We know that PCBA processing is a complicated process. As long as there is a mistake in one of the links, it will cause some continuous problems later. Today we will mainly understand the causes of tin beads in PCBA processing.

1. When it comes to tin beads, the first thing we think of is solder paste. In the PCBA processing process, the selection of solder paste directly affects the quality of soldering to a large extent, including the metal content in the solder paste, the degree of oxidation of the metal powder, the size of the metal powder, etc., all of which can affect the quality of the solder beads. produce. The following Sanjing will be introduced in turn.

①The metal content in the solder paste: the mass ratio of the metal content in the solder paste is about 88% to 92%, and the volume ratio is about 50%. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during the preheating process. The increase of the metal content can make the metal powder arranged tightly, and it is easier to combine without being blown off when melting. In addition, the increase in the metal content of the solder paste can also reduce to a certain extent the phenomenon of "collapse" after the solder paste is printed, thereby effectively reducing or avoiding the generation of tin beads.

②The oxidation degree of metal powder in the solder paste: the higher the oxidation degree of the metal powder in the solder paste, the greater the resistance of the metal powder during soldering, and the solder paste is not easily wetted with the pads and components, resulting in reduced solderability. Experiments show that the production rate of tin beads is directly proportional to the degree of oxidation of the metal powder. Generally, the oxidation degree of the solder in the solder paste is controlled below 0.05%, and the maximum limit is 0.15%.

③The size of the metal powder in the solder paste: the smaller the particle size of the metal powder in the solder paste, the larger the overall surface area of the solder paste, which leads to a higher degree of oxidation of the finer powder, which intensifies the phenomenon of tin beading. Experiments have shown that when using finer-particle solder paste, solder beads are more likely to be produced.

④Flux activity and dosage in the solder paste: When the flux activity is weak, the ability to remove oxidation is weaker, so it is easy to produce tin beads; when the amount of flux is too much, the solder paste will collapse locally, and it is easier to produce tin beads.

In addition to the above, we also need to pay attention to the fact that if the solder paste is taken out from the refrigerator and opened for use without being warmed up, the solder paste will absorb moisture, and the solder paste will splash during preheating and cause tin beads. At the same time, if the room is relatively humid, PCB is damp, excessive thinner is added to the solder paste, or the machine is stirred for too long, it will promote the production of tin beads.

2. PCBA processing placement machine placement pressure. If the pressure is too high during mounting, the solder paste will easily be squeezed onto the solder mask under the component. During reflow soldering, the solder paste will melt and run around the component to form tin beads. At this time, it is necessary to reduce the mounting pressure , Use a suitable stencil opening form to prevent the solder paste from being squeezed out of the pad.

3. PCBA processing furnace temperature curve setting. Tin beads are generally produced during reflow soldering. During the preheating stage, the temperature of the solder paste, PCB and components will rise to between 120°C and 150°C to reduce the thermal shock of the components during reflow. At this stage, the flux in the solder paste begins to vaporize, so that the small particles of metal powder separate and run to the bottom of the component, and when the flow is added, it runs around the component to form tin beads. Therefore, the preheating temperature and the preheating speed of the reflow soldering should be adjusted in time to control to reduce or avoid the production of tin beads.

The above is about the reasons why tin beads are produced in PCBA processing, I hope it can be helpful to everyone!