Lead-free SMT Soldering Defects

Lead-free may increase the uplifting of smaller components. This is due in part to the reduced wetting behavior of lead-free alloys. Component placement is more important with lead-free alloys since less centering will occur during reflow. This can increase the incidence of tombstones. SAC305 tends to reduce tombstones, this alloy has a concentration of 96.5 Tin, 3.0 Silver and 0.5 Copper and has melting range of 217-220°C. Because of the small pasty range the component prone to tombstone is tacked by the initial melting phase of the alloy.A solder paste, which exhibits excessive out-gassing during the initial stages of the melting of the solder powder, will also increase tombstone defects. The paste manufacturer must carefully choose resins and solvents, which do not decompose or vaporize at the melting point of the alloy.