PCB surface treatment and advantages and disadvantages.

- Feb 03, 2018-

1. Hot air leveling (spray tin)
The general process of hot air leveling process is: microerosion, preheating, coating flux, spray tin, cleaning.
Hot air leveling and hot air solder leveling (commonly known as spray tin), it is coated on the PCB surface heating molten tin solder (lead) and total compressed air (blow) flat craft, make its form a layer of copper oxidation resistance, and can provide good weldability of the coating layer. The solder and copper are formed in the joint to form a compound. The PCB should be immersed in molten solder during hot air conditioning. The wind knife flushes the liquid solder before the solder solidifies. The wind knife can minimize the bending of the solder on the copper surface and prevent welding bridge.
The hot air is evenly divided into vertical and horizontal types, which is generally considered to be good, which is mainly composed of horizontal hot air leveling coating, which can realize automatic production.
Advantages: longer storage time; After PCB is finished, the copper surface is completely wetted (completely covered with tin before welding); Suitable for lead-free soldering; Mature process, low cost, suitable for visual inspection and electrical measurement.
Disadvantages: not suitable for line binding; Due to the surface flatness problem, there are also limitations on SMT; Not suitable for contact switch design. The copper will dissolve in tin, and the board will undergo a high temperature. Especially thick or thin plate, spray tin is limited, production operation is inconvenient.

2. Organic weldable protective agent (OSP)
The general process is: degreasing -> microcorrosion -> pickling -> pure water cleaning -> organic coating -> cleaning, process control is relatively easy to show processing technology.
OSP is a process that meets the RoHS directive requirement for PCB copper foil surface treatment. OSP is an abbreviation of Organic Solderability preservative. It is also known as the Organic soldering film, also known as the copper protector, and also known as Preflux in English. In a nutshell, the OSP is a chemically modified layer of organic skin on the surface of clean, bare copper. This film has anti-oxidation, thermal shock and moisture resistance, which can protect the copper surface from rust (oxidation or vulcanization) in normal environment. But in the subsequent welding heat, the protective film and must be quickly removed by flux easily, so just can make clean copper surface of the show is in a very short period of time with molten solder immediately become a solid solder joints.
Advantages: simple process, very flat surface, suitable for lead-free soldering and SMT. Easy to rework, easy to operate, suitable for horizontal line operation. The board is suitable for multiple processes (such as OSP+ENIG). Low cost and environmentally friendly.
Disadvantages: limit of the number of reflow soldering (multiple welding thick, the film will be destroyed, basically 2 times no problem). Not suitable for pressing technology, line binding. Visual inspection and electrical testing are not convenient. N2 gas protection is required for SMT. SMT rework is not suitable. Storage conditions are high.

3. Full plate nickel-plated gold.
Nickel-plated nickel plating is plated on the surface of PCB and then plated with a layer of gold. The nickel plating is mainly to prevent the diffusion of gold and copper. Now there are two types of electroplating nickel gold: soft gold plating (gold, gold surface looks not bright) and hard gold plating (surface is smooth and hard, wear-resisting, contain other elements such as cobalt, gold looks more light). Soft gold is mainly used for chip packaging gold wire; The hard gold is mainly used for electrical interconnection in non-welding areas.
Advantages: longer storage time >12 months. Suitable for contact switch design and gold wire binding. Suitable for electrical testing
Weakness: high cost, gold is thicker. Electroplated fingers require additional design lines to conduct electricity. Because the gold thickness is not always applied in welding, it may cause the welding point to be brittle and affect the strength. Plating surface uniformity problem. The nickel plating is not covered by the line. Not suitable for aluminum wire binding.

4, zedoary
The general process is: pickling and cleaning -> microetching -> presoaking -> activation -> electroless nickel -> chemical immersion gold; There are six chemical channels in the process, involving nearly 100 chemicals, and the process is complicated.
The sinking gold is coated with a thick, electrically good nickel-gold alloy on the copper surface, which can protect PCB for a long time. In addition, it has the tolerance of other surface treatment technologies. In addition, sinking gold can also prevent the dissolution of copper, which will be beneficial to lead-free assembly.
Advantages: not easy to oxidize, can be stored for a long time, surface is flat, suitable for welding fine gap pin and welding spot smaller components. The first choice for PCB boards (such as mobile phone boards). It can be repeated several times that reflow soldering is less likely to reduce its weldability. It can be used as the base material for a Chip On Board.

5, sink a tin
Since all solders are based on tin, the tin layer can match any type of solder. Tin process between flat copper tin compounds can be formed, this feature makes heavy tin has like hot air leveling of good weldability and no hot air leveling flatness of headache problem; The tin plate cannot be stored for too long and must be assembled according to the order of settling tin.
Advantages: suitable for horizontal line production. Suitable for fine line processing, suitable for lead-free soldering, especially suitable for pressing technology. Very good flatness, suitable for SMT.
Disadvantages: good storage conditions, preferably not more than 6 months, to control the growth of tin. Not suitable for contact switch design. In the production process, the resistance welding film process requirements are high, otherwise, it will cause the soldering film to fall off. For multiple welding, the best N2 gas protection. Electrical testing is also a problem.

6, heavy silver
The silver process is between organic coating and electroless nickel plating. The process is simple and fast. Even when exposed to heat, humidity and pollution, silver can maintain good weldability but will lose its luster. The silver does not have the good physical strength of the chemical plating nickel/sinking gold because there is no nickel under the silver layer.
Advantages: simple process, suitable for lead-free soldering, SMT. The surface is very smooth, low cost, suitable for very fine lines.
Disadvantages: high storage condition, easy to pollute. Welding strength is prone to problem (microcavity problem). It is easy to show the phenomenon of electrical migration and the phenomenon of the cavitating phenomenon in the copper under the resistance welding film. Electrical testing is also a problem.

7. Chemical nickel palladium.
Compared with gold deposit, the chemical nickel palladium is a layer of palladium between nickel and gold. Palladium can prevent the corrosion phenomenon caused by the substitution reaction, and make full preparation for the sinking. Gold is closely covered with palladium, providing a good interface.
Advantages: suitable for lead-free soldering. The surface is very smooth and suitable for SMT. The through-hole can also be made into nickel gold. Long storage time, storage conditions are not harsh. Suitable for electrical testing. Suitable for switch contact design. Suitable for aluminum wire binding, suitable for thick plate, resistance to environmental attack.

8. Plating hard gold
In order to improve the wear-resisting property of the product, increase the insertion number and the plating hard gold.
The changes in the surface treatment process of PCB are not very large, as if they are far away, but it should be noted that long-term slow changes will lead to great changes. The surface treatment process of PCB is sure to change dramatically in the face of increasing environmental demand.