PCB Technical - Ground processing PCB surface technology

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process appear to be particularly prominent. At present, the topic of lead and bromine is the most popular; lead-free and halogen-free will affect the development of PCB in many aspects. Although at present, the changes in PCB surface treatment process are not very big, it seems to be a relatively remote 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 will definitely undergo tremendous changes in the future.

How to "advance with the times" to deal with PCB surface technology?

Purpose of surface treatment

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 original copper for a long time, so other treatments are needed for copper. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxides, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.

Five common surface treatment processes

There are many PCB surface treatment processes, the common ones 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

Also known as hot-air solder leveling, it is a process of coating molten tin-lead solder on the surface of the PCB and flattening (blowing) it with heated compressed air to form a coating that is resistant to copper oxidation and provides good solderability. Cladding. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the joint. The thickness of the solder to protect the copper surface is about 1-2 mils. The PCB should be immersed in molten solder during hot air leveling; 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. There are two types of hot air leveling: vertical and horizontal. Generally, the horizontal type is considered to be better. The main reason is that the horizontal hot air leveling is more uniform and can realize automated production.

The general process of the hot air leveling process is: micro-etching-preheating-coating flux-spraying tin-cleaning.

2. Organic coating process

Different from other surface treatment processes, it acts as a barrier between copper and air; the organic coating process is simple and low in cost, which makes it widely used in the industry. The early organic coated molecules were imidazole and benzotriazole, which played a role in rust prevention, and the newest molecules were mainly benzimidazole, which was copper that chemically bonded nitrogen functional groups to the PCB. In the subsequent soldering process, if there is only one organic coating layer on the copper surface, it will not work, there must be many layers. This is why copper liquid is usually added to the chemical tank. After coating the first layer, the coating layer adsorbs copper; then the organic coating molecules of the second layer are combined with copper until twenty or even hundreds of organic coating molecules gather on the copper surface, which can ensure that multiple cycles are performed. Flow welding.

Tests have shown that the latest organic coating process can maintain good performance during multiple lead-free soldering processes.

The general flow of the organic coating process is: degreasing-micro-etching-pickling-pure water cleaning-organic coating-cleaning. The process control is easier than other surface treatment processes.

3. Electroless nickel plating / immersion gold chemical nickel plating / immersion gold process

It is not as simple as organic coating. Electroless nickel/immersion gold seems to put a thick armor on the PCB; in addition, the electroless nickel/immersion gold process is not like organic coating as an anti-rust barrier layer, it can be used on PCB for a long time The process is useful and achieves good electrical performance. Therefore, electroless nickel/immersion gold is to wrap a thick, good electrical nickel-gold alloy on the copper surface, which can protect the PCB board for a long time; in addition, it also has environmental protection that other surface treatment processes do not have. Patience.

The reason for nickel plating: Since gold and copper will diffuse each other, the nickel layer can prevent the diffusion between gold and copper. In addition, electroless nickel/immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.

The general process of electroless nickel plating/gold immersion process is: acid cleaning-micro-etching-pre-dipping-activation-electroless nickel plating-chemical immersion gold. There are mainly 6 chemical tanks, involving nearly 100 chemicals, so the process control is compared difficulty.