1. Solder beading mainly appear on sides of chip resistor and capacitor, and in some cases they also appear around the pins of chip ICs. Tin beads not only affect the appearance of PCBA board products, but more importantly, due to the dense components on the printed board, there is a risk of short circuits in the application process, which in turn harms the quality of electronic products. There are many reasons for solder beading, and they are often caused by one or more factors. Therefore, prevention and improvement must be done one by one so that they can be better controlled.
2. Solder beading refers to some large solder balls occur after reflow soldering. When printing solder paste, the solder paste may be out of the printed pad due to various reasons such as collapse and squeeze. During reflow soldering, this solder paste that exceeds the pad cannot be melted together with the solder paste on the pad, and Solder beading is formed under the component or the periphery of the pad.
3. However, in PCBA processing, most of the solder beading are produced on both sides of the chip components. Take the chip components with the pad design in square shape for example. After the solder paste printed, if the solder paste exceeds, it will be very easy to cause tin beads. Melting with the solder paste on the pad will not produce solder beading.
However, when the amount of solder is large, the component placement pressure will squeeze the solder paste under the component (insulator), and it will melt during reflow soldering. Due to the surface tension, the melted solder paste gathers into a large ball, which tends to raise the components, but this force is very small, they are squeezed to both sides of the components by the gravity of the components, separated from the pads, and tin beads are produced when they are cooled. If the gravity of the component is large and the solder paste is squeezed out, multiple solder balls may even be produced.
4. According to the causes of tin beads, the main factors that form tin beads in the SMT processing are:
a. Graphic design of stencil openings and pads.
b. Stencil cleaning.
c. The accuracy of SMT placement machine.
d. Temperature Profile of reflow oven.
e. Placement pressure of SMT machine.
f. The amount of solder paste outside the pad.