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Nickel electroplating process for PCB printed circuit boards

Updating:2019-11-21 17:43:49  Hits:

1. Function and characteristic PCB (printed by PrintedCircuieBoard in English The abbreviation of circuit board) uses nickel plating as the substrate coating of precious metals and base metals, and it is also often used as the surface layer for some single-sided printed boards. For some surfaces that are worn under heavy load, such as switch contacts, contacts or plug gold, using nickel as the gold substrate plating can greatly improve the wear resistance. When used as a barrier layer, nickel can effectively prevent the diffusion between copper and other metals. Dumb nickel/gold composite coating is often used as an anti-etching metal coating, and can meet the requirements of hot-press welding and brazing. Only nickel can be used as an anti-corrosive coating of ammonia-containing etchant, without hot-press welding. For PCBs that require bright plating, light nickel/gold plating is usually used. The thickness of the nickel coating is generally not less than 2.5 microns, usually 4-5 microns. The deposited layer of PCB low-stress nickel is usually plated with a modified Watt nickel plating solution and some sulfamate nickel plating solutions with stress-reducing additives. We often say that PCB nickel plating has bright nickel and dumb nickel (also called low-stress nickel or semi-bright nickel), which usually require uniform and fine plating, low porosity, low stress, and good ductility.

2, Nickel sulfamate (nickel ammonia) Nickel sulfamate is widely used as the substrate plating on metallized hole plating and printed plug contacts. The obtained deposited layer has low internal stress, high hardness, and extremely superior ductility. Add a stress relief agent to the plating solution, and the resulting coating will have a little stress. There are many different formulations of sulfamate baths. The typical formulations of nickel sulfamate baths are as follows. Due to the low stress of the coating, it has been widely used, but the stability of nickel sulfamate is poor and its cost is relatively high. 3. Modified Watt Nickel (Sulfur Nickel) Modified Watt Nickel formula uses nickel sulfate, together with nickel bromide or nickel chloride. Due to internal stress, nickel bromide is mostly used. It can produce a semi-bright, slightly internal stress, good ductility coating; and this coating is easy to activate for subsequent electroplating, and the cost is relatively low. 4. The role of each component of the plating solution: the main salt—nickel sulfamate and nickel sulfate are the main salts in the nickel solution. The nickel salt mainly provides the nickel metal ions required for nickel plating and also acts as a conductive salt. . The concentration of nickel plating solution varies slightly with different suppliers, and the allowable content of nickel salt varies greatly. The content of nickel salt is high, higher cathode current density can be used, and the deposition speed is fast. It is often used for high-speed thick nickel plating. However, if the concentration is too high, the cathodic polarization will be reduced, the dispersion ability will be poor, and the carry-out loss of the plating solution will be large. The nickel salt content is low and the deposition rate is low, but the dispersing ability is very good, and the crystalline fine and bright coating can be obtained. Buffer-Boric acid is used as a buffer to keep the pH of the nickel plating solution within a certain range. Practice has proved that when the pH value of the nickel plating solution is too low, the cathode current efficiency will decrease; and when the pH value is too high, due to the continuous precipitation of H2, the pH value of the liquid layer close to the cathode surface will increase rapidly, resulting in Ni The formation of (OH)2 colloid, and the inclusion of Ni(OH)2 in the coating increases the brittleness of the coating. At the same time, the adsorption of Ni(OH)2 colloid on the electrode surface will also cause the retention of hydrogen bubbles on the electrode surface. The porosity of the coating increases. Boric acid not only has a PH buffering effect, but also can improve the cathode polarization, thereby improving the performance of the plating solution and reducing the "scorching" phenomenon under high current density. The presence of boric acid also helps to improve the mechanical properties of the coating. Anode activator──Except for sulfate-type nickel plating solutions that use insoluble anodes, other types of nickel plating processes all use soluble anodes. The nickel anode is very easy to passivate during the electrification process. In order to ensure the normal dissolution of the anode, a certain amount of anode activator is added to the plating solution. Through experimentation, it is found that CI—chloride ion is the best nickel anode activator. In the nickel plating bath containing nickel chloride, nickel chloride not only serves as the main salt and conductive salt, but also acts as an anode activator. In the electroplating nickel solution that does not contain nickel chloride or its low content, a certain amount of sodium chloride needs to be added according to the actual conditions. Nickel bromide or nickel chloride is also commonly used as a stress relief agent to maintain the internal stress of the coating and give the coating a semi-bright appearance. Additive——The main component of the additive is a stress reliever. The addition of the stress reliever improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. As