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In SMT chip processing, reflow soldering is a very important process. It is a soldering process in which the chip components are combined with the pads on the circuit board through high temperature and then cooled together, which has great stability in the use of the circuit board. Influence. Some process defects are also prone to occur in reflow soldering, and the reasons need to be analyzed and resolved in a targeted manner to ensure product quality. The following is mainly for you to organize and introduce the common defects and cause analysis of SMT reflow soldering.
1. Tin beads
Reasons: 1. The silk screen hole and the pad are not aligned, and the printing is not accurate, which makes the solder paste dirty the PCB.
2, the solder paste is exposed to too much in an oxidizing environment, and too much water in the air is sucked.
3. The heating is not precise, too slow and uneven.
4. The heating rate is too fast and the preheating interval is too long.
5. The solder paste dries too fast.
6. The flux activity is not enough.
7. Too many tin powders with small particles.
8. The flux volatility is inappropriate during the reflow process.
The process approval standard for tin beads is: when the distance between the pads or printed wires is 0.13mm, the diameter of the tin beads cannot exceed 0.13mm, or there can be no more than five tin beads within a 600mm square range.
Two, open the way
Reasons: 1. The amount of solder paste is not enough.
2. The coplanarity of the component pins is not enough.
3. The tin is not wet enough (not enough to melt, and the fluidity is not good), and the tin paste is too thin to cause tin loss.
4, the pin sucks tin (like rush grass) or there is a connection hole nearby. The coplanarity of the pins is particularly important for fine-pitch and ultra-fine-pitch pin components. One solution is to apply tin on the pads in advance. The pin sucking can be prevented by slowing down the heating speed and heating the bottom surface more and less heating on the top surface.
Three, solder cracks
Reasons: 1. The peak temperature is too high, causing the solder joints to suddenly cool down, and the solder cracks are caused by excessive thermal stress caused by chilling;
2, the quality of the solder itself;
Four. Hollow
Reasons: 1. The influence of materials. The solder paste is damp, the metal powder in the solder paste has high oxygen content, the use of recycled solder paste, the component pins or the pads of the printed circuit board substrate are oxidized or contaminated, and the printed circuit board is damp.
2. The influence of soldering process: the preheating temperature is too low and the preheating time is too short, so that the solvent in the solder paste cannot escape in time before hardening and enter the reflow area to generate bubbles.
Five, Tin Bridge
Generally speaking, the cause of solder bridges is that the solder paste is too thin, including low metal or solid content in the solder paste, low thixotropy, easy squeezing of the solder paste, too large solder paste particles, and too small flux surface tension. Too much solder paste on the pad, too high peak reflow temperature, etc.
SMT reflow soldering is a relatively complicated process, which is susceptible to various factors, and various defects appear, which are generally difficult to eliminate. Understand the common defects of SMT reflow soldering and their causes, and pay more attention to the operation process to avoid defects., And once there is a problem, it can be analyzed and solved in time.
