The ultimate coating process produced by circuit boards has undergone significant changes in recent years. These changes are an increasing result of the constant demand for HASL (hot air solder leveling) limitations and the HASL alternative approach.
The final coating is used to protect the surface of the copper foil of the circuit. Copper (Cu) is a good surface of welded components, but easy to oxidize. Copper oxide hinders solder from melting (wetting). Although gold (Au) is used to cover copper because gold does not oxidize, gold and copper will rapidly diffuse and permeate each other. Any exposed copper will quickly form copper oxide unweldable. One method is to use the nickel (Ni) barrier layer, which prevents the transfer of gold from copper and provides a durable, electrically conductive surface for assembly of components.
The ultimate goal of a circuit is to form a connection between a circuit board and a component that has high physical strength and good electrical characteristics. If there is any oxide or contamination on the circuit board, this welding connection will not happen with today's weak soldering flux.
Gold deposits naturally on nickel and does not oxidize in long storage. However, gold does not precipitate on the oxidized nickel, so the nickel must be kept pure between the bath bath (nickel) and the gold dissolution. In this way, the first requirement for nickel is to keep the oxide free for a long time to allow gold precipitation. The development of chemical bath components, phosphorus content to allow 6~10% in nickel precipitation in. The phosphorus content of electroless nickel coating is as carefully balanced bath control, oxides, and electrical and physical properties of the considered.