Overview of the final coating of multilayer PCB circuit boards
The final coating process produced by multilayer PCB circuit board has undergone important changes in recent years. These changes are increasingly the result of the constant need to overcome the limitations of HASL (hot air solder leveling) and the HASL alternative approach.
The final coating is used to protect the surface of the circuit copper foil. Copper (CU) is a good surface of the welding element, but it is easy to oxidize, and the copper oxide prevents the solder from melting (wetting). Although Gold (AU) is now used to cover copper, gold does not oxidize, and gold and copper diffuse rapidly to each other. Any exposed copper will soon form an weldable copper oxide. One method is to use a nickel (NI) "barrier layer", which prevents the transfer of gold and copper and provides a durable, conductive surface for assembly of components.
Requirements of multilayer PCB board on non-electrolytic nickel coating
Non-electrolytic nickel coating should be completed several functions:
The surface of gold precipitation
The ultimate aim of the circuit is to form a high physical strength and good electrical connection between PCB circuit boards and components. If there is any oxide or contamination on the surface of the multilayer PCB circuit board, this welding connection will not occur with today's weak flux.
Gold naturally precipitates on the nickel and does not oxidize in long-term storage. However, gold does not precipitate on the nickel oxide, so the nickel must remain pure between the nickel Bath (nickel bath) and the gold dissolution. In this way, the first requirement of nickel is to keep the oxidation long enough to allow the gold to precipitate. The component developed a chemical immersion bath to allow $number phosphorus content in the deposition of nickel. The phosphorus content in non-electrolytic nickel coatings is considered as a careful balance of immersion control, oxides, and electrical and physical properties.
Non-electrolytic nickel coating surfaces are used in many applications requiring physical strength, such as car-driven bearings. The need of multilayer PCB circuit board is far from these application is strict, but for lead bonding (wire-bonding), touch point of touch pad, plug-in connector (edge-connetor) and processing sustainability, certain hardness is important.
Lead bonding requires a nickel hardness. If the lead causes the precipitate to deform, the loss of friction may occur and it helps the lead to "melt" onto the substrate. SEM photographs showed no penetration into the surface of the planar nickel/gold or nickel/palladium (PD)/gold.
Because it is easy to make, copper is the metal which is chosen as the circuit. The conductivity of copper is superior to almost every kind of metal. Gold also has good conductivity and is the perfect choice for outermost metals because electrons tend to flow on the surface of a conductive route ("surface" efficiency).
Gold 2 4µωcm
Non-electrolytic nickel coating 55~90µωcm
Although the electrical properties of most production boards are unaffected by the nickel layer, nickel can affect the electrical properties of high-frequency signals. The microwave multilayer PCB circuit board's signal loss may surpass the designer's specification. This phenomenon is proportional to the thickness of the nickel-the circuit needs to reach the solder point through the nickel. In many applications, electrical signals can be specified by a nickel precipitation of less than 2.5µm restored to design specifications.