Solder bead and its control method caused by SMT raw material problem

Solder beads caused by raw material problems in SMT processing and control methods:

1. PCB quality, components, PCB PAD planning is not reasonable, if the components are pressed too much on the PAD and the solder paste is extruded too much, tin beads may occur. When planning PCB, it is necessary to select suitable component package and suitable PAD. PCB solder film printing is not good, rough surface, resulting in the backflow of tin beads, it is necessary to strictly control the incoming PCB material inspection, solder film is severe bad, need to batch back or scrap processing. Solder pad has water or dirt, resulting in tin beads, need to carefully remove water or dirt on PCB, and then put into production. In addition, we often encounter substitution requirements for components with different package sizes from customers, which will lead to mismatch between the components and PAD and easily produce tin beads. Therefore, substitution should be avoided as far as possible.

2. Solder paste selection, solder paste significantly affects the welding quality, metal content, oxide content, metal powder particle size, solder paste activity, etc., have varying degrees of influence on the composition of solder beads. If flux share is too much, solder paste viscosity is reduced, in the preheating area, flux gasification force is too large to occur. Particle size and uniformity of metal powder. Metal powder is a very fine spherical particle, whose shape, diameter and uniformity affect its printing performance. The oxide content in fine particles is higher, if the fine particles share a large amount, there will be better printing clarity, but it is easy to collapse edge, so that the increase of tin beads; The larger particle share is large, so that the increase of even tin, its uniformity difference is large, will lead to the increase of tin beads. Solder paste activity. Tin paste activity is not good, dry too fast, if excessive diluent is added, in the preheating area, the force of diluent gasification is too large to occur tin beads. If you encounter bad tin paste, you can immediately stop replacing good activity.

3. PCB is affected by moisture, and too much moisture in PCB. When the PCB is installed in reflux furnace, gas will occur due to rapid expansion of moisture and tin beads will be generated. PCB is required to be dry and vacuum packed before it is put into SMT. If it is affected by moisture, it needs to be used after baking in oven. For organic bonding film (OSP) boards, baking is not allowed. According to the production cycle, OSP board does not exceed 3 months can be online production, more than 3 months need refueling.