With the continuous improvement of human requirements for the living environment, the environmental problems involved in the production process of PCB manufacturers are particularly prominent. The topic of lead and bromine is the most popular, and lead-free and halogen-free will affect the development of PCB circuit boards in many ways.
Although at present, the changes in the surface treatment process of PCB boards are not very large, it seems to be a relatively distant thing, but it should be noted that long-term slow changes will lead to huge changes. With the increasing demand for environmental protection, the surface treatment process of PCB proofing will definitely change dramatically in the future.
The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as primary copper for a long time. Therefore additional treatment of the copper is required. Although strong flux can be used to remove most copper oxides in subsequent assembly, strong flux itself is not easy to remove, so strong flux is generally not used in the industry.
There are many surface treatment processes for PCB boards. The five common processes are hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one below.
1. Hot air leveling (spray tin)
Hot air leveling, also known as hot air solder leveling (commonly known as tin spraying), is a process of coating molten tin (lead) solder on the surface of the PCB and leveling (blowing) it with heated compressed air to form a layer that is resistant to copper oxidation. It can also provide a good solderability coating. Solder and copper form a copper-tin intermetallic compound at the joint during hot air leveling. When the PCB is hot air leveled, it sinks in the molten solder; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent solder bridging.
2. Organic Solderability Preservative (OSP)
OSP is a process for the surface treatment of printed circuit board (PCB) copper foil that meets the requirements of the RoHS directive. OSP is the abbreviation of Organic Solderability Preservatives. It is translated as Organic Solderability Preservatives in Chinese, also known as copper protection agent, and it is also called Preflux in English. Simply put, OSP is to chemically grow an organic film on a clean bare copper surface.
This layer of film has anti-oxidation, thermal shock resistance, and moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in a normal environment; but in the subsequent welding high temperature, this protective film must be very strong It is easy to be quickly removed by flux, so that the exposed clean copper surface can be combined with molten solder in a very short time to form a firm solder joint.
3. Nickel-plated gold on the whole board
The nickel-gold plating of the pcb board is to plate a layer of nickel on the surface conductor of the PCB and then plate a layer of gold. The nickel plating is mainly to prevent the diffusion between gold and copper. Now there are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains other elements such as cobalt, and the gold surface looks brighter). Soft gold is mainly used for making gold wires in chip packaging; hard gold is mainly used for electrical interconnection at non-welding places.
4. Shen Jin
Immersion gold is to wrap a thick layer of nickel-gold alloy with good electrical properties on the copper surface, which can protect the PCB for a long time; in addition, it also has environmental tolerance 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.
5. Shen Tin
Since all current solders are tin-based, the tin layer matches any type of solder. The immersion tin process can form a flat copper-tin intermetallic compound. This feature makes immersion tin have the same good solderability as hot air leveling without the troublesome flatness problem of hot air leveling; immersion tin boards cannot be stored for too long , the assembly must be carried out according to the sequence of immersion tin.