the concentration of the stress reliever changes, the internal stress of the coating can be increased. Change from tensile stress to compressive stress. Commonly used additives are: naphthalenesulfonic acid, p-toluenesulfonamide, saccharin and so on. Compared with nickel plating without stress relief agent, adding stress relief agent to the plating solution will obtain a uniform, fine and semi-bright plating layer. Usually the stress relief agent is added at an ampere hour (the current general-purpose combination special additives include anti-pinhole agents, etc.). Wetting agent —— During the electroplating process, the precipitation of hydrogen on the cathode is inevitable. The precipitation of hydrogen not only reduces the cathode current efficiency, but also causes pinholes in the plating layer due to the retention of hydrogen bubbles on the electrode surface. The porosity of the nickel plating layer is relatively high. In order to reduce or prevent the generation of pinholes, a small amount of wetting agent, such as sodium lauryl sulfate, sodium diethylhexyl sulfate, and n-octyl sulfate should be added to the plating solution. Sodium sulphate, etc., it is an anionic surface active substance that can be adsorbed on the surface of the cathode, reducing the interfacial tension between the electrode and the solution, and reducing the wetting contact angle of hydrogen bubbles on the electrode, thereby making bubbles easy Leaving the surface of the electrode prevents or reduces the generation of pinholes in the coating. 5. Maintenance of plating solution a) Temperature —— Different nickel processes have different plating solution temperatures. The influence of temperature changes on the nickel plating process is more complicated. In a nickel plating solution with a higher temperature, the obtained nickel plating layer has low internal stress and good ductility. When the temperature is increased to 50 degrees C, the internal stress of the plating layer becomes stable. The general operating temperature is maintained at 55-60 degrees C. If the temperature is too high, the nickel salt solution will be hydrolyzed, and the generated nickel hydroxide colloid will trap the colloidal hydrogen bubbles, causing pinholes in the coating, and at the same time reducing the cathode polarization. Therefore, the operating temperature is very strict and should be controlled within the specified range. In actual work, the room temperature controller is used to maintain the stability of its operating temperature according to the optimal temperature control value provided by the supplier. b) PH value——Practical results show that the pH value of the nickel-plated electrolyte has a great influence on the performance of the coating and the performance of the electrolyte. In the strong acidic electroplating solution of PH≤2, there is no deposition of metallic nickel, but light gas is precipitated. Generally, the pH value of PCB nickel plating electrolyte is maintained between 3 & mdash; 4. The nickel plating solution with higher PH value has higher dispersion power and higher cathode current efficiency. However, when the pH is too high, the cathode will continuously release light gas during the electroplating process, which will increase the pH value of the coating near the cathode surface. Pinholes appear in the plating. The inclusion of nickel hydroxide in the coating will also increase the brittleness of the coating. The nickel plating solution with lower pH has better anode dissolution, which can increase the content of nickel salt in the electrolyte, allowing the use of higher current density, thereby enhancing production. However, if the pH is too low, the temperature range for obtaining bright coatings will be narrowed. Add nickel carbonate or basic nickel carbonate, the PH value will increase; add sulfamic acid or sulfuric acid, the PH value will decrease, check and adjust the pH value every four hours during the working process. c) Anode——The conventional nickel plating on PCBs that can be seen at present all use soluble anodes, and it is quite common to use titanium baskets as anodes to install nickel corners. The advantage is that the anode area can be made large enough without changing, and the anode maintenance is relatively simple. The titanium basket should be put into the anode bag woven of polypropylene material to prevent the anode mud from falling into the plating solution. And should regularly clean and check whether the holes are unblocked. The new anode bag should be soaked in boiling water before use. d) Purification —— When there is organic contamination in the plating solution, it should be treated with activated carbon. However, this method usually removes a part of the stress relief agent (additive), which must be supplemented. The treatment process is as follows: (1) Take out the anode, add 5ml/l of impurity-removing water, and heat (60-80°C) to inflate (agitate) for 2 hours. (2) When there are many organic impurities, first add 3—5ml/lr of 30% hydrogen peroxide for treatment, and stir for 3 hours. (3) Add 3—5g/l powdered activity under constant stirring, continue air stirring for 2 hours, turn off the stirring and let it stand for 4 hours, add filter powder and use the spare tank to filter and clean the tank at the same time. (4) Clean and maintain the anode and hang it back, use a nickel-plated corrugated iron plate as the cathode, and drag the cylinder for 8-12 hours at a current density of 0.5-0.1 A/dm (when there is inorganic contamination in the plating solution) When the quality is affected, it is often used) (5) Change the filter