PCB Technical - Circuit board factory: the purpose of nickel plating on parts or circuit boards

Circuit board factory: the purpose of nickel plating on parts or circuit boards

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, to protect the copper layer from oxidation, and to prevent the conductivity and solderability from cracking. 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, InterMetallic Compound), although the strength of this IMC is not as strong as that produced by OSP surface treatment Cu6Sn5, but it is enough to meet the needs of most current products.

In addition, in order to achieve a certain mechanical strength for the pins of electronic parts, "brass" is often used instead of "pure copper" as the substrate. 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, and a layer of "nickel" must be plated as a barrier layer. In order to successfully 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.

Is it possible to use nickel plating directly for soldering? The answer is basically no, because "nickel" is also very easy to passivation (Passivation) in the atmospheric environment, and it will also have an extremely adverse effect on the solderability. Therefore, generally a layer of pure tin is plated on the outside of the nickel layer. 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.

In order 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 a period of 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 plating layer of ENIG PCB is generally recommended to fall between 2μ"～5μ" (0.05μm～0.125μm), and the thickness of the chemical nickel layer should fall between 3μm (118μ") and 6μm (236μ"). It is recommended to use short pins for wave soldering to avoid short circuit problems, and it is recommended that the length of the pins should not exceed 2.54mm.