1, function and characteristics
Nickel plated on PCB is used as the base coating for precious metals and base metals, and for certain single-sided printed boards, they are often used as surface coatings. For heavy duty wear surfaces such as switch contacts, contacts or plug gold, nickel is used as the base coating of the gold to improve wear resistance. When used as a barrier, nickel can effectively prevent the diffusion of copper and other metals. Dumb nickel / gold composite coating is often used as an etching resistant metallic coating, and can adapt to the hot pressure welding and braze welding, only read only nickel can be used as anti-corrosive coating of ammonia etchant, without hot welding and bright coating PCB, commonly used light nickel / gold plating. Nickel coating thickness is generally not less than 2.5 microns, usually 4-5 microns.
PCB deposited layer of low stress nickel is usually plated with modified nickel metal nickel plating solution and some amino sulfonic acid nickel plating solution with additive of reducing stress.
We often say that PCB nickel plated, bright nickel and dumb nickel (also called low stress nickel or semi bright nickel) usually require uniform and fine coating, low porosity, low stress and good ductility.

2, nickel - amino - sulfonic acid (ammonia, nickel)
Nickel tin sulfonate FA is used as a metallized hole for electroplating and printing the substrate coating on plug contacts. The obtained deposits have low internal stress, high hardness and excellent ductility. Adding a de - stress agent into the plating solution results in a little stress on the resulting coating. There are a variety of different formulations of amino sulfonate plating solution, typical nickel sulfonic acid nickel plating solution is formulated as follows. Because of the low stress of the coating, it has been widely used, but the stability of nickel sulfonic acid is poor and its cost is relatively high.

3, modified watt nickel (sulfur nickel)
Modified watt nickel formulations were prepared using nickel sulfate, together with the addition of nickel bromide or nickel chloride. Because of the internal stress, most of them are made of nickel bromide. It can produce 1.5 bright, slightly a little internal stress, good ductility of the coating, and this kind of coating for subsequent electroplating is easy to activate, cost relative to the bottom.

4, the effect of each component of plating bath:
The main salt nickel and nickel sulfate are the main salts of the nickel solution. The nickel salt mainly provides the nickel metal ion needed for nickel plating and acts as the conductive salt simultaneously. The concentration of nickel plating solution varies slightly with the supplier, and the allowable content of nickel salt varies greatly. High nickel content, high cathode current density, fast deposition rate, and high speed nickel plating. However, the concentration is too high, it will reduce the cathodic polarization, poor dispersion ability, and the loss of the plating liquid is large. Nickel salt is low in content and low in deposition speed, but it has good dispersing ability and can obtain fine crystalline bright coating.
Buffer boric acid is used as buffer to keep the pH value of nickel plating liquid within a certain range. Practice has proved that when the bath pH value is too low, the cathode current efficiency decreased; while the pH value is too high, due to the precipitation of H2, near the close to the cathode surface layer of the liquid pH value increased rapidly, leading to Ni (OH) 2 generation of colloids, and Ni (OH) 2 in the coating with the coating. Increased brittleness (OH) and Ni 2 colloid adsorption on the electrode surface, but also cause hydrogen bubbles on the electrode surface retention, the coating porosity increased. Boric acid not only has PH buffering effect, but also improves cathode polarization, thus improving bath performance and reducing "burning" phenomenon at high current density. The presence of boric acid is also beneficial to improve the mechanical properties of the coating.
The anode activator, in addition to the insoluble anode used in the sulfate type nickel plating solution, soluble anode is adopted in other types of nickel plating process. The nickel anode is easy to be passivated in the process of electrifying. In order to ensure the normal dissolution of the anode, a certain amount of anodic activator is added into the bath. It is found that CI - CL is a good activator for nickel anode. In the nickel plating solution containing nickel chloride, nickel chloride acts as an anode activator in addition to being the main salt and the conductive salt. A certain amount of sodium chloride should be added according to the actual conditions in the electroplating nickel solution without nickel chloride or its lower content. Nickel bromide or nickel chloride is also often used to stress agent used to keep the internal stress of coating, and gives a coating with semi bright appearance.
Main ingredients: additive additive is stress relieving agent, stress relieving agent, the cathodic polarization of the plating solution is improved, reducing internal stress of the coating with stress relieving agent concentration changes, the internal stress of coatings by tensile stress changed into compressive stress. Commonly used additives are naphthalene sulfonic acid, p-toluene sulfonamide, saccharin and so on. Compared with the nickel coating without the stress relief agent, the addition of the stress relief agent in the plating solution will result in even, fine and semi bright coating. Normally, the stress removing agent is added to the ampere for an hour (now used as a special additive, including anti pinhole agents).
Wetting agent -- in the electroplating process, the cathode hydrogen hydrogen evolution is inevitable, not only reduces the cathodic current efficiency, but also because of hydrogen bubbles on the electrode surface on the retention, will also make the coating appear pinhole. The porosity of the nickel layer is relatively high, in order to reduce or prevent the generation of pinhole, shall apply to the bath by adding a small amount of wetting agent, such as sodium dodecyl sulfate, twelve two ethylhexyl octyl sodium sulfate, sodium sulfate, it is a kind of anionic surfactant, can be adsorbed on the surface of the cathode. The interfacial tension between solution and electrode decreased, wetting contact hydrogen bubbles on the electrode angle decreased, so that the bubble easy to leave the electrode surface, prevent or reduce the generation of pinhole coating.

