2. These changes address the limitations of HASL (hot air solder leveling) and the increasing range of alternative methods to HASL.
3. The final PCB surface treatment process protects the surface of the circuit copper foil.
4. Copper (Cu) is a good surface for soldering but is prone to oxidation, which hinders solder wetting.
5. Although gold (Au) is now used to cover copper because it does not oxidize, gold and copper quickly diffuse into each other.
6. Any exposed copper will soon develop non-weldable copper oxide.
7. One solution is to use a “barrier layer” of nickel (Ni), which prevents the diffusion of gold and copper, providing a durable and conductive surface for component assembly.
8. Requirements for the surface treatment process of non-electrolytic nickel PCBs for multilayer PCBs.
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**The surface treatment process of non-electrolytic nickel PCB should fulfill several key functions:**
**Gold Precipitation on the Surface**
The ultimate goal of the circuit is to establish a high-strength, electrically reliable connection between multilayer PCB circuit boards and components. If there is any oxide or contamination on the surface of a multilayer PCB, the welding connection may not be effective with today’s weak fluxes. Gold naturally precipitates on nickel and resists oxidation during long-term storage. However, gold will not precipitate on oxidized nickel, so the nickel must remain pure between the nickel bath and gold dissolution stages. Therefore, the first requirement for nickel is to stay free of oxidation long enough to enable gold precipitation. The process uses a chemical immersion bath with a phosphorus content of 6-10% in the nickel precipitation. This phosphorus content represents a careful balance of bath control, oxides, and electrical and physical properties in the surface treatment of non-electrolytic nickel PCBs.
**Hardness**
The non-electrolytic nickel PCB surface treatment process is employed in various applications that require physical strength, such as automobile transmission bearings. While the demands for multilayer PCBs are less stringent compared to these applications, a certain level of hardness remains important for wire bonding, touchpad contact points, edge connectors, and overall processing durability. Wire bonding requires nickel to have sufficient hardness; if the lead deforms the nickel, friction loss may occur, causing the lead to “melt” into the substrate. SEM photos have shown no penetration into planar nickel/gold or nickel/palladium (PD)/gold surfaces.
**Electrical Characteristics**
Copper is chosen for circuit formation due to its ease of use and superior conductivity compared to almost every other metal. Gold, with good conductivity, is also ideal for the outermost metal layer because electrons prefer to flow along the surface of a conductive path (“surface” benefit).
Copper: 1.7 µΩ·cm
Gold: 2.4 µΩ·cm
Nickel: 7.4 µΩ·cm
Non-electrolytic nickel coating: 55-90 µΩ·cm
Although the electrical characteristics of most production boards are not significantly affected by the nickel layer, nickel can influence the electrical performance of high-frequency signals. Signal loss in microwave multilayer PCBs can exceed the designer’s specifications. This issue is proportional to the thickness of the nickel layer—the circuit must traverse the nickel to reach the solder joint. In many cases, specifying a nickel layer thickness of less than 2.5 µm can restore the electrical signal to the design specifications.
**Contact Resistance**
Contact resistance differs from weldability in that the nickel/gold surface must maintain its conductivity throughout the lifespan of the end product. Nickel/gold must continue to conduct external contacts effectively even after long-term environmental exposure. Antler’s 1970 research quantified the contact requirements for nickel/gold surfaces, studying various end uses.
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