Eight PCB surface treatment processes
With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process are particularly prominent. Lead and bromine are the hottest topics, and lead-free and halogen-free will affect PCB development in many ways.
Although the current surface treatment process of PCBs is not very large, it seems to be a relatively distant problem, but it should be noted that long-term slow changes will lead to huge changes. In the case of increasingly high environmental requirements, the surface treatment process of PCB will definitely change greatly in the future.
The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist in the form of oxides in the air, it is unlikely to remain as raw copper for a long time, so other treatments of copper are required. Although in the subsequent assembly, a strong flux can be used to remove most of the copper oxide, the strong flux itself is not easily removed, so the industry generally does not use a strong flux.
There are many PCB surface treatment processes, such as hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one.
1, hot air leveling (spray tin)
Hot air leveling, also known as hot air solder leveling (commonly known as tin-spraying), is the process of applying molten tin (lead) solder to the surface of a PCB and heating (compressing) by heating compressed air to form copper-resistant oxides. Layer. It also provides a good solderability coating. When the hot air is flattened, the solder and copper form a copper-tin intermetallic compound at the junction. During the hot air leveling process, the PCB is immersed in the molten solder. The air knife blows away the liquid solder before the solder solidifies; the air knife minimizes the solder meniscus on the copper surface and prevents solder bridging.
2. Organic solderability protectant (OSP)
OSP is a process that meets the requirements of the RoHS Directive for the surface treatment of printed circuit board (PCB) copper foil. OSP is an abbreviation for organic solderability preservatives. The Chinese translation is an organic solder mask, also known as a copper protective layer, also known as Preflux in English. In short, OSP chemically grows organic films on clean bare copper surfaces.
The film has oxidation resistance, thermal shock resistance and moisture resistance. It is used to protect the copper surface from rust (oxidation or vulcanization) under normal conditions. However, in the subsequent high temperature soldering, the protective film must be very high. It is easily removed by the flux so that the exposed clean copper surface can be combined with the molten solder to form a strong solder joint in a short period of time.
3, full plate nickel-plated gold
Nickel-plated gold is plated with a layer of nickel on the surface conductor of the PCB and then plated with a layer of gold. Nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of nickel-plated gold: gold-plated (pure gold, gold surface does not look bright) and hard gold (smooth and hard surface, wear-resistant, containing other elements such as cobalt) and gold surface looks brighter). Soft gold is mainly used for gold wire bonding in chip packages; hard gold is mainly used for electrical interconnection of non-welded parts.
4, Shen Jin
Shenjin is a thick nickel-gold alloy with good electrical properties wrapped around the copper surface, which can protect the PCB for a long time. In addition, it has environmental tolerances that are not found in other surface treatment processes. In addition, immersion gold also prevents copper from melting, which will facilitate lead-free assembly.
5, deep creek
Since all current solders are based on tin, the tin layer can be matched to any type of solder. The immersion tin process can form a flat copper-tin intermetallic compound. This feature allows the immersion tin to have the same weldability as hot air leveling without causing hot air leveling. The tablet cannot be stored for too long. Assembly must be done in the order of tin.
The silver process is between the organic coating and the chemical nickel/immersion gold. The process is simple and fast. Even when exposed to heat, moisture and pollution, silver can maintain good solderability but loses its luster. . Silver does not have good physical strength for electroless nickel/immersion gold because there is no nickel below the silver layer.
7. Chemical nickel palladium
Compared to immersion gold, chemical nickel-palladium has an additional layer of palladium between nickel and gold. Palladium prevents corrosion caused by the displacement reaction and is well prepared for immersion gold. Gold is tightly covered on top of the palladium to provide a good contact surface.
8, electroplated hard gold
In order to improve the wear resistance of the product, the number of insertions and removals is increased and hard gold is plated.
As the requirements of users become higher and higher, the requirements for the environment are becoming more and more strict, and the surface treatment process is also increasing. The choice of surface treatments with broad prospects and versatility is a bit dazzling and chaotic. . The future development direction of PCB surface treatment process will not be accurately predicted. In any case, you must first meet the user requirements and protect the environment!