False soldering is the most common problem in SMD processing. Sometimes after welding, it seems to weld the front and rear steel strips together, but in fact, it does not reach the level of integration. The strength of the joint surface is very low. The weld must go through a variety of complex technological processes on the production line, especially through the high-temperature furnace area and the high-tension tension tension straightening area. Therefore, the faulty weld on the production line is very easy to cause the belt breaking accident, which has a great impact on the normal operation of the production line. The essence of faulty welding is that the temperature of the joint surface of the weld is too low, the size of the nugget is too small, and even does not reach the melting level. It just reaches the plastic state. After rolling, it is reluctantly combined, so it seems to be welded well, but in fact it is not fully fused.

SMT quality management workshop

The causes and steps of faulty soldering in SMD processing are analyzed as follows:

1. First, check whether the welding joint surface has rust, oil stain and other impurities, or uneven, poor contact, which 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 conforms to the process regulations, and whether the current setting does not increase with the change of the product thickness, resulting in insufficient current during welding and poor welding.

Chip processing components

3. Check whether the overlap of the weld is normal and whether the overlap of the drive side is reduced or cracked. The reduction of overlap will make the joint area of the front and rear steel strips too small, so that the total stress surface will be reduced and can not bear large tension. In particular, the crack phenomenon at the driving side will cause stress concentration, which will make the crack more and more large, and finally break.

4. Check whether the welding roller pressure is reasonable. If the pressure is not enough, the actual current will decrease due to the excessive contact resistance. Although the welding controller has a constant current control mode, the resistance increases beyond a certain range (generally 15%), which will exceed the limit of current compensation. The current cannot increase with the increase of the resistance, and cannot reach the set value. In this case, the system will give an alarm when it works normally. In actual operation, if the exact cause of the faulty welding cannot be analyzed at the moment, the head and tail of the steel strip can be cleaned up, the welding overlap can be increased, the welding current and welding roller pressure can be properly increased for another welding, and the formation state of the weld seam can be closely observed during the welding. In most cases, the problem can be solved in an emergency.

SMT mounting line

The main reasons for insufficient solder paste in SMT chip processing are as follows:

1. When the printing press is working, solder paste is not added in time

2. The solder paste is of abnormal quality, mixed with hard lumps and other foreign matters

3. The previously unused solder paste has expired and has been reused

4. The quality of the circuit board is poor. There are inconspicuous covers on the pad, such as solder resist (green oil) printed on the pad

5. The fixing clamp of the circuit board in the printer is loose

6. The thickness of solder paste leakage screen is uneven

The accuracy of the mounter determines the quality

7. There are pollutants on the solder paste leakage screen or circuit board (such as PCB packaging, screen wiping paper, foreign matters floating in the ambient air, etc.)

8. The solder paste scraper is damaged and the screen is damaged

9. The pressure, angle, speed and demoulding speed of solder paste scraper are not set properly

10. Solder paste was accidentally knocked off due to human factors after printing