Troubleshooting of electroless nickel plating process for multilayer printed boards

Troubleshooting of electroless nickel plating process for multilayer printed boards
1. Functions and characteristics of nickel plating on the surface Multilayer PCB
Nickel plating is used as the base coating of precious and base metals on multilayer PCB. It is also commonly used as the surface layer of some single-sided printed circuit boards. For some surfaces worn under heavy load, such as switch contacts or plug gold of contacts, nickel is used as the base coating of gold, which can greatly improve the wear resistance. When used as a barrier, nickel can effectively prevent the diffusion between copper and other metals. The dumb nickel/gold composite coating is usually used as a corrosion resistant metal coating, which can meet the requirements of hot pressing welding and soldering. Only nickel can be used as an anti-corrosion coating containing ammonia etchant without hot pressing welding. For multilayer PCB with bright coating, light nickel/gold coating is usually used. The thickness of the nickel coating is usually not less than 2.5 microns, usually 4-5 microns.
Low stress nickel deposits on multilayer printed circuit boards are usually plated with modified Watt nickel plating solution and some sulfamic acid nickel plating solutions containing stress reducing additives.
We often say that nickel plating on multilayer PCB has the characteristics of smooth nickel and dumb nickel (also known as low stress nickel or semi bright nickel). Generally speaking, the coating shall be uniform, fine, low porosity, low stress and good ductility
2. Nickel sulfamate
Nickel sulfamate is widely used as the base coating for metallized hole electroplating and printing plug contacts. The deposition layer has low internal stress, high hardness and good ductility. When a stress relieving agent is added to the bath, the resulting coating will be subject to slight stress. There are many different formulations of sulfamate nickel plating solutions. Typical formulations of sulfamate nickel plating solutions are shown in the table below. Due to the low stress of the coating, it has been widely used, but nickel sulfamate has poor stability and relatively high cost.
3. Modified Watt Nickel
The modified Watt nickel formula uses nickel sulfate and adds nickel bromide or nickel chloride. Due to internal stress, nickel bromide is widely used. It can produce a semi bright coating with a small amount of internal stress and has good ductility; In addition, the coating is easy to be activated for subsequent electroplating, and the cost is relatively low.
Printed circuit board

4. Functions of each component of plating solution:
The main salts nickel sulfamate and nickel sulfate are the main salts in nickel solution. Nickel salt mainly provides nickel metal ions required for nickel plating and also acts as conductive salt. The nickel plating solution concentration of different suppliers is slightly different, and the allowable content of nickel salt varies greatly. High nickel salt content, high cathodic current density and fast deposition rate can be used. It is usually 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 bath will be large. The nickel salt content is low, the deposition rate is low, but the dispersion ability is good, and fine and bright crystal coatings can be obtained.
Buffer Solution - Use boric acid as the buffer solution to keep the pH value of nickel plating solution within a certain range. Practice has proved that when the pH value of nickel plating solution is too low, the cathodic current efficiency will be reduced; When the pH value is too high, due to the continuous precipitation of H2, the pH value of the liquid layer near the cathode surface increases rapidly, resulting in the formation of Ni (OH) 2 colloid. Adding Ni (OH) 2 to the coating will increase the brittleness of the coating. At the same time, the adsorption of Ni (OH) 2 colloid on the electrode surface will also cause hydrogen bubbles to remain on the electrode surface and increase the porosity of the coating. Boric acid not only has pH buffer effect, but also can improve cathodic polarization, thus improving bath efficiency and reducing "coking" phenomenon under high current density. The presence of boric acid is also conducive to improving the mechanical properties of the coating.
Anode Activator - Soluble anodes are also used in other types of nickel plating processes, except for insoluble anodes used in sulfate nickel plating solutions. Nickel anodes are easily passivated during energization. In order to ensure the normal dissolution of the anode, a certain amount of anode activator is added to the plating solution. The results show that chloride ion is the best nickel anode activator. In nickel plating solution containing nickel chloride, nickel chloride is not only the main salt and conductive salt, but also the anode activator. In the nickel plating solution without nickel chloride or with low content, a certain amount of sodium chloride shall be added according to the actual situation. Nickel bromide or nickel chloride is also often used as a stress relieving agent to maintain the internal stress of the coating and make the coating appear semi bright.
