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What is the Purpose of Plating Nickel on PCB?

by: Aug 06,2020 2299 Views 0 Comments Posted in PCB Manufacturing Information

ENIG Nickel Plating IMC Passivation Matte Tin Surface Treatment OSP

The chemical element symbol of nickel is Ni. Because of its excellent physical, mechanical, and chemical properties, nickel has many applications in engineering and industry, such as anti-corrosion, increased hardness, wear-resistance, and magnetism. It is mainly used in alloy formulations, such as nickel steel, nickel-chromium steel, nickel-copper, etc. to increase its corrosion resistance and oxidation resistance. Because of its good oxidation resistance, it was often used to make coins in the early days.

Nickel itself is very active to oxygen, which means that nickel itself is easily oxidized in the atmosphere, but because the oxides (NiO, Ni (OH) 2) produced by itself have excellent fineness. A film can be formed to cover itself to isolate it from continuing contact with the air, so it can resist continued oxidation, which means that nickel will oxidize itself to form a protective film before it can protect itself and the bottom metal from continued oxidation. This point should be paying attention during use.

The behavior of forming a protective film after oxidation to prevent further oxidation is called "metal passivation." Therefore, nickel is often used to be plated on other metal surfaces to protect the underlying metal from contact with air and cause oxidation. However, the plating layer must be "defect-free" to a certain degree of protection, in the early nickel plating process, because the process is not very mature, small holes often occur, so that the bottom metal material under the coating cannot be completely sealed, resulting in the oxidation of the bottom metal after nickel plating. The current nickel plating process has matured day by day, and a plugging agent is generally added to the plating bath to solve the porosity problem.

In addition to the effect of preventing oxidation on the metal surface, nickel plating can also enhance the following mechanical properties:

1. Tensile strength

2. Elongation

3. Hardness

4. Internal stress

5. Fatigue life

6. Hydrogen embrittlement

Besides, nickel plating also has good chemical corrosion resistance, and is often used in the chemical, petroleum, food and beverage industries to prevent corrosion, prevent product contamination, and maintain product purity. However, it should be noted that when the nickel oxide protective film is invaded by the chloride solution, pinhole corrosion will be formed. Generally, the nickel plating layer will not have much problem in neutral or alkaline solutions, but most of the problems are encountered. Minerals will be corroded.

What is the purpose of plating nickel on PCB?

The main purpose of plating nickel on the ENIG (nickel immersion gold) circuit board is to prevent the migration and diffusion between copper and gold. As a barrier layer and anti-corrosion protective layer, it protects the copper layer from oxidation, and can also prevent cracking of conductivity and solderability. According to the IPC-4552 recommendations for ENIG nickel plating, its thickness must be at least 3μm (micrometer)/118μ" for protection function. In the process of soldering or SMT reflow, the nickel layer will combine with the tin in the solder paste to form Ni3Sn4 InterMetallic Compound (IMC), although the strength of this layer of IMC is not as strong as the Cu6Sn5 produced by the OSP surface treatment. But is sufficient to meet the requirements of most current products.

Besides, to achieve a certain mechanical strength for the pins of electronic parts, brass is often used instead of pure copper as the substrate metal. However, because brass contains a large amount of zinc, it will greatly hinder the solderability, so it is not possible to plate tin directly on the brass, but a layer of nickel must be plated as a barrier layer to complete the task of welding.

Please note: Do not plate tin directly on the brass surface, because the brass is a copper-zinc alloy, otherwise the copper will peel off directly after remelting, and there will be false welding.

Can nickel plating be used instead of soldering tin ? The answer is basically no, because nickel is also very easy to passivation in the atmospheric environment, and it will also have an extremely adverse effect on the solderability, so generally, a layer of pure tin is plated outside the nickel layer to improve the solderability of the part feet. Unless the packaging of the finished parts can ensure that the air is isolated, and the user can ensure that the nickel layer is not oxidized before welding, once the nickel layer is oxidized, even if it is welded, its welding strength will continue to deteriorate and eventually break.

To prevent the migration and diffusion of zinc and tin in brass, in addition to pre-plating a layer of nickel, some people also choose to pre-plating pure copper as a barrier layer and a protective layer against corrosion. And then tinned to strengthen the soldering ability.

It is common for some tin-plated parts to oxidize after being placed for some time. Most of them are caused by no pre-plated copper or pre-nickel, or the thickness of the pre-plated layer is not enough to prevent the above problems.

If the purpose of tinning is to strengthen the solder, matte tin plating is generally recommended instead of bright tin plating.

According to the requirements of IPC4552, the thickness of the gold layer of ENIG circuit board is recommended between 2μ"~5μ" (0.05μm~0.125μm), and the thickness of the chemical nickel layer should between 3μm(118μ")~6μm(236μ").

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