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SMT electrostatic protection and the cause of virtual welding
SMT electrostatic protection and the cause of virtual welding

SMT electrostatic protection and the cause of virtual welding


1. What is static electricity?

The static electricity of the SMT patch is the relative static charge on the surface of an object, which is excessive or insufficient, and it is a form of electrical energy. Static electricity is the result of the loss of balance between positive and negative charges in a local area, and is formed by electron transfer. These unbalanced charges generate a PCB electric field that can be measured, called an electrostatic field, which can affect other objects within a certain distance and cause them to be charged. The distance is related to the amount of electricity.

2. Electrostatic discharge (ESD) phenomenon

Electrostatic discharge (ESD) is the transfer of charges between entities with different electrostatic potentials. E.g:

1. Thunder and lightning.

2. Small experiment: Plexiglass will generate static electricity after rubbing with silk or cotton cloth, which can absorb small paper scraps.

3. In the winter when the air is relatively dry, there will be a crackling sound when the synthetic fiber clothes are taken off, and sparks can be seen at night (the air breakdown field strength is 30KV/cm).

4. Why is it that wearing chemical fiber underwear is prone to skin allergies? Try changing to cotton underwear!

What is the cause of the virtual welding in the patch processing

pcb board

Virtual soldering is the most common problem in SMT processing. Sometimes after PCB welding, it may seem that the front and rear steel strips are welded together, but in fact they have not reached the level of integration. The strength of the joint surface is very low. The welding seam has to go through various complicated processes on the production line, especially It is necessary to pass through the high temperature furnace zone and the high tension straightening zone, so the weld seam of the virtual welding is very easy to cause the broken belt accident 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 the joint surface of the weld is too low during welding, and the size of the nugget is too small or even to the extent of melting, but it has reached a plastic state. After rolling, it is barely joined together, so it looks good. , Actually failed to fully integrate.

Analysis of the causes and steps of virtual soldering in PCBA patch processing:

1. First check whether the weld joint surface has impurities such as rust, oil, or unevenness, or poor contact. This will increase the contact resistance, reduce the current, and the temperature of the welding joint surface is not enough.

2. Check whether the current setting complies with the process regulations, and whether the current setting does not increase correspondingly when the thickness of the product changes, resulting in insufficient current during welding and poor welding.

3. Check whether the overlap of the weld is normal, and whether the overlap of the drive side is reduced or cracked. The reduction in the amount of overlap will make the combined area of the front and rear steel strips too small, so that the total bearing surface is reduced and cannot withstand greater tension. In particular, the driving side cracking phenomenon will cause stress concentration, and the cracking will become larger and larger, and finally it will be broken.

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%) ), the current compensation limit will be exceeded, and the current will not increase with the increase of resistance, and will not reach the set value. In this case, an alarm will be issued when the system is working normally. In actual operation, if the exact cause of the false welding cannot be analyzed for a while, the head and tail of the steel strip can be cleaned up, the welding overlap amount can be increased, the welding current and the welding wheel pressure should be appropriately increased, and the welding should be performed again on the PCB. Pay close attention to the formation state of the weld during welding, and in most cases, the problem can be dealt with in an emergency.