There are various surface treatment processes for PCB.
1. Bare copper plate
Advantages: low cost, flat surface, good solderability (when not oxidised).
Disadvantages: susceptible to acid and humidity, can not be stored for a long time, need to be used up within 2 hours after unpacking, as copper is easily oxidised when exposed to air, can not be used for double sided PCBs. If there is a test point, it must be printed with solder paste to prevent oxidation, otherwise it will not be able to make good contact with the probe subsequently.
Pure copper is easily oxidised if exposed to air, and the outer layer must be protected as described above, so surface treatment is required in circuit board processing.
2. OSP process PCB board
OSP is different from other surface treatment processes: its role is to act as a barrier between copper and air, in short, OSP is a clean bare copper surface, chemically grow a layer of organic film. Because it is organic, not metal, it is cheaper than the tin spray process.
The only purpose of this organic film is to ensure that the inner copper foil is not oxidised before soldering. As soon as it is heated during soldering, the film evaporates and the solder is able to bond the copper wires and components together. But this layer of organic film is not corrosion-resistant, a piece of OSP circuit board, exposed to air for ten days, can not be soldered components.
Advantages: all the advantages of bare copper board soldering, expired boards can also be re-do a surface treatment.
Disadvantages: 1. OSP transparent and colourless, so it is difficult to check, it is difficult to identify whether the OSP treatment.
2.OSP itself is insulated, non-conductive, will affect the electrical test. So the test point must be open stencil with printing paste to remove the original OSP layer to contact the needle point for electrical testing, OSP can not be used as a treatment of electrical contact surfaces, such as the keyboard surface of the keys.
3. OSP is easily affected by acid and temperature. OSP is susceptible to acid and temperature effects. When used for secondary reflow, it needs to be completed within a certain period of time, and the results of the second reflow are usually poorer. If the storage time exceeds three months, it must be re-surfaced and used within 24 hours after opening the package.
3.Hot air surface levelling (HASL)
Hot air levelling, also known as hot air solder levelling, is a process whereby molten tin-lead solder is coated on the surface of a PCB and levelled (blown flat) with heated compressed air to form a coating layer that is both resistant to copper oxidation and provides good solderability. Hot air levelling solder and copper in the bond to form a copper-tin metal compound, the thickness of about 1 to 2 mil.
In situations where through-hole devices dominate, wave soldering is the best soldering method, and HASL is sufficient to meet the process requirements of wave soldering. Of course, when high node strength requirements are required, electroplating nickel/gold is often used.
Advantages: low cost
Disadvantages: 1. HASL technology to deal with the pad is not flat enough, coplanarity can not meet the fine-pitch pad process requirements.
2. Not environmentally friendly, lead is harmful to the environment.
4. Gold-plated PCB board
Gold plating uses real gold, even if only a very thin layer is plated, it already accounts for nearly 10% of the cost of circuit boards. The use of gold as a coating is for the convenience of soldering and for corrosion prevention. Even the golden fingers of memory modules that have been used for several years still shine brightly as before. If copper, aluminum, and iron were used for the same amount of time, they have now rusted into a pile of scrap.
The gold plating layer is widely used in the solder pads, gold fingers, connector shrapnel and other positions of circuit boards. The motherboards of the most widely used mobile phone circuit boards are mostly gold-plated or Immersion Gold, while computer motherboards, audio and small digital circuit boards are generally not gold-plated.
Advantages: Strong conductivity, good oxidation resistance, and long service life. The coating is dense and relatively wear-resistant, and is generally used in bonding, Soldering, and plugging situations.
Disadvantages: High cost and poor soldering strength.
5. ENIG/Immersion Gold
ENIG, also known as Nickel Immersion Gold, abbreviated as Nickel Immersion Gold. ENIG is a chemical method that wraps a thick layer of nickel gold alloy with good electrical properties on a copper surface, which can protect PCBs for a long time. The deposition thickness of the inner layer of nickel is generally 120-240 μ in (about 3-6 μ m), and the deposition thickness of the outer layer of gold is generally 2-4 μ in (0.05-0.1 μ m). Unlike OSP, which only serves as an anti rust and barrier layer, it can achieve good electrical performance during long-term use of PCBs. In addition, it also has environmental tolerance that other surface treatment processes do not possess.
Advantages: 1. The surface of the PCB treated with ENIG is very flat and has good coplanarity, making it suitable for use on key contact surfaces. 2. ENIG has excellent solderability, and gold quickly integrates into the melted solder, forming Ni/Sn metal compounds with the solder.
Disadvantages: The process flow is complex, and strict control of process parameters is required to achieve good results. The most troublesome thing is that the surface of the PCB treated with EING is prone to producing black disk effects during ENIG or soldering processes. The direct manifestation of black plates is excessive oxidation of Ni and excessive gold, which can cause embrittlement of solder joints and affect reliability.
6. Electroless Nickel Palladium Immersion Gold (ENEPIG)
Compared with electroless nickel gold, ENEPIG has an extra layer of palladium between nickel and gold. In the deposition reaction of displacement gold, the chemically plated palladium layer will protect the nickel layer from excessive corrosion by the replacement gold. While preventing the corrosion caused by the replacement reaction, palladium is fully prepared for immersion gold. The deposition thickness of nickel is generally 120~240μin (about 3~6μm), and the thickness of palladium is 4~20μin (about 0.1~0.5μm). The deposition thickness of gold is generally 1~4μin (0.02~0.1μm).
Advantages: It has a very wide range of applications. At the same time, chemical nickel palladium gold surface treatment can effectively prevent the connection reliability problems caused by black pad defects compared to electroless nickel gold surface treatment, and can replace electroless nickel gold.
Disadvantages: Although ENEPIG has many advantages, palladium is expensive and is a scarce resource. At the same time, like electroless nickel gold, its process control requirements are strict.
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