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 10–15 microinches of silver. Some dissolves into the bath, limits exceeding buildup is rare.
* Immersion tin: does not contaminate high-tin baths.
* OSP (Organic solderability preservative): usually imidazole-class compounds forming a thin layer on the copper surface. Copper readily dissolves in high-tin baths.