PCB Tech - Introduction to Static Electricity of SMT Processing Technology

1. How is static electricity generated in SMT

Matter is composed of molecules, and molecules are composed of atoms, which are composed of negatively charged electrons and positively charged protons. Under normal conditions, the number of protons of an atom is the same as the number of electrons, and the positive and negative balances are balanced, so it appears uncharged to the outside. However, the electrons surround the nucleus and deviate from the orbit by external force, leave the original atom and invade other atoms B. The A atom is positively charged due to the lack of electrons, which is called a cation. The B atom is charged due to the increase in the number of electrons. The phenomenon of negative electricity is called anion.

The reason for the unbalanced electron distribution is that the electrons depart from the orbit by an external force. This external force contains various energies (such as kinetic energy, potential energy, thermal energy, chemical energy, etc.). In daily life, after any two objects of different materials come into contact If you separate it again, you can generate static electricity. Solid, liquid and gas will be charged with static electricity due to contact and separation, so our surrounding environment and even our body will have different levels of static electricity. When the static electricity accumulates to a certain level, discharge will occur.

2. How high is the static electricity on the human body

In the dry season, if you wear fiber clothing and walk on the insulated ground, the static electricity on the human body can reach thousands of volts or even tens of thousands of volts.

3 What are the forms of electrostatic damage to SMT electronic products

The basic physical properties of static electricity are: attraction or repulsion, and there is a potential difference with the earth, which will generate a discharge current. These three characteristics have three effects on electronic components:

1. Electrostatic adsorption of dust reduces the insulation resistance of the components (shortens the life).

2. Electrostatic discharge destroys, so that the components are damaged and cannot work (complete destruction).

3. The electromagnetic field generated by electrostatic discharge has a large amplitude (up to several hundred volts/meter), causing interference or even damage to electronic products (electromagnetic interference).

If all the components are damaged, they must be detected and eliminated in production and quality management, and the impact will be small. If the components are slightly damaged, it is not easy to find under normal inspection. In this case, it is often caused by multiple layers of processing. The damage was discovered when it was already in use. Not only was it not easy to check, but the loss was also difficult to predict.

4 What does ESD mean

ESD stands for "Electro Static Discharge" in English. ESD was formed since the middle of this century to study the generation and attenuation of static electricity, electrostatic discharge models, electrostatic discharge effects such as current thermal (spark) effects (such as fire and explosion caused by static electricity) and electromagnetic effects (such as electromagnetic interference), etc. Subject. In recent years, with the rapid development of science and technology, the widespread application of microelectronics technology and the increasingly complex electromagnetic environment have brought more and more attention to the magnetic field effects of electrostatic discharge, such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC).

5 What problems should be paid attention to in the use of anti-static wrist strap

If the wristband is not fastened tightly, the contact resistance between the human body and the wristband becomes larger. The wristband should be connected to the ground with a special grounding wire with a socket. It should not be clamped on the metal body on the table or the side of the table, because the resistance of these metal bodies to the ground may be very large. At the same time, the resistance of the wristband should be checked frequently.

6 Human safety issues in the use of anti-static wrist straps

When considering the anti-static point of SMT patch processing, the total resistance of the human body to ground is as small as possible, but the minimum value is restricted by safety. The human body must have a certain value of resistance to ground in case of metal equipment or equipment. In the case of short-circuiting with a power frequency source, the resistor can limit the current flowing through the human body during operation. The minimum value should not be less than 100K ohms, and the current limiting resistance of the wrist strap is usually 1M ohms.