The final coating process for PCB manufacturing has undergone important changes in recent years. These changes are the result of the continued need to overcome the limitations of HASL (hot air solder leveling) and the growing number of HASL alternatives.
The final coating is used to protect the surface of the circuit copper foil. Copper (Cu) is a good surface for soldering components, but is easily oxidized. Copper oxide hinders the wetting of solder. Although gold (Au) is now used to cover copper, gold does not oxidize. Gold and copper interdiffusion and penetrate rapidly. Any exposed copper will quickly form unsolderable copper oxide. One approach is to use a “barrier layer” of nickel (Ni), which prevents gold and copper transfer and provides a durable, conductive surface for component assembly.
PCB requirements for electroless nickel coating
The electroless nickel coating should perform several functions:
1. Surface of Gold Precipitation
The ultimate purpose of the circuit is to form a connection with high physical strength and good electrical characteristics between the PCB and the components. This soldered connection would not have occurred with today’s weak fluxes if there were any oxides or contamination on the PCB surface.
Gold naturally precipitates on top of nickel and does not oxidize during long-term storage. However, gold does not precipitate on the oxidized nickel, so the nickel must remain pure between the nickel bath and the dissolution of the gold. As such, the first requirement for nickel is to remain free of oxidation long enough to allow precipitation of gold. Components developed chemical dip baths to allow 6-10% phosphorus content in nickel precipitation. This phosphorus content in the electroless nickel coating is considered as a careful balance of bath control, oxide, and electrical and physical properties.
Electroless nickel-coated surfaces are used in many applications requiring physical strength, such as bearings in automotive transmissions. PCB requirements are far less stringent than these applications, but some stiffness is still important for wire-bonding, touch pad contact points, edge-connector and handling sustainability.
Wire bonding requires a hardness of nickel. If the lead deforms the deposit, a loss of friction can occur, which helps the lead “melt” to the substrate. The SEM pictures showed no penetration into the planar nickel/gold or nickel/palladium (Pd)/gold surfaces.
3. Electrical Characteristics
Copper is the metal of choice for circuit formation due to its ease of fabrication. Copper conducts electricity better than almost every metal. Gold also has good electrical conductivity and is a perfect choice for the outermost metal, as electrons tend to flow on the surface of a conductive path (the “skin” benefit).
Copper 1.7 µΩcm
Gold 2.4 µΩcm
Nickel 7.4 µΩcm
Electroless nickel plating 55~90 µΩcm
Although the electrical characteristics of most production boards are not affected by the nickel layer, nickel can affect the electrical characteristics of high frequency signals. The signal loss of a microwave PCB can exceed the designer’s specification. This phenomenon is proportional to the thickness of the nickel – the circuit needs to pass through the nickel to reach the solder point. In many applications, electrical signals can be restored to within design specifications by specifying nickel deposits of less than 2.5µm.
4. Contact Resistance
Contact resistance is not the same as solderability because the nickel/gold surface remains unwelded throughout the life of the end product. Nickel/gold must maintain conductivity to external contacts after prolonged environmental exposure. Antler’s 1970 book expresses contact requirements for nickel/gold surfaces in quantities. Various end-use environments were studied: 3″ 65°C, a normal maximum temperature for electronic systems operating at room temperature, such as computers; 125°C, the temperature at which general-purpose connectors must operate, often specified for military applications; 200 °C, a temperature that is becoming increasingly important for flight equipment.”
For low temperature environments, a nickel barrier is not required. As the temperature increases, the amount of nickel required to prevent nickel/gold transfer increases.
Nickel barrier layer Satisfactory contact at 65°C Satisfactory contact at 125°C Satisfactory contact at 200°C
0.0 µm 100% 40% 0%
0.5 µm 100% 90% 5%
2.0 µm 100% 100% 10%
4.0 µm 100% 100% 60%
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