1. Bare copper plate
The advantages and disadvantages are obvious:
Advantages: low cost, smooth surface, good weldability (without being oxidized).
Disadvantages: easily affected by acid and humidity, can not be stored for a long time, need to be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to the air; cannot be used for double-sided panels, because the second side after the first reflow soldering already oxidized. If there are test points, solder paste must be added to prevent oxidation, otherwise it will not be able to make good contact with the probe.
Pure copper is easily oxidized if exposed to air, and the outer layer must have the above protective layer. Therefore, surface treatment is required in circuit board processing.
2. OSP process board
The difference between OSP and other surface treatment processes is that its function is to act as a barrier layer between copper and air. Simply put, OSP is to chemically grow an organic film on the clean bare copper surface. Because it is organic, not metal, it is cheaper than tin spraying.
The only function of this organic film is to ensure that the inner copper foil will not be oxidized before soldering. As soon as it is heated during soldering, the film volatilizes. Solder is able to solder copper wires and components together. However, this layer of organic film is not resistant to corrosion. An OSP circuit board cannot be soldered after being exposed to the air for ten days. Many computer motherboards use the OSP process. Because the board area is too large, OSP is more economical.
Advantages: It has all the advantages of bare copper board welding, and the expired board can also be resurfaced.
1. OSP is transparent and colorless, so it is difficult to check, and it is difficult to distinguish whether it has been treated by OSP.
2. OSP itself is insulating and non-conductive, which will affect the electrical test. Therefore, the test point must be opened with a stencil and printed with solder paste to remove the original OSP layer in order to contact the pin point for electrical testing. OSP cannot be used to handle electrical contact surfaces, such as keyboard surfaces for keys.
3. OSP is easily affected by acid and temperature. When used for secondary reflow soldering, it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be poor. If the storage time exceeds three months, it must be resurfaced. Use within 24 hours after opening the package
3. Hot air leveling (HASL)
Hot air leveling, also known as hot air solder leveling, is a process of coating molten tin-lead solder on the surface of the PCB and leveling (blowing) with heated compressed air to form a layer that resists copper oxidation and provides good solderability. Sexual coating. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction with a thickness of about 1-2 mils.
In the case 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, for the case of high junction strength requirements, the method of electroplating nickel/gold is often used.
Advantage: low cost
1. The pads processed by HASL technology are not flat enough, and the coplanarity cannot meet the process requirements of fine-pitch pads.
2. Not environmentally friendly, lead is harmful to the environment
4. Gold-plated plate
Gold plating uses real gold, which, even in a thin layer, already accounts for nearly 10% of the cost of the board. The use of gold as a coating is one for the convenience of welding and the other for anti-corrosion. Even the golden fingers of memory sticks that have been used for several years are still as shiny as ever. If they used copper, aluminum, and iron for the same period of time, they will now rust into a pile of waste.
The gold-plated layer is widely used in the component pads, gold fingers, connector shrapnel and other positions of the circuit board. Most of the motherboards of the most widely used mobile phone circuit boards are gold-plated boards, immersion gold boards, computer motherboards, audio and small digital circuit boards are generally not gold-plated boards.
Advantages: strong conductivity, good oxidation resistance, long life. The coating is dense and wear-resistant, generally used in bonding, welding and plugging occasions
Disadvantages: high cost, poor welding strength.
5. Chemical Gold/Immersion Gold (ENIG)
Nickel immersion gold (ENIG), also known as nickel gold, immersion nickel gold, referred to as gold and immersion gold. ENIG is to chemically wrap a thick layer of nickel-gold alloy on the copper surface with good electrical properties and can protect the PCB for a long time. The deposition thickness of the inner layer of nickel is generally 120 240μin (about 36μm ), and the deposition thickness of the outer layer of gold is generally 24μinch (0.05 0.1μm). Unlike OSP, which only acts as a rust barrier, it can be useful and achieve good electrical performance during long-term use of the PCB. In addition, it also has environmental tolerance that other surface treatment processes do not have.
1. The surface of the PCB treated by ENIG is very flat and has good coplanarity, which is suitable for the contact surface of keys.
2. ENIG has excellent solderability, gold will quickly melt into the molten solder, and the solder and Ni form a Ni/Sn metal compound.
