PCB board surface of the final coating types introduced
PCB manufacture of the final coating process has undergone important changes in recent years. These changes are to overcome the HASL (hot air solder leveling) requirements and limitations of the continuous HASL alternatives more and more results.
Final coating is used to protect the surface of copper foil circuits. Copper (Cu) is weld components with good surface, but easily oxidized; hinder solder copper oxide wetting (wetting). Although the use of gold (Au) to cover the copper, because gold is not oxidized; gold and copper will rapidly interdiffusion penetration. Any exposed copper will soon not be welded to form copper oxide. One way is to use the Nickel (Ni) of the "barrier layer", which prevent the transfer of gold and copper and for the assembly of components to provide a durable, conductive surface.
Electroless nickel coating on PCB requirements, non-electrolytic nickel coating should be completed in several functions:
The surface precipitation of gold
The ultimate goal of the circuit is formed between the PCB and the components of physical strength, electrical characteristics of a good connection. If there is any surface oxide or PCB contamination, the connection of the welding flux is weak today will not happen.
Natural precipitation in the nickel-gold top, and not long-term storage of oxidation. However, gold is not deposited on top of nickel oxide, it must be in the nickel bath of nickel (nickel bath) and between dissolved pure gold. This is the first requirement of nickel oxide-free long enough to keep time, to allow the precipitation of gold. Components to develop chemical bath, to allow the deposition of nickel in 6 to 10% of the phosphorus content. Electroless nickel coating in the bath as the phosphorus content control, oxides, and electrical and physical properties of the careful balance of considerations.
Non-electrolytic nickel coating requires physical strength used in many applications, such as automotive drive bearings. PCB needs of these applications is far from strict, but for wire bonding (wire-bonding), touch pad contact points, plug-in connectors (edge-connetor) and deal with sustainability, a certain hardness is important.
Wire bonding requirements of the hardness of a nickel. If the lead to sediment deformation, friction losses may occur, it helps lead "melting" to the substrate. SEM photographs show not penetrate into the surface of nickel / gold or nickel / palladium (Pd) / gold surface.
Due to the easy production, copper is the metal selected for the circuit formation. Conductivity of copper is superior to almost every kind of metal (Table I) 1,2. Kim also has a good conductivity, is the perfect choice for the outermost layer of the metal, because the electron tend to surface in a flow of conductive line ("layer" efficiency).
Table I, PCB metal resistivity
Copper 1.7 μΩcm
Gold 2.4 μΩcm
Nickel 7.4 μΩcm
Electroless nickel plated 55 ~ 90 μΩcm
Although most of the electrical characteristics of the production board is not affected nickel, nickel can affect the electrical characteristics of high frequency signals. Microwave PCB signal loss may exceed the designer's specifications. This phenomenon is proportional to the thickness of Ni - Nickel circuit needs to reach across the solder points. In many applications, electrical signals can be provided back to the Ni precipitation is less than 2.5μm within design specifications.
Contact resistance and weldability different, because the Ni / Au surface in the life of the end products remain welding. Ni / Au in the long-term environmental exposure to external contacts must be maintained after the conductivity. Antler's book in 1970 to quantify the Ni / Au surface contact requirements. Of a variety of end-use environment: 3 "65 ° C, at room temperature, the electronic system to work a normal maximum temperature, such as computers; 125 ° C, universal connector to work in temperatures often required for military applications; 200 ° C, the temperature on the flight equipment is becoming increasingly important. "
For the low-temperature environment, no nickel barrier. As the temperature increases, the requirement to prevent the Ni / Au transfer increase in the number of nickel (Table II).
Table II, Ni / Au contact resistance (1000 hours results)
Ni barrier layer of satisfaction when the 65 ° C 125 ° C when the exposure to the satisfaction of the exposure to 200 ° C when exposed satisfaction
0.0 μm 100% 40% 0%
0.5 μm 100% 90% 5%
2.0 μm 100% 100% 10%
4.0 μm 100% 100% 60%
Antler study in the use of nickel is plated. Will be from non-electrolytic nickel is expected to be improved, as evidenced by Baudrand 4. However, the result is 0.5 μm of gold, where planes often precipitated 0.2 μm. Plane can be deduced for the 125 ° C operation in the contact element is sufficient, but higher temperature components will require special testing.
Antler advice: "Nickel and thicker, the better the barrier, is true in all cases, but the actual situation of PCB manufacturing to encourage engineers needed only precipitates of nickel content. Flat Ni / Au is now for those who use touch pad contact point cellular phones and pagers. Such component specifications is at least 2 μm nickel.
Electroless nickel / immersion gold for use in containing the spring with, press-fit, low slide together without solder connectors other circuit board production.
Plug-in connectors require a longer physical durability. In these cases, non-electrolytic nickel coating for PCB applications, strength is adequate, but the leaching of gold is not enough. Thin gold (60 ~ 90 Knoop) repeated friction loss from the friction out of nickel. When the gold removed, exposure to nickel oxide quickly, results in increased contact resistance.