Generation, influence and protection of static electricity

1、 Generation of static electricity:

1. Friction: In daily life, any two objects made of different materials can generate static electricity after contact and separation. The most common way to generate static electricity is to generate electricity by friction. The better the insulation of materials, the easier it is to use friction to generate electricity. In addition, static electricity can also be generated when objects of any two different substances are separated after contact.

2. Induction: For conductive materials, because electrons can flow freely on their surfaces, if they are placed in an electric field, positive and negative electrons will transfer due to the repulsion of the same and the attraction of the different.

3. Conduction: For conductive materials, since electrons can flow freely on their surfaces, if they contact charged objects, charge transfer will occur.

2、 Influence of static electricity on electronic industry

The circuit size, withstand voltage and line area of integrated circuit components are reduced, which weakens the static impact resistance of components. Electrostatic field and current become the fatal killer of these high-density components. At the same time, a large number of plastic products and other high insulation materials are widely used, resulting in a great increase in the chance of generating static electricity. Static electricity can be generated in daily life such as walking, air flow, transportation, etc. It is generally believed that only CMOS chips are sensitive to static electricity. In fact, highly integrated components and circuits are very sensitive.

A. Influence of static electricity on electronic components

1. Static electricity absorbs dust, changes the impedance between lines, and affects the function and life of the product.

2. The insulation or conductor of the element is damaged due to electric field or current, making the element inoperable (completely damaged).

3. The component can still work and its life is damaged due to the heat generated by the instantaneous electric field or current.

B. Characteristics of electrostatic damage:

1. The concealed human body cannot directly perceive the static electricity, unless the static electricity discharge occurs, but the human body may not feel electric shock when the static electricity discharge occurs. This is because the electrostatic discharge voltage perceived by the human body is 2-3KV.

2. Latent Some electronic components have no obvious performance degradation after being damaged by static electricity, but multiple cumulative discharges will cause internal injury to the device and form hidden dangers, and increase the sensitivity of the device to static electricity. There is no cure for the problems that have arisen.

3. Under what circumstances will random electronic components be damaged by static electricity? It can be said that all processes from the production of a component to its damage are threatened by static electricity, and the generation of these static electricity is also random. The generation and discharge of static electricity are instantaneous and difficult to predict and protect.

4. The complicated electrostatic discharge damage is divided into plates. Because of the fine and small structure characteristics of electronic products, it takes time, trouble and money. Complex technologies often require the use of precision instruments such as scanning electron microscopy. Even so, some electrostatic damage phenomena are difficult to distinguish from the damage caused by other reasons, which makes people mistake the electrostatic damage failure as other failures. This is before they fully understand the electrostatic discharge damage, It is often attributed to early failure or failure with unknown conditions, thus unconsciously covering up the real cause of failure.

5. On the surface, the serious ESD problem only affects the users of finished products, but in fact it also affects manufacturers at all levels, such as warranty fees, maintenance and the company's reputation.

3、 Three types of ESD

1. Human body type refers to the friction charge generated by the friction between the body and clothes during human activities. When people hold ESD sensitive devices without first dragging charges to the ground, friction charges will move to ESD sensitive devices and cause damage.

2. The electrification type of microelectronic devices refers to these ESD sensitive devices, especially for plastic parts, which will generate friction charges during the automatic production process, and these friction charges will quickly drain to the highly conductive and firmly grounded surface through low resistance lines, thus causing damage; Or the metal part of the ESD sensitive device may be electrified by induction to cause damage.

3. The field induction type is surrounded by a strong electric field, which may come from plastic materials or people's clothes, and electronic conversion will occur across the oxide layer. If the potential difference exceeds the dielectric constant of the oxide layer, an arc will be generated on the side to destroy the oxide layer, resulting in a short circuit.

4、 Electrostatic protection

1. Grounding

Grounding is to directly discharge static electricity through a wire connection to the ground, which is the most direct and effective anti-static measures. For conductors, grounding methods are usually used, such as manual anti-static wrist strap and worktable grounding.

Grounding is implemented by the following methods:

1) The human body is grounded through the wrist strap.

2) Human body is grounded through anti-static shoes (or shoelaces) and anti-static floor.

3) The worktable is grounded.

4) Test instrument, tool clamp and soldering iron are grounded.

5) Anti static floor, grounding mat.

6) The anti-static transfer vehicle, box and frame shall be grounded as far as possible.

7) The anti-static chair is grounded.

2. Electrostatic shielding

Electrostatic sensitive components will be exposed to electrostatic areas during storage or transportation. Electrostatic shielding can weaken the impact of external static electricity on electronic components. The most common method is to use electrostatic shielding bags and anti-static turnover boxes for protection. In addition, anti-static clothing has a certain shielding effect on human clothes.

3. Ion neutralization

Insulators are often prone to generate static electricity, and the grounding method is invalid for eliminating the static electricity of insulators. The commonly adopted method is ion neutralization (partially shielded), that is, use ion fans in the working environment to provide a working area with an equal potential.

Therefore, anti-static materials and anti-static facilities are products derived from these three ways, which can be divided into anti-static instruments, anti-static products of grounding system, anti-static shielding packaging, transportation and storage of anti-static materials, neutralization type static elimination equipment, and other anti-static products.