Disadvantages: The process flow is complex, and to achieve good results, it is necessary to strictly control the process parameters. The most troublesome thing is that EING-treated PCB surfaces are prone to black disk benefits during the ENIG or soldering process. The direct manifestation of the black plate is excessive oxidation of Ni and too much gold, which will embrittle the solder joints and affect reliability.
6. Electroless nickel palladium immersion gold (ENEPIG)
Compared with nickel and gold, ENEPIG has an additional layer of palladium between nickel and gold. In the deposition reaction of gold replacement, the electroless palladium layer will protect the nickel layer to prevent it from being excessively corroded by the replacement gold, and the palladium will prevent the replacement reaction from occurring. At the same time, it is fully prepared for immersion gold. The deposition thickness of nickel is generally 120 240 μin (about 3 6 μm), and the thickness of palladium is 4 20 μin (about 0.1 0.5 μm). Gold deposition thickness is generally 14μin (0.02 0.1μm).
Advantages: It has a very wide range of applications. At the same time, the surface treatment of chemical nickel palladium gold can effectively prevent the connection reliability problems caused by black pad defects, and can replace nickel gold.
Disadvantages: Although ENEPIG has many advantages, palladium is expensive and is a scarce resource. At the same time, the same as nickel gold, its process control requirements are strict.
Seven, spray tin circuit board
The silver board is called spray tin board. Spraying a layer of tin on the outer layer of the copper circuit can also help with soldering. But it cannot provide long-term contact reliability like gold. It has no effect on the components that have been soldered, but for the pads exposed to the air for a long time, the reliability is not enough, such as ground pads, spring pin sockets, etc. Long-term use is easy to oxidize and rust, resulting in poor contact. Basically, the circuit boards used for small digital products, without exception, are spray tin boards, because they are cheap.
Advantages: low price, good welding performance.
Disadvantages: It is not suitable for soldering pins with fine gaps and components that are too small, because the surface flatness of the tin-sprayed board is poor. Solder beads are easily generated in PCB processing, which is more likely to cause short circuits to fine pitch components. When used in the double-sided SMT process, because the second side has undergone high temperature reflow soldering, it is very easy to re-melt the tin spray and produce tin beads or similar water droplets that are affected by gravity into spherical tin points that drop, causing the surface to be even more unstable. Flattening in turn affects welding problems.
8. Immersion silver
The immersion silver process is between OSP and electroless nickel/immersion gold, and the process is relatively simple and fast. Immersion silver is a displacement reaction, which is almost a submicron coating of pure silver (5 15μin, about 0.1 0.4μm). Sometimes the immersion silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate the problem of silver migration. Generally, it is difficult to measure the organic matter in this thin layer. Analysis shows that the weight of the organic matter is less than 1%. Provides good electrical properties and maintains good solderability even when exposed to heat, humidity and pollution, but tarnishes. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel/immersion gold.
Advantages: The immersion silver welding surface has good solderability, good coplanarity, and at the same time, there is no conductive barrier like OSP, but when used as a contact surface (such as a button surface), its strength is not as good as gold.
Disadvantages: An important problem with immersion silver is the electron migration of silver. When exposed to a humid environment, silver will generate electron migration under the action of voltage. By adding organic components to silver, the electron migration problem can be reduced.
Nine, immersion tin
Since all current solders are based on tin, the tin layer can be matched to any type of solder. However, the previous PCB is prone to tin whiskers after the tin immersion process, and the migration of tin whiskers and tin during the soldering process will bring reliability problems, thus limiting the use of the tin immersion process. After adding organic additives to the tin immersion solution, the tin layer has a granular structure, which overcomes the previous problems and has good thermal stability and solderability.
The biggest weakness of immersion tin is the short life, especially when stored in a high temperature and high humidity environment, the Cu/Sn intermetallic compound will continue to grow until it loses solderability. It is reasonable to deposit tin at a thickness of not less than 40 μin (1.0 μm) to provide a pure tin surface to meet solderability requirements.
Disadvantages: The biggest weakness of immersion tin is short life, especially when stored in a high temperature and high humidity environment, the Cu/Sn intermetallic compound will continue to grow until it loses solderability. There is also no diffusion problem between electroless nickel/immersion gold metal; just the immersion tin plate cannot be stored for too long
Each surface treatment process has its own unique features, and the scope of application is not the same. But Nickel Palladium Immersion Gold (ENEPIG) is an all-purpose processing method, which can meet the requirements of various assembly occasions.