Cause of SMT Electrostatic Protection and Virtual Welding

Cause of SMT Electrostatic Protection and Virtual Welding
1 What is static electricity?
The electrostatic SMT patch of the battery is the relative electrostatic charge on the surface of the object. It is a form of electrical energy Electrostatics is the result of the imbalance of positive and negative charges in local areas and is formed by electron transfer These unbalanced charges generate measurable PCB electric field, called electrostatic field, which will affect other objects within a certain distance and lead to their electrification The distance is related to the quantity of electricity
2. Electrostatic discharge phenomenon
Electrostatic discharge (ESD) is the charge transfer between entities with different electrostatic potentials. For example:
1. Lightning.
2. Small experiment: The friction between plexiglass and silk or cotton cloth will generate static electricity, which can absorb small paper scraps.
3. When the air is relatively dry in winter, the clothes made of synthetic fibers will burst when taken off, and sparks will appear at night (the air breakdown field strength is 30KV/cm).
4. Why is wearing chemical fiber underwear easy to cause skin allergy? Try changing into cotton underwear!
What is the reason for virtual welding in patch processing

Virtual welding is the most common problem in SMT processing Sometimes after PCB welding, it seems that the front and rear steel strips are welded together, but in fact, they have not yet reached the fusion level The strength of the joint surface is very low The weld must go through various complex processes on the production line, especially through the high-temperature furnace area and high-pressure straightening area. In conclusion, the virtual weld is very easy to cause belt breaking accidents on the production line, which has a great impact on the normal operation of the production line The essence of virtual welding is that the temperature of weld joint surface is too low during welding, and the size of nugget is too small, even to the extent of melting, but it has reached a plastic state After rolling, it is hardly connected, so it looks good, In fact, it is not completely integrated
Analysis of causes and steps of virtual welding in PCBA chip processing:
1. First, check whether there are impurities such as rust, oil stain, unevenness or poor contact on the weld surface. This will increase the contact resistance, reduce the current, and the temperature of the welded joint surface is not enough.
2. Check whether the current setting conforms to the process regulations. When the product thickness changes, whether the current setting is not added correspondingly, which leads to insufficient current during welding and poor welding.
3. Check whether the weld overlap is normal and whether the drive side overlap is reduced or cracked. The reduction of the overlap will make the combined area of the front and rear steel strips too small, thus reducing the entire bearing surface and unable to withstand greater tension. In particular, the cracking on the drive side will cause stress concentration, and the cracking will become larger and larger, which will eventually break.
4. Check whether the welding wheel pressure is reasonable If the pressure is not enough, the actual current will decrease due to excessive contact resistance Although the PCB welding controller has a constant current control mode, the PCB resistance increases beyond a certain range (generally 15%), will exceed the current compensation limit, and the current will not increase with the increase of the resistance, and will not reach the set value In this case, when the system works normally, an alarm will be given In actual operation, if the exact cause of false welding cannot be analyzed temporarily, the head and tail of the steel strip can be cleaned, and the welding overlap can be increased. The welding current and welding wheel pressure should be increased appropriately, and the PCB should be welded again Pay close attention to the formation state of the weld during welding. In most cases, this problem can be solved in an emergency

The above is the explanation given by the editor of pcb circuit board company.
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