compounds are formed, because gold preferentially reacts with tin, forming the compound. Particles of disperse in the solder matrix, forming preferential cleavage planes, significantly lowering the mechanical strength and therefore reliability of the resulting solder joints. If the gold layer does not completely dissolve into the solder, then slow intermetallic reactions can proceed in the solid state as the tin and gold atoms cross-migrate. Intermetallics have poor electrical conductivity and low strength. The ongoing intermetallic reactions also cause Kirkendall effect, leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known as purple plague. A 2–3 µm layer of gold dissolves completely within one second during typical wave soldering conditions. Layers of gold thinner than 0.5 µm (0.02 thou) also dissolve completely into the solder, exposing the underlying metal (usually nickel) to the solder. Impurities in the nickel layer can prevent the solder from bonding to it. Electroless nickel plating contains phosphorus. Nickel with more than 8% phosphorus is not solderable. Electrodeposited nickel may contain nickel hydroxide. An acid bath is required to remove the passivation layer before applying the gold layer; improper cleaning leads to a nickel surface difficult to solder. A stronger flux can help, as it aids dissolving the oxide deposits. Carbon is another nickel contaminant that hinders solderability. See also Daniel Davis Jr. - inventor who was first to do gold plating as a business Gold filled jewelry References External links Electronics manufacturing Coatings Jewellery Metal plating de:Vergolden#Galvanische Verfahren