Electrostatic knowledge of SMT training materials

I. Knowledge of static electricity

The so-called static electricity is a static charge or a non-flowing charge (the flowing charge forms a current). Static electricity is formed when electric charges accumulate on an object or surface, and electric charges are divided into positive and negative charges, that is to say, electrostatic phenomena are also divided into positive and negative charges. When a positive charge gathers on an object, it forms positive static electricity, and when a negative charge gathers on an object, it forms negative static electricity. However, whether it is positive static electricity or negative static electricity, when an electrostatically charged object touches an object with zero potential (grounded object) or an object with potential difference with it, charge transfer will occur, that is, we see the spark discharge phenomenon everyday.

For example, in winter in northern China, when the weather is dry, the human body is prone to static electricity, and discharge will occur when it comes into contact with others or metal conductors. People will feel the tingling of electric shock, and they can see sparks at night, which is the reason why chemical fiber clothing rubs against the human body and brings positive static electricity to the human body. (With basic physical knowledge, we know that the rubber stick rubs against the fur, the rubber stick is negatively charged, and the fur is positively charged).

Second, the generation of static electricity

Static electricity is not static electricity, but electricity that temporarily stays somewhere macroscopically. When a person stands on a carpet or sofa, the voltage of the human body can also be higher than 10,000 volts, while the static electricity in the rubber and plastic film industry can be as high as 100,000 volts.

Any two objects made of different materials contact and then separate, and static electricity can be generated. The common method of generating static electricity is triboelectricity. The better the insulation of the material, the easier it is to generate static electricity. Because air is also composed of atoms, it can be said that static electricity may be generated at any time and any place in people's lives.

It is almost impossible to completely eliminate static electricity, but some measures can be taken to control static electricity so that it will not cause harm.

When two different objects touch and rub against each other, the electrons of one object are transferred to the other object, which is positively charged because of the lack of electrons, while the other object gets some surplus electrons and is negatively charged, and the object is charged with static electricity.

Third, the harm of static electricity

There are many hazards of static electricity, and some of them come from the interaction of charged bodies.

The second hazard of static electricity is that it is possible for some flammable objects to ignite and explode due to static sparks.

The generation of static electricity is inevitable in industrial production, and its harm can be mainly attributed to the following two mechanisms.

First, the harm caused by electrostatic discharge (ESD):

(1) Cause the malfunction or misoperation of electronic equipment, and cause electromagnetic interference.

(2) Breakdown of integrated circuits and precision electronic components, or aging of components, reducing the yield of production.

(3) High voltage electrostatic discharge causes electric shock, endangering personal safety.

(4) It is easy to cause explosion and fire in the production place with many inflammable and explosive articles or dust and oil mist.

Second, the harm caused by electrostatic attraction (ESA):

(1) Electronic industry: adsorption of dust, resulting in pollution of integrated circuits and semiconductor components, greatly reducing the rate of finished products.

(2) Film and plastic industry: uneven rolling of film or film; Film and CD plastic discs are contaminated with dust, which affects the quality.

(3) Paper-making and printing industry: the paper is not rolled up evenly, the overprinter is not allowed, the dirt is sucked seriously, and even the paper sticks, which affects the production.

(4) Textile industry: causing harm such as fluttering of roots, broken ends of flowers and tangled yarns.

Four, the method of eliminating static electricity

Use anti-static appliances: electrostatic bracelet, electrostatic gloves, electrostatic clothes, electrostatic cap, electrostatic table mat and electrostatic floor.

Increasing humidity: Washing hands, bathing and changing clothes frequently can effectively eliminate static electricity accumulated on the surface of human body. Humidity control in workshop.

Grounding: To keep the human body connected to the earth, the static electricity of the human body can be conducted into the earth.

Use antistatic materials: electrostatic rubber basin, electrostatic material box and electrostatic cart.

Discharge of static electricity: Holding the metal door frame can discharge static electricity from your body at any time when friction generates static electricity.