A. Anti static instrument

1. The wrist strap/foot strap/anti-static shoes comprehensive tester is also called the human body comprehensive tester - purpose: to check whether the wrist strap, foot strap and anti-static shoes meet the requirements.

2. When testing foot straps and anti-static shoes, it is necessary to add a metal plate and a conductor connected to the instrument.

3. Electrostatic ion blower detector - purpose: regularly detect and verify the balance and decay time of the ion blower to ensure that the ion blower works within the safe index range.

4. Electrostatic field detector is also called electrostatic tester or electrostatic voltmeter - purpose: measure the electrostatic field to reflect the existence of static electricity, read in the form of voltage, and test the electrostatic intensity of the environment. It is generally affected by the environment and the transient characteristics of static electricity, and it is difficult to truly reflect the actual situation.

5. Electrostatic shielding bag tester - purpose: used to test the shielding effect of electrostatic shielding bags.

6. Surface resistance meter - purpose: used to measure the surface resistance and volume resistance of materials.

B. Grounding anti-static products

1. Anti static wrist strap: It is widely used in various operation stations. There are many types of wrist straps. It is recommended to use wrist straps with 1 megohm resistance, and a certain margin should be left for the wire length.

2. Anti static watch: it needs other anti-static measures (such as adding ion blower, wearing anti-static heel strap, etc.) to achieve good anti-static effect. It is recommended not to wear anti-static watches in large quantities.

3. Anti static foot strap/anti-static shoes: after using the anti-static ground in the workshop, the anti-static shoe strap or anti-static shoes should be worn. It is recommended that the workshop mainly wear anti-static shoes to reduce the introduction of dust. It will be better if operators and employees wear anti-static wrist strap.

4. Anti static pad: used for laying the surface of each workbench. Each pad is connected to the anti-static ground reliably after being connected with a 1 megohm resistance.

5. Anti static floor: Anti static floor is divided into PVC floor, polyamine floor and raised floor.

6. Anti static wax and anti-static paint: anti-static wax can be used to increase anti-static function on various floor surfaces and make the floor more bright and clean. Anti static paint can be used on various floor surfaces, as well as on various shelves, turnover boxes and other containers.

C. Shielding anti-static packaging, transportation and storage materials

1. Anti static turnover box and anti-static component box: used for turnover, transportation and storage of single boards and components in the workshop.

2. Anti static shielding bag: used for packaging, transportation and storage of veneer and components, with certain moisture-proof effect.

3. Anti static tape: used for various packing boxes, etc

4. Anti static IC strips and IC trays: used for the storage and handling of IC components in the production workshop. It is prohibited to store IC in the open air before use; Or unpack for transportation.

5. Anti static shelves, trolleys and workbenches: Anti static shelves and trolleys are widely used for the turnover and handling of boards and components in electronic assembly workshops. The anti-static shelf and workbench shall be connected with anti-static ground, and the anti-static pad on the trolley shall be in contact with the anti-static ground with a metal chain

6. Anti static work clothes and work shoes: In the processing workshop with static sensitive components and certain cleanliness requirements, employees should generally be strictly required to wear anti-static work clothes and work shoes

7. Anti static finger sleeve: If the operator needs to hold the workpiece or static sensitive components frequently, it is necessary to wear an anti-static finger sleeve.

D. Neutralization equipment

1. Ion fans: according to the placement mode, there are table type, horizontal type and suspension type. According to the number of holes, there are single head ion fans, double head ion fans, three head ion fans, four head ion fans, five head ion fans, as well as ion fans that are suitable for harsh environments and need wide range and rapid elimination of static electricity. Table type ion fans, horizontal ion fans and single head ion fans can be placed on the workbench or installed in suspension mode, Double head, three head, four head and five head are suspension type ion fans. The ion fans can be used only by plugging in 220V power supply, and the air volume can be adjusted. Except for the ion air blower, due to the limitation of air volume, the ion fans are suitable for places where static electricity is highly required to eliminate the residual voltage of static electricity, and the dust removal function will be less effective than other static electricity removal equipment;

2. Ion air gun: it can be divided into ordinary ion air gun, plastic gun body ion air gun, big head ion air gun, TOP GUN ion air gun (also called high-precision ion air gun, suitable for use in purification rooms with high requirements). Ion air guns are generally handheld, and electrostatic elimination and dust removal are carried out at the same time, requiring external compressed air, The air source is required to be free of water and oil (generally, air compressors are equipped with oil and water filtering devices, if not, they can be purchased at the hardware store, and each one costs 30-100 yuan);

3. Ion air nozzle: there are common ion air nozzle, inductive ion air nozzle (automatic induction object blowing), which are generally fixed on the assembly line for use;

4. Ion wind snake: similar to the ion wind nozzle, the difference is that it has a base, the air duct can be folded, and the blowing direction can be changed at will;

5. Ionic wind rod: according to the material, there are copper rods (ionic copper rods, ionic wind copper rods), aluminum alloy rods (ionic aluminum rods, ionic aluminum rods, ionic wind curtains). According to the mode of ion generation, there are ionization type and induction type. Copper rods generally have electric ions. Aluminum alloy rods are generally induction type, and the length is generally customized as required. There are wind and no wind, and they are generally installed on the equipment for use;

6. Static electricity removal net: generally installed at the air inlet at the top of the workshop. The purification room is well installed and can cover the whole room. The size is customized as required.