The presence of solder balls on the circuit board after reflow soldering indicates that the process is not completely correct, and there is a danger of short circuits in electronic products. This is a serious defect, so it needs to be solved and eliminated. The internationally recognized standard for solder balls is: printed circuit components cannot have more than 5 solder balls within 600. There are many reasons for solder balls, and the root of the problem needs to be found. Guang Shengde is here to share with you the solution to the solder ball on the circuit board after reflow soldering.

SMT solder ball

1. The mechanism of reflow solder ball formation

The solder balls produced during reflow soldering are often hidden in the sides or between the fine pitch pins between the two ends of the rectangular chip component. During the component mounting process, the solder paste is placed between the pins and the pads of the chip components. As the printed board passes through the reflow oven, the solder paste melts and becomes liquid. If the wetting is not good, the liquid solder will shrink and make the welding seam insufficiently filled, and all the solder particles cannot aggregate into a solder joint. Part of the liquid solder will flow out of the solder joints and form solder balls. Therefore, poor wettability of solder with pads and device pins is the root cause of the formation of solder balls.

2. Cause analysis and control method of tin ball after reflow soldering

There are many reasons for the poor wettability of reflow solder. The following mainly analyzes the reasons and solutions related to related processes:

1. Improper setting of reflux temperature curve. The reflow of solder paste is a function of temperature and time. If it does not reach a sufficient temperature or time, the solder paste will not reflow. The temperature rise in the preheating zone is too fast, and the time to reach the top temperature is too short, so that the moisture and solvent in the solder paste are not completely volatilized. When it reaches the reflow soldering temperature zone, the moisture and solvent will boil and the solder balls will be splashed. Practice has proved that it is ideal to control the temperature rise rate of the preheating zone at 1～4℃/s.

2. If solder balls always appear in the same position, it is necessary to check the metal plate design structure. The corrosion accuracy of the template opening size does not meet the requirements. For the large pad size and soft surface material (such as copper template), the outline of the missing solder paste is not clear and bridges each other. This situation often occurs in the right When the pads of fine-pitch devices are missed, a large number of tin beads between the pins will inevitably be produced after reflow soldering. Therefore, according to the different shapes and center distances of the pad patterns, appropriate template materials and template manufacturing processes should be selected to ensure the quality of solder paste printing.

3. If the time from mounting to reflow soldering is too long, due to the oxidation of solder particles in the solder paste, the flux will deteriorate and its activity will decrease, which will cause the solder paste to not reflow and solder balls will be produced. Choosing a solder paste with a longer working life (we think at least 4 hours) will reduce this effect.

4. In addition, the printed board with the wrong solder paste is not cleaned sufficiently, leaving the solder paste on the surface of the printed board and the through holes. Before reflow soldering, the placed components are realigned and placed to deform the missing solder paste. This is also the cause of solder balls.

Therefore, it is necessary to strengthen the sense of responsibility of the reflow soldering machine operators and process personnel in the production process, strictly follow the process requirements and operating procedures for production, and strengthen the quality control of the process.