Electroplating a Solderable Deposit
Electroplating is often used to produce a clean, pristine metal surface upon which to solder. Many of the electrodeposited metals are capable of being soldered upon. These include gold, silver, nickel, electroless nickel, cadmium, copper and tin. If the soldering is to be performed on the ultimate layer such as nickel, this layer must be kept devoid of oxidation and transient surface contaminants. Careful rinsing of the surface after plating and packaging of the parts to protect them from surface contamination is critical. Sealed nitrogen bagging serves this application very well to protect the plated surface.
Because it is difficult to keep the final deposit in a pristine state, the most common scheme to promote solderability is to deposit the surface upon which to solder as the penultimate layer and then top coat with a metal that will amalgamate into the solder (tin, gold or silver). The best known version of this is to copper plate or nickel plate a substrate and then apply a matte tin as the final coating. The matte tin should be devoid of any codeposited organics from the electrolyte. In addition, careful rinsing of the surface is critical to prevent organic surface contamination that can impede soldering.
In soldering of tin electrodeposits, the tin becomes part of the solder joint and the bond occurs between the solder and the deposit or base material beneath the tin. For gold or silver soldering the same principle applies – the gold or silver forms and amalgam with the solder in the solder joint. Since high additions of silver or gold in a solder joint can cause embrittlement, thin gold or silver topcoats are preferable in solderable applications. As with matte tin, a high purity soft gold (99.9% pure) or matte silver (99.9% pure) are preferred for solderable applications.