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As electronics continue to shrink, the solder interconnections must also scale down — a challenge that’s especially evident when trying to print small, fine-feature components. For PCB assembly, this is a key area where solder paste plays a critical role.
In the SMT world, a paste’s particle size is one of its most important properties. Solder paste is thixotropic, meaning that it has a low viscosity at rest, but becomes much thicker when shear stress is applied (such as when the squeegee prints the paste onto the printed circuit board).
Paste particle size is measured in microns, and smaller pastes typically have lower viscosities than larger ones. Pastes with very large particle sizes are generally used for larger feature printing, while those with smaller particles are favored for fine-feature applications. The size of the particle influences how well the paste can release from the stencil, as the spherical shape of the powder has the effect of a smooth surface that facilitates the process of stencil release.
The size of the particles can also influence how easy it is to print on top of fine features, as the larger spheres have more surface area. This may create an opportunity for the formation of oxidation, which can affect paste performance by creating a layer that inhibits the wetting of the flux medium. This can lead to reflow defects such as solder balls and graping.
Increasing the size of the powder particles can help with these fine-feature printing challenges, but it’s a mistake to simply switch to a finer paste type without first auditing your printing processes and ensuring that they are stable and optimized for fine-feature printing. This is why it’s always good to ask for advice from your paste and stencil suppliers; they deal with these types of questions on a daily basis and have many solutions at the ready.