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solder paste particle size impacts all stages of surface mount assembly from screen/stencil printing to component placement and production yield. Generally, the lower the solder paste particle size, the larger the powder particles within the paste. This can be confusing because the particle size varies by alloy composition and flux type.
Particle size influences paste viscosity and shear thinning (slagging). The tendency to shear thin, however, is more related to the roundness of the powder than to its oxide content. Therefore, pastes of similar powder weight percent and flux but rounded powder have less resistance to viscous flow than pastes of same particle weight and size with irregular shape.
A typical solder paste will feature a specialized flux vehicle (rosin-based, water-soluble, or no clean) that flows through the paste. The flux vehicle is a rosin-based solvent that is used to bind the solder spheres together and prevent them from being separated during printing and reflow welding.
Typically, the solder spheres are composed of a mixture of metals that form a fusible metal alloy. The main alloying elements in these alloys include tin, copper, and lead. These alloys can be formulated with other materials to increase strength and reliability of the solder joint.
Aside from the alloy, there are a few other parameters that affect the wetting properties of the solder paste. These include oxidation behavior, melting temperatures, flow within the joint, and joint strength. These can be evaluated by a trained technician using formal testing procedures.