element (usually use a set of cotton cores and a set of carbon cores to filter continuously in series, changing periodically can effectively delay the large processing time and improve the stability of the plating solution), analysis Trial plating can be done by adjusting the parameters and adding the additive wetting agent. e) Analysis —— The plating solution should use the key points of the process regulations stipulated by the process control, regularly analyze the composition of the plating solution and the Hull cell test, and guide the production department to adjust the parameters of the plating solution according to the obtained parameters. f) Stirring —— The nickel plating process is the same as other electroplating processes. The purpose of stirring is to accelerate the mass transfer process to reduce concentration changes and increase the allowable current density upper limit. Stirring the plating solution also has a very important effect, which is to reduce or prevent pinholes in the nickel plating layer. Because, during the electroplating process, the plating ions near the cathode surface are poor, and a large amount of hydrogen is precipitated, which increases the pH value and produces nickel hydroxide colloid, which causes the retention of hydrogen bubbles and produces pinholes. The above phenomenon can be eliminated by strengthening the stirring of the remaining plating solution. Commonly used compressed air, cathode movement and forced circulation (combined carbon core and cotton core filtration) stirring. g) Cathode current density —— Cathode current density has an impact on cathode current efficiency, deposition rate and coating quality. The test results show that when nickel is plated with an electrolyte with a lower pH, in the low current density area, the cathode current efficiency increases with the increase of the current density; in the high current density area, the cathode current efficiency has nothing to do with the current density, and when used At higher PH electroplating bath nickel, the cathode current efficiency has little relationship with the current density. Like other plating species, the cathode current density range selected for nickel plating should also depend on the composition, temperature and stirring conditions of the electroplating solution. Due to the large PCB panel area, the current density of the high current area and the low current area The difference is very large, generally 2A/dm2 is appropriate. 6. Causes and troubleshooting a) Pit: Pit is the result of organic pollution. Large pits usually indicate oil pollution. If the stirring is not good, the air bubbles cannot be expelled, which will form pits. Wetting agents can be used to reduce its impact. We usually call small pits as pinholes. Poor pretreatment, metal, low boric acid content, and too low bath temperature will all cause pinholes. The bath is maintained. And process control is the key, and the anti-pinhole agent should be used as a process stabilizer to supplement it. b) Roughness and burrs: Roughness means that the solution is dirty, which can be corrected by adequate filtration (the pH is too high to easily form hydroxide precipitation and should be controlled). If the current density is too high, the anode mud and the impurity of the supplementary water will bring in impurities, which will cause roughness and burrs in severe cases. c) Low adhesion: If the copper coating is not fully deoxidized, the coating will peel off, and the adhesion between copper and nickel will be poor. If the current is interrupted, it will cause the nickel coating itself to peel off at the interrupted place, and peel off when the temperature is too low. d) The coating is brittle and poor weldability: When the coating is bent or worn to a certain degree, it will usually show that the coating is brittle. This indicates that there is organic or heavy metal pollution. Too much additives, entrained organics and electroplating resists are the main sources of organic pollution, which must be treated with activated carbon. Insufficient additives and high pH will also affect the brittleness of the coating. e) Dark plating and uneven color: Dark plating and uneven color indicate metal contamination. Because copper is generally plated first and then nickel is plated, the copper solution brought in is the main source of pollution. It is important to reduce the copper solution on the hanger to a minimum. In order to remove the metal contamination in the tank, especially the copper removal solution should use a corrugated steel cathode. At a current density of 2 to 5 amperes/square foot, 5 amperes per gallon of solution is plated for one hour. Poor pretreatment, poor low plating, too small current density, too low main salt concentration, and poor contact of the electroplating power circuit will all affect the color of the plating. f) Coating burns: possible causes of coating burns: insufficient boric acid, low metal salt concentration, too low working temperature, too high current density, too high pH or insufficient stirring. g) Low deposition rate: Low pH or low current density will cause low deposition rate. h) Blistering or peeling of the coating: poor pre-treatment, excessively long intermediate power-off time, organic impurities pollution, excessive current density, too low temperature, too high or too low pH, and serious influence of impurities will cause blistering Or peeling phenomenon. I) Anode passivation: the anode activator is insufficient, the anode area is too small and the current density is too high.

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