Additives - The main components of additives are stress relievers. The addition of stress relief agent improves the cathodic polarization of the bath and reduces the internal stress of the coating. With the change of the concentration of stress relieving agent, the internal stress of the coating can change from tensile stress to compressive stress. Common additives include naphthalene sulfonic acid p-toluenesulfonamide saccharin, etc. Compared with the nickel coating without stress relief agent, the homogeneous, fine and semi bright coating can be obtained by adding stress relief agent in the plating solution. Generally, the stress relieving agent is added for one hour according to the amperage (currently, special additives include pinhole inhibitor, etc.).
Wetting Agent - During electroplating, hydrogen is inevitably deposited on the cathode. Hydrogen precipitation not only reduces the cathodic current efficiency, but also causes pinholes in the coating due to hydrogen bubbles remaining on the electrode surface. The porosity of nickel plating layer is relatively high. To reduce or prevent pinholes, a small amount of wetting agent, such as sodium dodecyl sulfate, should be added to the plating solution. Sodium diethylhexyl sulfate octyl sulfate is an anionic surfactant, which can be adsorbed on the cathode surface, reduce the interface tension between the electrode and the solution, and reduce the wetting contact angle of hydrogen bubbles on the electrode, so that the bubbles can easily leave the electrode surface, and prevent or reduce the generation of coating pinholes.
5. Bath maintenance
a. Temperature - Different nickel processes use different bath temperatures. The influence of temperature change on nickel plating process is complex. In high temperature nickel plating solution, the nickel coating obtained has low internal stress and good ductility. When the temperature rises to 50 ° C, the internal stress of the coating becomes stable. Generally, the operating temperature is maintained at 55-60 ° C. If the temperature is too high, the nickel salt will be hydrolyzed, and the generated nickel hydroxide colloid will retain colloidal hydrogen bubbles, resulting in pinholes in the coating and reducing cathodic polarization. In this case, the working temperature is very strict and should be controlled within the specified range. In practice, the normal temperature controller is used to maintain the stability of its operating temperature according to the best temperature control value provided by the supplier.
B. PH value - The practice results show that the PH value of nickel plating electrolyte has a great influence on the performance of the coating and electrolyte. In the strong acidic electroplating solution with pH value of 2, there is no nickel deposition, but light gas will be precipitated. Generally, the pH value of nickel plating electrolyte of multilayer PCB is kept between 3-4. The higher the pH value is, the greater the dispersion force of nickel plating solution is, and the higher the cathode current efficiency is. However, when the pH value is too high, the pH value of the coating near the cathode surface increases rapidly due to the continuous release of light gas from the cathode during the electroplating process. When it is greater than 6, a light nickel oxide colloid will be formed, thus retaining hydrogen bubbles and pinholes in the coating. Nickel hydroxide in the coating will also increase the brittleness of the coating. The nickel plating solution with low pH value has good anodic solubility, which can increase the nickel salt content in the electrolyte and allow higher current density to be used, thus strengthening production. However, if the pH value is too low, the temperature range for obtaining a bright coating will narrow. Adding nickel carbonate or basic nickel carbonate will increase the pH value; Add sulfamic acid or sulfuric acid to reduce the pH value. During operation, check and adjust the pH value every four hours.
c. Anode - At present, soluble anode is used for traditional nickel plating of multilayer PCB, and it is very common to use titanium basket as anode to install nickel corner. The utility model has the advantages that the anode area can become large enough without changing, and the anode maintenance is relatively simple. The titanium basket shall be put into the anode bag made of polypropylene materials to prevent the anode mud from falling into the plating solution. Regularly clean and check whether the hole is unblocked. The new anode bag shall be soaked in boiling water before use.
d. Purification - When the electroplating solution is polluted by organic substances, it should be treated with activated carbon. However, this method usually removes some stress relievers (additives) and must be supplemented. The processing is as follows:;
(1) Take out the anode, add 5ml/L of impurity removal water, heat (60-80 ℃), and extract air (air stirring) for 2h.
