1. PCB quality, components
PCB pads (PAD) design is not reasonable, if the components of the body of too much pressure on the PAD to squeeze out too much solder paste, may produce tin beads.
PCB design, we must choose the right components package and the right PAD.
PCB solder resist film printing is not good, rough surface, resulting in reflow tin beads, must be stricter PCB incoming material into the inspection, solder resist film is seriously bad, must be batch back or scrap processing.
Solder pads with water or dirt, resulting in tin beads, must carefully remove the water or dirt on the PCB, and then put into production.
In addition, often encounter customers to material for different package size device substitution requirements, resulting in devices and PAD does not match, easy to produce tin beads, so should try to avoid substitution.
2. PCB is damp
PCB in moisture too much, after mounting over reflow furnace, due to moisture rapid expansion produce gas, produce tin beads. Requires the PCB must be dry vacuum packaging before putting into SMT production, such as moisture needs to be used after baking with oven. For organic solder film (OSP) board, baking is not allowed. According to the production cycle, OSP board is not more than 3 months can be on line production, more than 3 months need to change the material.
3. Solder paste selection
Solder paste significantly affects the quality of soldering, the metal content of the paste, oxide content, metal powder particle size, paste activity, etc. all affect the formation of tin beads to varying degrees.
Metal content, viscosity. Under normal circumstances, the volume ratio of metal content in the solder paste is about 50%, the mass ratio is about 89% to 91%, and the rest is flux (Flux), rheology regulator, viscosity control agent, solvent, etc.. If the proportion of flux is too much, the viscosity of the solder paste is reduced, and in the preheating area, the force generated by the flux vaporization is too large to produce tin beads. Solder paste viscosity is an important factor affecting the printing performance, usually between 0.5 ~ 1.2 K Pa-s, stencil printing, the best paste viscosity of about 0.8 KPa-s. Metal content increases, the paste viscosity increases, can more effectively resist the force generated by vaporization in the preheat zone, can also reduce the paste after printing collapse trend, can reduce the beads.
Oxide content. Oxide content in the solder paste also affects the soldering effect. The higher the oxide content, the greater the resistance of the metal powder melting combined with the process, reflow stage, the metal powder surface oxide content will also increase, not conducive to pad "wetting" and produce tin beads. Therefore, in the metal powder (Powder) process must require vacuum operation to prevent Powder oxidation.
The particle size of metal powder, uniformity. Metal powder is very fine spherical particles, its shape, diameter size and uniformity are affecting its printing performance. Finer particles in the oxide content is higher, if the proportion of fine particles, there will be better printing clarity, but it is easy to produce collapsed edges, so that the increase in tin beads; a large proportion of larger particles, so that even tin increased, its uniformity difference is large, will lead to an increase in tin beads.
Solder paste activity. Solder paste activity is not good, dry too fast, if you add too much thinner, in the preheating area, the force generated by the thinner vaporization is too large easy to produce tin beads. If you encounter bad activity of the solder paste, it is best to immediately stop using the replacement activity of good.
