How to Select Solder Paste for SMT Printing?

When selecting solder paste, the first thing to observe is whether the packaging is completely sealed to ensure that there is not the slightest leakage. Then, read the product description in depth, because the solder paste specifications are many, so the choice is to make sure to choose their own models, production to meet the process requirements. Generally speaking, SMT printing is the first choice for soldering, and solder paste must be the first choice, and then based on the composition of the solder paste alloy, particle size, viscosity and other key parameters, one by one to select the specifications and type of solder paste.

First, SMT printing solder paste of radiant classification:

Class A: common rosin easy to clean type solder paste

This category of solder paste, soldering time to show excellent “tin rate”, and to ensure that the welding results of excellence. After the completion of welding, PCB board rosin left on the board, but after the appropriate cleaning agent treatment, the board surface is clean and flawless, insulation impedance performance is excellent, can easily pass all kinds of electrical performance cough cough research and testing.

Category B: no-clean environmentally friendly solder paste

This type of solder paste, after the completion of welding, PCB board surface clean as a mirror, the residue can be almost ignored, electrical performance is excellent, without additional cleaning steps, both to shorten the production process, but also to accelerate the production schedule, soldering quality and production efficiency go hand in hand.

Category C: Environmentally friendly water-soluble solder paste

In the past, the production of solder paste, due to technical limitations, PCB board surface residue is very much, electrical performance is poor, the quality is greatly reduced. Generally use industrial cleaners to cope with, and later due to environmental restrictions, many countries are static use. In response to market demand, water-soluble solder paste came into being, the residue of water can be removed, both to reduce customer costs, but also in line with environmental requirements.

Second, SMT printing solder paste selection of the golden rule:

Careful matching of alloy components

Under normal circumstances, Sn63/Pb37 alloy components can meet the needs of most soldering. However, for silver (Ag) or palladium (Pd) coating device soldering, Sn62/Pb36/Ag2 alloy components are more appropriate. As for PCB soldering of components that are not resistant to thermal shock, solder powders containing Bi are preferred.

Viscosity ratios need to be rigorously tested, take the experimental values

SMT process, since the solder paste printing (or point injection) to component placement, and then to reflow heating, during which the PCB needs to go through the process of moving, placing and so on. In order to ensure the stability of the solder paste form, the component does not shift, the solder paste in the reflow soldering before the need to have a good viscosity and durability. Viscosity is often measured in Pa-S. 200-600 Pa-S is suitable for needle dispensing or highly automated production; while 600-1200 Pa-S is more suitable for manual or mechanical printing. The viscosity of the high and low, each has its own merits: high viscosity, the solder joints into a pile-shaped effect is good, suitable for fine-pitch printing; low viscosity, the drop is rapid, time and labor-saving, easy to clean the tools. In addition, the viscosity also changes with the mixing, after resting, it will be restored to its original state, this point is particularly important in the selection of different viscosity solder paste. Also note that the viscosity is closely related to the temperature, with the temperature rise and gradually decline.

Tin powder mesh fine screening

Domestic solder paste manufacturers mostly use “granularity” to categorize solder paste, while foreign or imported solder paste often adopts the concept of “Mesh (MESH)”. The larger the mesh, the finer the particles of tin powder; conversely, the coarser. Selection, should be based on the PCB on the minimum solder joint spacing: spacing is large, you can choose a smaller number of mesh of the solder paste; spacing is small, you should choose a larger number of mesh of the solder paste. Generally speaking, the diameter of the tin powder particles should not be greater than one-fifth of the template opening.