(2) When there are many organic impurities, 3-5 ml/LR 30% hydrogen peroxide is added for treatment and stirred with air for 3 hours.
(3) Add 3-5 g/L powdered active agent under continuous mixing, continue to stir with air for 2 hours, close the mixing and stand for 4 hours, add filter aid powder, and filter and clean the gas cylinder with standby tank at the same time.
(4) Clean and maintain the anode, use nickel plated corrugated iron plate as the cathode, and drag the cylinder for 8-12 hours under the current density of 0.5-0.1 A/dm2 (it is also often used when inorganic pollution exists in the plating solution and affects the quality)
(5) Replace the filter element (usually, a group of cotton cores and a group of carbon cores are used in series for continuous filtration. They can be replaced regularly, which can effectively delay a large processing time and improve the stability of the plating solution. Analyze and adjust various parameters. Add additive wetting agent to test the coating.
e) Analysis - The plating solution shall use the key points of the process specification specified in the process control, regularly analyze the bath composition and Hull cell test, and guide the production department to adjust the bath parameters according to the obtained parameters.
f) Stirring - As with other electroplating processes, the purpose of stirring is to accelerate the mass transfer process, reduce concentration changes, and increase the upper limit of allowable current density. Stirring the plating solution also plays a very important role in reducing or preventing pinholes in the nickel plating layer. In the electroplating process, the electroplating ions close to the cathode surface are poor, and a large amount of hydrogen is precipitated, which will increase the pH value and produce nickel hydroxide colloid, leading to the retention of hydrogen bubbles and pinholes. The above phenomena can be eliminated by strengthening the stirring of the remaining plating solution. Commonly used compressed air cathode movement and forced loop (combined with carbon core and cotton core filtration and stirring).
g) Cathodic current density the effect of cathodic current density on cathodic current efficiency deposition rate and coating quality are affected. The results show that the cathodic current efficiency increases with the increase of current density in the low current density region when nickel is plated in low pH electrolyte; In the area of high current density, the cathodic current efficiency is independent of current density, but it has little relationship with current density when nickel plating solution with high pH value is used.
As with other types of electroplating, the range of cathodic current density selected for nickel plating should also depend on the composition of the bath, temperature and mixing conditions. Due to the large area of multilayer PCB, the current density of high current area and low current area is very different, which is generally applicable to 2A/DM2.
6. Fault causes and troubleshooting
a) Ma Hang: Ma Hang is the result of organic pollution. Large pits usually indicate oil contamination. If the mixture is not good, bubbles cannot be discharged, thus forming pits. Wetting agents can be used to reduce their effects. We usually call it pinholes, and the metal with poor pretreatment is the one with too low boric acid content, which will produce pinholes when the bath temperature is too low. Bath maintenance and process control are key. Pinhole inhibitors should be used as process stabilizers.
b) Roughness and burr: Roughness means that the solution is very dirty, which can be corrected by full filtration (if the pH value is too high, it is easy to form hydroxide precipitation, which should be controlled. If the current density is too high, impure anode mud and make-up water will bring impurities, and in serious cases, roughness and burr will occur.
c) Low adhesion: If the copper coating is not completely oxidized, the coating will peel off, and the adhesion between copper and nickel is poor. If the current is interrupted, the nickel coating will self peel when interrupted, and peel when the temperature is too low.
d) Poor weldability of coating brittleness: when the coating is bent or worn to a certain extent, the coating brittleness will usually be exposed. This indicates the presence of organic or heavy metal pollution and excessive additives. The entrained organics and electroplating corrosion inhibitors are the main sources of organic pollution and must be treated with activated carbon. Insufficient addition and high pH value will also affect the brittleness of the coating.

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