PCB board common surface treatment process and its advantages and disadvantages
There are many PCB surface treatment processes, common are hot air leveling, organic solderability protection agent (OSP), full-plate nickel-plated gold, heavy gold, heavy tin, heavy silver, chemical nickel-palladium gold, and hard gold plating. A variety of processes.
1, hot air leveling (spraying)
The general process of the hot air leveling process is: micro-etching → pre-heating → coating flux → spraying tin → cleaning.
Hot air leveling, also known as hot-air solder leveling (commonly known as spray tin), is the process of applying a molten tin (lead) solder on the surface of a PCB and heating (or blowing) the compressed air to form a layer that resists copper oxidation. It also provides a good solderability coating. When the hot air is leveled, the solder and copper form a copper-tin intermetallic compound at the junction. When the PCB is hot-air levelled, it sinks into the molten solder; the air knife blows the liquid solder before the solder solidifies; the air knife minimizes the meniscus of the solder on the copper surface and prevents solder bridging.
Hot air leveling is divided into vertical and horizontal types. It is generally considered that the horizontal type is better, mainly because the horizontal hot air leveling coating is relatively uniform and can realize automatic production.
Advantages: Longer storage time; After the PCB is completed, the copper surface is completely wet (completely covered with tin before welding); suitable for lead-free welding; mature technology, low cost, suitable for visual inspection and electrical testing
Disadvantages: not suitable for wire bonding; due to surface flatness issues, there are limitations on SMT; not suitable for contact switch design. When the tin was sprayed, the copper dissolved, and the board was subjected to a high temperature. Specially thick or thin plates, sprayed tin has limitations, and production operations are inconvenient.
- Organic Solderability Protection Agent (OSP)
The general process is: degreasing -> micro-etching -> pickling -> pure water cleaning -> organic coating -> cleaning, process control compared to other shows that the processing process is relatively easy.
The OSP is a process that complies with the RoHS directive for surface treatment of printed circuit board (PCB) copper foils. OSP is the abbreviation of Organic Solderability Preservatives. Chinese translation is organic solder mask, also known as copper protector. English is also known as Preflux. Simply put, OSP is to chemically grow a layer of organic film on the surface of clean bare copper. This layer of film has anti-oxidation, thermal shock resistance, moisture resistance, to protect the copper surface in the normal environment no longer continue rust (oxidation or vulcanization, etc.); but in the subsequent welding temperature, this protective film must be very It is easily removed by the flux quickly, so that the exposed clean copper surface can be immediately combined with the molten solder in a very short time to become a solid solder joint.
Advantages: Simple process, very smooth surface, suitable for lead-free soldering and SMT. Easy to rework, easy to operate, suitable for horizontal line operation. On the board for a variety of processing coexist (for example: OSP + ENIG). Low cost and friendly environment.
Disadvantages: Limits on the number of reflows (Multiple welds are thick, the film is destroyed, basically no problems with 2). Not suitable for crimping technology, wire bonding. Visual inspection and electrical measurement are inconvenient. S2 requires N2 gas protection. SMT rework is not suitable. Storage requirements are high.
3, full plate nickel-plated gold
Nickel-plated gold is plated with a layer of nickel on the surface of the PCB and then plated with gold. Nickel plating is mainly used to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold: gold-plated (pure gold, gold surface does not look bright) and gold plated (surface smooth and hard, wear-resistant, containing cobalt and other elements, the gold surface looks brighter). Soft gold is mainly used for gold wire in chip packaging; hard gold is mainly used in non-welded electrical interconnections.
Advantages: Longer storage time >12 months. Suitable for contact switch design and gold wire binding. Suitable for electrical testing
Weaknesses: Higher costs, gold is thicker. Electrode gold finger requires extra design line conductivity.
Due to the fact that the thickness of gold is not always applied during welding,
the solder may be embrittled due to too thick gold and affect the strength. Plating surface uniformity problems.
Electroplated nickel gold does not cover the edges of the wire. Not suitable for aluminum binding.
4, Shen Jin
The general process is: deacidification and cleaning -> micro-etching -> pre-dipping -> activation -> electroless nickel -> chemical leaching gold; in the process there are 6 chemical tanks, involving nearly 100 kinds of chemistry Products, the process is more complicated.
Immersion gold is a thick layer of nickel-gold alloy coated on the copper surface, which can protect the PCB in the long-term; in addition it also has the endurance to the environment that other surface treatment processes do not have. In addition, Immersion Gold can also prevent the dissolution of copper, which will benefit lead-free assembly.
Advantages: It is not easy to oxidize, can be stored for a long time, and has a smooth surface. It is suitable for welding fine gap pins and components with small solder joints. There is a preference for the keypad PCB (such as a mobile phone board). Repeated reflow soldering can also be repeated less often to reduce solderability.
It can be used as a substrate for COB (Chip On Board) wire bonding.
Disadvantages: High cost, poor welding strength, because the use of electroless nickel process, easy to have black disc problems. The nickel layer will oxidize over time, and long-term reliability is a problem.
- Shen Tin
Since all current solders are based on tin, the tin layer can match any type of solder. Immersion tin process can form a flat copper-tin intermetallic compound, this feature makes it possible to have the same good solderability as hot air leveling without the headache flatness problem of hot air leveling; it can not be stored for too long.
The assembly must be carried out according to the order of sinking tin.
Advantages: Suitable for horizontal production. Suitable for fine line processing, suitable for lead-free soldering, especially for crimping technology. Very good flatness, suitable for SMT.
Disadvantages: Good storage conditions are required. It is best not to exceed 6 months to control whisker growth. Not suitable for contact switch design. The production process requires a relatively high solder mask process, otherwise it will cause the solder mask to fall off. When multiple welding, N2 gas protection is preferable. Electrical measurement is also a problem.
6, Shen Yin
The silver immersion process is between organic coating and electroless nickel/immersion gold. The process is simple and fast. Even if exposed to heat, humidity and pollution, silver can maintain good solderability but it will lose its luster. . Shen Yin does not have the good physical strength of electroless nickel/immersion gold because there is no nickel below the silver layer.
Advantages: simple process, suitable for lead-free soldering, SMT. The surface is very smooth, low cost, and suitable for very fine lines.
Disadvantages: High storage conditions and easy contamination.
The welding strength is prone to problems (micro-void problems).
It is prone to electromigration and the phenomenon of Giovanni’s gnawing of copper under the solder mask.
Electrical measurement is also a problem
7, chemical nickel palladium gold
The chemical nickel-palladium-gold has a layer of palladium between nickel and gold compared with the gold-immersed gold. Palladium can prevent the phenomenon of corrosion caused by the substitution reaction and make full preparation for the heavy gold. Gold is tightly covered on the palladium, providing a good contact surface.
Advantages: Suitable for lead-free soldering. The surface is very smooth and suitable for SMT. Through holes can also be applied to nickel gold. Longer storage time, storage conditions are not harsh. Suitable for electrical testing. Suitable for switch contact design. Suitable for aluminum wire binding, suitable for thick plates, strong against environmental attacks.
8, plating hard gold
In order to improve the wear resistance of the product, the number of plugs and pulls is increased and the hard gold plating is performed.