials, Neutral-type static elimination equipment, as well as other anti-static products.
A. Anti-static meter
1. Wrist strap / foot strap / anti-static shoes comprehensive detector - use: for the detection of wrist straps, foot straps, anti-static shoes meet the requirements.
2. When testing the foot strap and anti-static shoes, it is necessary to add a metal plate and the wires connected to the instrument.
3. In addition to electrostatic ion fan detector - use: Regularly check and verify the ion fan balance and decay time to ensure that the ion fan works in a safe range of indicators.
4. Electrostatic Field Detector - Use: Measure the electrostatic field to reflect the presence of static electricity, read in voltage form, and test the electrostatic strength of the environment. Generally, it is difficult to truly reflect the actual situation due to the environmental impact and the instantaneous characteristics of static electricity.
5. Electrostatic shielding bag tester - use: used to detect the shielding effect of the electrostatic shielding bag.
6. Surface resistance measuring instrument - use: used to measure material surface resistance, volume resistance.
B. Grounding anti-static products
1. Anti-static wrist strap: It is widely used in various operating positions. There are many types of wrist straps. It is recommended to use a wrist strap with a resistance of 1 mega ohm. The length of the cable should have a certain margin.
2. Anti-static watch: Remedial of other anti-static measures (such as adding an ion fan, wearing an anti-static heel strap, etc.) to achieve better anti-static effect. It is not recommended to wear a large number of anti-static watches.
3. Anti-static foot strap / anti-static shoes: After using anti-static floor, the factory should wear anti-static shoelaces or anti-static shoes. It is recommended to wear anti-static shoes in the workshop to reduce the introduction of dust. It is better for the operator to combine the anti-static wrist strap with the effect.
4. Anti-static mat: It is used for the laying of the surface of each workbench. Each mat has a 1 megaohm resistor on the string and is reliably connected with anti-static ground.
5. Anti-static floor:
Anti-static floor is divided into: PVC floor, polyurethane floor, raised floor.
Anti-static wax and anti-static paint: Anti-static wax can be used on various floor surfaces to increase anti-static function and make the floor brighter and cleaner.
Anti-static paint can be applied to a variety of floor surfaces, as well as to a variety of shelves, totes and other containers.
C. Shielded anti-static packaging transportation and storage materials
1. Anti-static turnover box, anti-static assembly box: used for the turnover, transportation and storage of the veneer and parts of the workshop.
2. Anti-static shielding bag: used for packaging, transportation and storage of veneers and parts, with a certain moisture-proof effect.
3. Anti-static tape: used in various packaging boxes, etc.
4. Anti-static IC strips and IC trays: used for storage and handling of IC components in production workshops. It is forbidden to store ICs in the open air before use; or unpack and transport.
5. Anti-static shelves, trolleys and workbench: Anti-static shelves and trolleys are widely used in the turnover and handling of veneers and components in electronic assembly workshops. Anti-static shelves and workbench should be connected statically. The anti-static mat on the trolley should have metal chain and anti-static contact.
6. Anti-static overalls work shoes: In the processing workshops with static sensitive components and certain cleanliness requirements, employees should generally be required to wear anti-static overalls work shoes.
7. Anti-static finger cot: If the operator of the operation station needs to hold the workpiece or static sensitive components frequently, it is necessary to wear an anti-static finger cot.
D. Neutralization equipment
Ion fan, air gun