5, maintenance of bath
A) the temperature is different from the nickel process, and the plating bath temperature is also different. The influence of temperature on the process of nickel plating is quite complex. In the nickel bath with higher temperature, the internal stress of nickel coating is low and the ductility is good. When the temperature is added to 50 degree C, the internal stress of the coating is stable. The general operating temperature is maintained at 55--60 degrees C. If the temperature is too high, there will be nickel salt hydrolysis, nickel hydroxide colloid colloid to generate hydrogen bubbles caused by coating pinholes, retention, but also reduce the cathode polarization. So the temperature is very strict, should be controlled within the specified range, in the actual work is quite good according to the value of supplier's control, stability by keeping the working temperature of the normal temperature controller.
B) pH value - the result of practice shows that the pH value of nickel plating electrolyte has great influence on the coating performance and electrolyte performance. In the PH below 2 strongly acidic plating solution, no deposition of nickel, but precipitation of light gas. Generally, the pH value of PCB nickel plating electrolyte is maintained between 3 - 4. The solution with higher pH value has higher dispersing power and higher cathode current efficiency. But PH is too high, because the cathode in the electroplating process of continuous precipitation of light gas to the vicinity of the cathode surface coating pH increased rapidly, when more than 6, there will be light nickel oxide colloid formation caused by hydrogen bubble retention, coating pinholes. With nickel hydroxide in the coating, the coating will increase the brittleness. PH lower nickel plating solution, anode dissolution is better, can improve the content of nickel salt in electrolyte, allowing the use of higher current density, thereby strengthening production. However, PH is too low and will narrow the temperature range of the bright coating. Add nickel carbonate or basic nickel carbonate, pH value increases; add amino sulfonic acid or sulfuric acid, pH value is reduced, every four hours in the work process check, adjust the pH value.
C) anode - the conventional PCB nickel plating can be used as a soluble anode. The titanium basket is used as the anode, and the nickel angle is quite common. The utility model has the advantages that the anode area can be large enough and does not change, and the anode maintenance is relatively simple. The titanium basket shall be packed in an anode bag made of polypropylene material to prevent the anode sludge from falling into the plating bath. And shall regularly clean and check whether the hole is unblocked. The new anode bag should be soaked in boiling water before use.
D) purification - when the plating solution is contaminated by organic matter, it should be treated with activated carbon. But this method usually removes part of the stress - removing agent (additive) and has to be supplemented. The treatment process is as follows;
(1) take out the anode, add water 5ml/l, heat (60 - 80 degrees C), air (air agitation) 2 hours.
(2) organic impurities for a long time, first add 3 - 5ml/l 30% hydrogen peroxide treatment, gas mixing for 3 hours.
(3) add the powder of 3 - 5g/l powder under constant stirring, continue stirring for 2 hours, turn off the mixture for 4 hours, add the filter powder, and use the spare tank to filter and clean the cylinder simultaneously.
(4) cleaning and maintenance of anode hanging back, with corrugated iron plating nickel as cathode, in 0.5 - 0.1 square decimeter ANN / current density under the drag cylinder 8 - 12 hours (when electroplating has effect of inorganic pollution, is often used)
(5) changes the filter (usually by a group of a group of cotton core carbon core series of continuous filtration, with periodic change at Li can effectively delay time, improve the stability of the bath), adjust the parameters, analysis of wetting agent can try plating additives.
E) analysis should be done with the key points of the technological process stipulated in the process control. The bath composition and the Hull trough test shall be analyzed periodically, and the parameters of the plating bath shall be adjusted by the production department in accordance with the parameters obtained.
F) mixing nickel plating process, like other plating processes, is designed to accelerate mass transfer processes to minimize concentration changes and to increase the allowable current density limit. A very important role in mixing the bath is to reduce or prevent pinholes. Because, in the electroplating process near the cathode surface plating ion is poor, precipitate a lot of hydrogen, the pH rise and produce nickel hydroxide colloid retention caused by hydrogen bubbles and pinholes. The phenomenon can be eliminated by strengthening the stirring of the bath. Common use of compressed air, cathode movement, and forced circulation (combined with carbon core and cotton core filtration) agitation.
(g) the cathode current density, the cathode current density, affects the cathodic current efficiency, deposition rate and the quality of the coating. The test results show that when using PH lower nickel electrolyte, at low current density region, the cathode current efficiency increases with the increase of current density; in the high current density region, cathodic current efficiency and current density independent, while PH with high nickel plating solution, is not the relationship between the cathodic current efficiency and current density. The.
Like other kinds of plating, cathodic current density range of the nickel plating solution should also be considered component, temperature and stirring conditions, due to the PCB plate area is large, the current density of high current region and low current region difference, generally used should be 2A/dm2.

