Reflow soldering is a very important process in SMT chip processing, which involves combining the chip components with the solder pads on the circuit board at high temperatures and then cooling them together, greatly affecting the stability of the circuit board. In reflow soldering, some process defects are also prone to occur, and the reasons need to be analyzed and targeted solutions are needed to ensure product quality. Below, the quick technician will mainly organize and introduce the common defects and cause analysis of SMT reflow soldering for everyone.

　　1、 Tin bead

　　Reason: 1. The silk screen holes are not aligned with the solder pads, and the printing is not accurate, causing solder paste to stain the PCB.

　　2. Solder paste is exposed too much in an oxidizing environment and absorbs too much water from the air.

　　3. Heating is inaccurate, too slow and uneven.

　　4. The heating rate is too fast and the preheating interval is too long.

　　5. The solder paste dries too quickly.

　　6. Insufficient activity of flux.

　　7. Too many small particles of tin powder.

　　8. Improper volatility of flux during reflow process.

　　2、 Open circuit

　　Reason: 1. Insufficient amount of solder paste.

　　2. Insufficient coplanarity of component pins.

　　3. Insufficient wetness of tin (insufficient melting, poor fluidity), and too thin solder paste can cause tin loss.

　　4. The pins are soldered (like rushes) or have wiring holes nearby. The coplanarity of pins is particularly important for closely spaced and ultra closely spaced pin components, and one solution is to pre solder the solder pads. Pin tin absorption can be prevented by slowing down the heating speed and heating more on the bottom and less on the top.

　　3、 Solder crack

　　Reason: 1. The peak temperature is too high, causing sudden cooling of the solder joint, resulting in excessive thermal stress and solder cracks due to quenching;

　　2. Quality issues with the solder itself;

　　4、 Void

　　Reason: 1. Material impact. Wet solder paste, high oxygen content of metal powder in solder paste, use of recycled solder paste, oxidation or contamination of solder pads on component pins or printed circuit board substrates, and moisture on printed circuit boards.

　　2. Welding process impact: The preheating temperature is too low and the preheating time is too short, causing the solvent in the solder paste to not escape in time before hardening, and entering the reflow zone to generate bubbles.

　　5、 Generally speaking, the factors that cause tin bridges are due to the thin solder paste, including low metal or solid content in the solder paste, low shaking solubility, easy squeezing of the solder paste, large solder paste particles, and low surface tension of the soldering flux. There is too much solder paste on the solder pad, and the peak reflow temperature is too high.

　　SMT reflow soldering is a relatively complex process that is easily affected by various factors, resulting in various defects that are generally difficult to eliminate. Understanding common defects and their causes in SMT reflow soldering, paying more attention during the operation process, can avoid the occurrence of defects, and once problems arise, they can be analyzed and solved in a timely manner.