by segregating impurities at the grain boundaries. However, excessive addition results in the formation of tin whiskers; it also results in spurious rare earth phases, which easily oxidize and deteriorate the solder properties. Phosphorus is used as antioxidant to inhibit dross formation. Decreases fluidity of tin-copper alloys. Impurities Impurities usually enter the solder reservoir by dissolving the metals present in the assemblies being soldered. Dissolving of process equipment is not common as the materials are usually chosen to be insoluble in solder. Aluminium – little solubility, causes sluggishness of solder and dull gritty appearance due to formation of oxides. Addition of antimony to solders forms Al-Sb intermetallics that are segregated into dross. Promotes embrittlement. Antimony – added intentionally, up to 0.3% improves wetting, larger amounts slowly degrade wetting. Increases melting point. Arsenic – forms thin intermetallics with adverse effects on mechanical properties, causes dewetting of brass surfaces Cadmium – causes sluggishness of solder, forms oxides and tarnishes Copper – most common contaminant, forms needle-shaped intermetallics, causes sluggishness of solders, grittiness of alloys, decreased wetting Gold – easily dissolves, forms brittle intermetallics, contamination above 0.5% causes sluggishness and decreases wetting. Lowers melting point of tin-based solders. Higher-tin alloys can absorb more gold without embrittlement. Iron – forms intermetallics, causes grittiness, but rate of dissolution is very low; readily dissolves in lead-tin above 427 °C. Lead – causes Restriction of Hazardous Substances Directive compliance problems at above 0.1%. Nickel – causes grittiness, very little solubility in Sn-Pb Phosphorus – forms tin and lead phosphides, causes grittiness and dewetting, present in electroless nickel plating Silver – often added intentionally, in high amounts forms intermetallics that cause grittiness and formation of pimples on the solder surface, potential for embrittlement Sulfur – forms lead and tin sulfides, causes dewetting Zinc – in melt forms excessive dross, in solidified joints rapidly oxidizes on the surface; zinc oxide is insoluble in fluxes, impairing repairability; copper and nickel barrier layers may be needed when soldering brass to prevent zinc migration to the surface; potential for embrittlement Board finishes vs wave soldering bath impurities buildup: HASL, lead-free (Hot Air Level): usually virtually pure tin. Does not contaminate high-tin baths. HASL, leaded: some lead dissolves into the bath ENIG (Electroless Nickel Immersion Gold): typically 100-200 microinches of nickel with 3-5 microinches of gold on top. Some gold dissolves into the bath, but limits exceeding buildup is rare. Immersion silver: typically