6, causes and troubleshooting
A) MA Keng: the pit is the result of organic pollution. Large pits usually indicate oil contamination. If the agitation is bad, the bubbles cannot be expelled, which will form a crater. You can use a wetting agent to reduce the influence of it, we usually think of small pits called pinhole, pretreatment, bad metal matter, what the boric acid content is too little, the bath temperature is too low will produce pinholes, bath maintenance and process control is the key technology for application of anti pinhole agent stabilizer added.
B) roughness and burr: roughness means that the solution is dirty and can be corrected by adequate filtration (PH is too high and easy to form
Hydroxide precipitation should be controlled. When the current density is too high, the anode mud and the filling water are impure, the impurities will be rough and burrs will be produced in severe cases.
C) low binding force: if the copper coating is not adequately oxidized, the coating will peel off and the adhesion between copper and nickel will be poor. If the current is interrupted, it will cause self flaking of the nickel coating at the point where the temperature is too low and spalling will occur in severe cases.
D) the coating is brittle and weldability is poor. When the coating is bent or subjected to some degree of wear, the coating is usually brittle. This indicates the presence of organic compounds or heavy metals and organic matter what pollution, excessive additives, electroplating entrainment resist, is the main source of organic pollution, must be treated with activated carbon, add economic problems and high PH will also affect the brittleness of the coating.
E) the coating is dark and uneven in color. The coating is dark and uneven in color, indicating metal contamination. Because copper is usually plated after nickel plating, so the copper solution is the main source of pollution. It is important to reduce the copper solution rack by James to a lower level. In order to remove the metal contamination in the tank, especially the copper removal solution, a corrugated steel cathode shall be used, with a 2~5 ampere / square foot current density of 5 amperes per gallon. Poor pre treatment, low coating, low current density, too low primary salt concentration, bad contact of plating power circuit will affect the coating color.
F) the reason why the coating is burnt is the lack of boric acid, the low concentration of metal salts, the low working temperature, the high current density, the high PH or the insufficient stirring.
G) low deposition rate: low pH or low current density, resulting in low deposition rates.
H) blisters on the coating or peeling serious adverse effects, the middle power: a long time, organic contamination, the current density is too large, the temperature is too low, the PH is too high or too low, impurity treatment before plating will be generated when the bubbling or peeling phenomenon.
I) anode passivation: insufficient anode activator, too small anode area, too high current density.