How to Deal With False Welding, Cold Welding and Wicking during SMT Chip Processing?

In the process of lead-free chip processing, SMT chip processing plants may encounter problems such as false soldering, cold welding and wicking. So how should chip processing factories solve these problems?

The first is the problem of false soldering, which is usually caused by the following reasons: poor solderability of components and pads, improper reflow soldering temperature and heating rate, incorrect printing parameters, excessive stagnation time after printing, and change in solder paste activity and other reasons.

Next is the problem of cold welding. The so-called cold welding is that the surface of the solder joint is dark and rough, and does not melt with the object to be welded. Generally speaking, the formation of cold welding in SMT chip processing is mainly caused by unsuitable heating temperature, deterioration of solder, long preheating time or high temperature.

General solution: According to the reflow temperature curve provided by the supplier, adjust the curve, and then adjust according to the actual situation of the production product. Replace with new solder paste. Check whether the equipment is normal and correct the preheating conditions.

Another problem is wicking. It used to be rare in Sn/Pb solder paste, but this problem often occurs when using lead-free solder paste, mainly because the wetting and spreading rate of lead-free solder paste is not as good as that of lead-containing solder paste. The main reason for the wicking phenomenon is that the thermal conductivity of the component pins is large and the temperature rises rapidly, so that the solder preferentially wets the pins, and the wetting force between the solder and the pins is much greater than that between the solder and the pads. The upturn of the pin will aggravate the wicking phenomenon.

General solution: During reflow soldering, the SMA should be fully preheated before being placed in the reflow furnace. Carefully check and ensure the solderability of the PCB pads. The coplanarity of the components to be welded cannot be ignored. Defective devices should not be used in production.