Particle Classification
Dimensional distributions of solder spheres define the printing capability of paste formulations used in fine-pitch electronic assembly. Stencil printing on micro-passive components requires powder type 5 particle size to achieve repeatable release through apertures smaller than 150 microns. This classification restricts the diameter of the metal spheres to a range between 15 and 25 microns.
Smaller particles improve stencil release but increase the total surface area of the metal in the paste.
Oxidation Risk
Solder balling and bead formation represent the primary defects associated with increased surface area in fine solder pastes. Due to the high surface-to-volume ratio of powder type 5 particle size, the paste contains a greater amount of oxide on the sphere surfaces before heating occurs. During reflow, this oxide must be fully reduced by the flux to allow the spheres to coalesce into a single joint.
If the flux chemistry is exhausted before the oxide is removed, uncoalesced solder spheres scatter across the solder mask, causing electrical shorts. The risk increases when boards sit on the line for extended periods between printing and reflow.
Print Performance
Paste transfer efficiency measures the volume of paste deposited relative to the aperture volume. Assembly engineers rely on this parameter to monitor the stencil printing process on high-density assemblies. Laser-cut stencils with smooth walls are used to assist the release of these fine particles.
