PCB requirements for non-electrolytic nickel coating

The electroless nickel coating should fulfill several functions:

Gold deposit surface

The ultimate goal of the circuit is to form a connection between the PCB and the components with high physical strength and good electrical characteristics. If there is any oxide or contamination on the PCB surface, this soldered connection will not happen with today’s weak flux.

Gold naturally precipitates on nickel and will not oxidize during long-term storage. However, gold does not precipitate on oxidized nickel, so nickel must remain pure between the nickel bath and the dissolution of gold. In this way, the first requirement of nickel is to remain free of oxidation long enough to allow the precipitation of gold. The component has developed a chemical immersion bath to allow 6-10% phosphorus content in the precipitation of nickel. This phosphorus content in the electroless nickel coating is considered as a careful balance of bath control, oxide, and electrical and physical properties.

pakeke

The non-electrolytic nickel coating surface is used in many applications that require physical strength, such as automotive transmission bearings. PCB needs are far less stringent than these applications, but for wire bonding

(Wire-bonding), touch pad contact points, plug-in connector (edge-connetor) and processing sustainability, a certain degree of hardness is still important. Wire bonding requires a nickel hardness. If the lead deforms the deposit, a loss of friction may occur, which helps the lead “melt” to the substrate. The SEM picture shows that there is no penetration into the surface of the flat nickel/gold or nickel/palladium (Pd)/gold.

Nga waahanga hiko

Because of its ease of fabrication, copper is the metal of choice for circuit formation. The conductivity of copper is superior to almost every metal. Gold also has good electrical conductivity and is the perfect choice for the outermost metal, because electrons tend to flow on the surface of a conductive path (“surface” 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 microwave PCB can exceed the designer’s specification. This phenomenon is proportional to the thickness of nickel-the circuit needs to pass through the nickel to reach the solder joints. In many applications, the electrical signal can be restored to within the design specification by specifying that the nickel deposit is less than 2.5 µm.

Whakaaetanga taatau

Contact resistance is different from solderability because the nickel/gold surface remains unsoldered throughout the life of the end product. Nickel/gold must maintain electrical conductivity to external contact after long-term environmental exposure. Antler’s 1970 book expresses the contact requirements of nickel/gold surfaces in quantitative terms. Various end-use environments are studied: 3″ 65°C, a normal maximum temperature for electronic systems that work at room temperature, such as computers; 125°C, the temperature at which general connectors must work, often specified for military applications; 200 °C, this temperature is becoming more and more important for flight equipment.”

For low temperature environments, no nickel barrier is 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%