Nickel plating process for PCB printed circuit board

- Sep 01, 2018-

1. Functions and characteristics of PCB (short for printed circuit board printed in English). Nickel plating is used as substrate coating for precious and base metals.For heavy duty wear on some surfaces, such as switch contacts, contact sheets or plug gold, nickel as the substrate coating of gold can greatly improve wear resistance.When used as a barrier, nickel effectively prevents diffusion between copper and other metals.Matte/gold composite coating is often used as an anti-etching metal coating, and can meet the requirements of hot pressing welding and brazing. Only nickel can be used as an anti-etching coating containing ammonia.Nickel plating thickness is generally not less than 2.5 microns, usually 4-5 microns.

The low stress nickel deposition layer of PCB is usually plated with modified watt nickel plating solution and some amino-sulfonate nickel plating solution with stress-reducing additives.

The PCB nickel plating we often say has the optical nickel and the dumb nickel (also called low stress nickel or semi-bright nickel), usually requires the coating to be uniform and meticulous, the porosity is low, the stress is low, and the ductility is good.

2. The amino-sulfonate nickel is widely used as the substrate coating on the metallized hole electroplating and the printed plug contact sheet.The obtained deposition layer has low internal stress, high hardness and excellent ductility.When a stress relieving agent is added to the plating solution, the coating obtained will be slightly stressed.There are a variety of sulfamate sulfonate plating solutions in different formulations. Typical sulfamate nickel plating solutions are listed below.Due to the low stress of the coating, it has been widely used, but the stability of nickel aminosulfonate is poor and its cost is relatively high.

3. Modified watt nickel (sulphur nickel) modified watt nickel formula, using nickel sulfate, together with the addition of nickel bromide or nickel chloride.Due to internal stress, nickel bromide is mostly used.It can produce a semi-bright, slightly stressed and malleable coating.And this coating is easily activated for subsequent plating and costs relatively low.

4, the function of each composition were: the main salt ─ ─ sulfamic acid nickel and nickel sulfate as nickel liquid the main salt, nickel salt mainly provide the necessary nickel plated nickel metal ions, and plays a role of conductive salt.The concentration of nickel plating solution varies slightly from supplier to supplier.High nickel salt content, high cathode current density can be used, deposition speed is fast, commonly used for high-speed thick nickel plating.However, too high concentration will reduce cathode polarization, poor dispersion ability, and the loss of the plating solution.The nickel salt content is low, the deposition speed is low, but the dispersing ability is very good, can obtain the crystal fine bright coating.

Buffer ─ ─ boric acid used as a buffer, the PH value of the nickel plating liquid to maintain in a certain range.It is proved that when the PH value of nickel plating solution is too low, the cathode current efficiency will be reduced.And PH value is high, due to the continuous precipitation of H2, make close to the PH of the liquid layer near the surface of the cathode rise rapidly, causing Ni (OH) 2 colloid generated, and Ni (OH) 2 in the coating of the inclusions, make coating brittleness increases, at the same time Ni (OH) 2 gel on the surface of the electrode, adsorption, still can cause hydrogen bubble on the surface of the electrode, retention and increase the coating porosity.Boric acid not only has a PH buffering effect, but also can improve cathode polarization, thus improving the properties of the plating solution and reducing the phenomenon of "burning" under high current density.The existence of boric acid is also beneficial to improve the mechanical properties of the coating.

Anode activator ─ ─ except sulfate type nickel plating liquid insoluble anode, the nickel plating technology adopts the other types of soluble anode.The nickel anode is easily passivated during electrification. In order to ensure the normal dissolution of the anode, a certain amount of anode activator is added to the plating solution.It was found that CI - chloride is the best nickel anode activator.In nickel chloride plating liquid, nickel chloride is not only used as main salt and conductive salt, but also ACTS as an anode activator.

A certain amount of sodium chloride should be added in the electroplating nickel solution without nickel chloride or with a lower content.Nickel bromide or nickel chloride are often used as de-stressing agents to maintain the internal stress of the coating and give the coating a semi-bright appearance.

Additive -- the main component of additive is stress relieving agent. The addition of stress relieving agent improves the cathode polarization of the plating solution and reduces the internal stress of the coating. As the concentration of stress relieving agent changes, the internal stress of the coating can be changed from tensile stress to compressive stress.Commonly used additives are: naphthalene sulfonic acid, toluene sulfonamide, saccharin and so on.Compared with the nickel coating without the stress reducing agent, adding the stress reducing agent in the plating solution will obtain uniform and fine coating with semi-gloss.Usually the destressing agent is added in one hour of ampere (now the general combination of special additives including pinhole prevention agent, etc.).

Wetting agent - during electroplating, it is inevitable that hydrogen is precipitated from the cathode, which not only reduces the current efficiency of the cathode, but also causes pinholes in the coating due to hydrogen bubbles on the electrode surface.Nickel plating layer porosity is relatively high, in order to reduce or prevent the formation of pinhole, shall be submitted to the plating solution to add a small amount of wetting agents, such as sodium dodecyl sulfate, sodium diethyl is based, is sodium octyl sulfate, etc., it is a kind of anionic surface active substance, can adsorption in the cathode surface, reduce the interfacial tension between electrode and solution, hydrogen bubble on the electrode of wetting contact Angle decreases, and thus make bubbles easily leave the electrode surface, and prevent or mitigate the plating pinhole.