Question 1

What is a typical transfer efficiency for solder paste printing?

Accepted Answer

Transfer efficiency typically ranges from 70% to 90%. Standard electroformed or laser-cut stencils achieve 75–85%, while nano-coated stencils can reach 85–95%. The value depends on aperture geometry (area ratio and aspect ratio), paste formulation, squeegee pressure, and print speed.

Question 2

Why does solder paste density vary between formulations?

Accepted Answer

Density varies primarily due to metal content (Type 3 vs Type 5 powder) and alloy composition. SAC305 (Sn96.5/Ag3.0/Cu0.5) paste typically has a density of 4.5–5.0 g/cm³, while SnPb (Sn63/Pb37) paste is denser at approximately 5.2–5.5 g/cm³ due to the higher atomic weight of lead.