Analysis of the main defects of reflow soldering!

• Solder Balls: Reasons:

• 1. The screen-printed holes are not aligned with the pads, and the printing is not accurate, which will make 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. Insufficient flux activity.

• 7. Too many tin powders with small particles.

• 8. The flux volatility is inappropriate during the reflow process. The process approval standard for solder balls is: when the distance between the pads or printed wires is 0.13mm, the diameter of the solder balls cannot exceed 0.13mm, or there can be no more than five solder balls within a 600mm square area.

• Bridging: Generally speaking, the cause of solder bridging 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, and too large solder paste particles. The surface tension of the flux is too small. Too much solder paste on the pad, too high peak reflow temperature, etc.

• Open: Reason:

• 1. The amount of solder paste is not enough.

• 2. The coplanarity of component pins is not enough.

• 3. The tin is not wet enough (not enough to melt and 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 ground more and less heating on the top. It is also possible to use a flux with a slower wetting speed and a high activation temperature or a solder paste with different ratios of Sn/Pb to retard melting to reduce the pin sucking.