When the PCB design is correct, components and printed boards are good, there may still be some problems with surface mounted technology (SMT). There can be something wrong during solder paste printing. What can affect assembly and soldering? Printing position (printing accuracy), the amount of solder paste, whether the solder amount is uniform, whether the solder paste pattern is clear, whether there is adhesion, whether the PCB surface is contaminated by solder paste, etc..

There are some things that affect printing quality. You want to improve them. Let's dive in.

Stencil

Stencil printing is contact printing, so the thickness and opening size of the stencil determine the amount of solder paste printed. Too much solder paste will cause bridging, while too little solder paste will cause insufficient solder or cold soldering. The opening shape of stencil and whether the opening is smooth will affect the demoulding quality. The opening must be flared downward. Otherwise, the solder paste will be brought out from the chamfer of the flare during demoulding.

Printing processability of solder paste

The viscosity, printability (rolling, transferability), thixotropy, and service life at room temperature can affect the printing quality. If the printability of solder paste is not good, in serious cases, the solder paste just slides on the steel mesh. In this case, the solder paste cannot be printed at all.

Printing process parameters

Solder paste is a non-Newtonian fluid with viscosity. When the scraper moves forward at a specific speed and angle, it will exert a certain pressure on solder paste, pushing solder paste to roll in front of the scraper, thereby injecting the pressure required for the mesh.

The viscous friction of solder paste produces shear at the intersection of the scraper and the steel mesh, and this shear force helps the solder paste to be smoothly injected into the mesh.

There is a certain mutual restriction between the scraper speed, scraper pressure, steel mesh angle and solder paste viscosity. Only by correctly controlling these parameters can the printing quality of solder paste be ensured.

For example, too much scraper pressure will cause the solder paste pattern to stick together. Too fast printing speed will easily lead to insufficient solder paste. If the residual solder paste at the bottom of steel mesh is not removed in time, the solder paste may contaminate areas outside the pad during printing. These may lead to issues such as bridging, cold soldering and solder beads.

Printing accuracy

When printing high-density and fine-pitch products, printing accuracy and repeatability of the printer are crucial. If the printer is not equipped with a visual alignment system, even if the manual pattern alignment is very accurate, and the PCB pad pattern and the steel mesh pattern are completely overlapped, it still cannot solve the problem of PCB processing errors.

Recycled solder paste and environmental factors

Ambient temperature, humidity and environmental hygiene are related to quality of solder joints. Recycled solder paste and new solder paste should be stored separately. Too high ambient temperature will reduce the viscosity of solder paste. When the humidity is too high, solder paste will absorb moisture in the air. When the humidity is too low, the solvent in solder paste will evaporate faster. Dust in the environment mixed with solder paste will cause pinholes in the solder joints.

By properly controlling these things, solder paste printing can be improved. PCB assembly and soldering quality can be improved.