Definition

Solder paste is a mixture of tiny solder spheres (typically SAC305 for lead-free) suspended in flux. It is applied to PCB pads through a stencil before component placement. During reflow, the paste melts to form solder joints. Paste is specified by alloy composition, particle size (Type 3 for standard, Type 4 or 5 for fine-pitch), and flux chemistry (no-clean, water-soluble, or rosin). For PCB designers, solder paste affects pad design: paste-to-pad ratio should be 0.5-1.5 by area, and stencil aperture design must account for aspect ratio limits. Paste deposits that are too large cause bridges; too small cause insufficient joints.