PCB Tech - Troubleshooting of electroless nickel plating process for Multilayer PCB

1. Function and characteristics of nickel plating on Multilayer PCB

Nickel plating is used as the substrate coating of precious and base metals on multi-layer PCB. It is also commonly used as the surface layer for some single-sided PCB. For some surfaces worn under heavy load, such as switch contacts For contact piece or plug gold, nickel is used as the substrate coating of gold, which can greatly improve the wear resistance. When used as a barrier, nickel can effectively prevent 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-corrosion coating containing ammonia etchant without hot press welding. For Multilayer PCB with bright coating, light nickel / gold coating is usually used. The thickness of nickel coating is generally not less than 2.5 microns, usually 4-5 microns.

The low stress nickel deposition layer of Multilayer PCB is usually plated with modified watt nickel plating solution and some sulfamic acid nickel plating solutions with 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, the coating is required to be uniform and fine, low porosity, low stress and good ductility

2. Nickel sulfamate (ammonia nickel)

Nickel sulfamate is widely used as substrate coating on metallized hole electroplating and printed plug contact. The internal stress of the deposited layer is low High hardness and excellent ductility. When a stress relieving agent is added to the bath, the resulting coating will be slightly stressed. There are many different formulations of sulfamate plating solution, and the typical formulation of sulfamate nickel plating solution is shown in the table below. 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)

The modified watt nickel formula adopts 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 A little internal stress Coating with good ductility; Moreover, this coating is easy to activate for subsequent electroplating, and the cost is relatively low.

4. Function of each component of plating solution:

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 plays the role of conductive salt. The concentration of nickel plating solution varies slightly with different suppliers, and the allowable content of nickel salt varies greatly. High nickel salt content, high cathode current density and fast deposition speed can be used. It is commonly 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 bring out loss of the plating solution will be large. The nickel salt content is low, the deposition speed is low, but the dispersion ability is very good, and the fine and bright crystalline coating can be obtained.

Buffer - boric acid is used as buffer to maintain 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 cathode 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 close to the cathode surface increases rapidly, resulting in the formation of Ni (OH) 2 colloid. 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 and increase the porosity of the coating. Boric acid not only has pH buffer effect, but also can improve cathodic polarization, so as to improve bath performance and reduce "coking" phenomenon under high current density. The existence of boric acid is also conducive to improve the mechanical properties of the coating.

Anode activator - except that insoluble anode is used in sulfate nickel plating solution, soluble anode is used in other types of nickel plating processes. The nickel anode is easy to passivate during power on. In order to ensure the normal dissolution of the anode, a certain amount of anode activator is added to the plating solution. It is found that CI - chloride ion is the best nickel anode activator. In the nickel plating solution containing nickel chloride, nickel chloride not only acts as the main salt and conductive salt, but also acts as an 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 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 the stress relief agent. The addition of the stress relief agent improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. With the change of the concentration of the stress relief agent, the internal stress of the coating can be changed from tensile stress to compressive stress. Common additives are naphthalene sulfonic acid P-toluenesulfonamide Saccharin, etc. Compared with nickel coating without stress relief agent, adding stress relief agent into the plating solution will obtain uniform, fine and semi bright coating. Usually, the stress relief agent is added according to ampere for one hour (now the general combination special additives include pinhole inhibitor, etc.).

Wetting agent - in 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 coating due to the retention of hydrogen bubbles on the electrode surface. The porosity of nickel plating layer is relatively high. In order 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 Sodium octyl sulfate is an anionic surfactant, which can be adsorbed on the cathode surface, reduce the interfacial tension between the electrode and the solution, and reduce the wetting contact angle of hydrogen bubbles on the electrode, so that the bubbles are easy to leave the electrode surface and prevent or reduce the generation of coating pinholes.

5. Maintenance of plating solution

a. Temperature - different nickel processes use different bath temperatures. The influence of temperature change on nickel plating process is complex. In the higher temperature nickel plating solution, the obtained nickel coating has low internal stress and good ductility. When the temperature is increased to 50 ° C, the internal stress of the coating reaches stability. Generally, the operating temperature is maintained at 55-60 ° C. If the temperature is too high, the hydrolysis of nickel salt will occur, and the generated nickel hydroxide colloid will retain the colloidal hydrogen bubbles, resulting in pinholes in the coating, and reduce the cathodic polarization. Therefore, the working temperature is very strict and should be controlled within the specified range. In actual work, the normal temperature controller is used to maintain the stability of its working temperature according to the optimal 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 properties of coating and electrolyte. In the strong acid electroplating solution with pH ≤ 2, there is no deposition of metal nickel, but light gas is precipitated. Generally, the pH value of nickel plating electrolyte for Multilayer PCB is maintained between 3-4. The nickel plating solution with higher pH value has higher dispersion force and higher cathode current efficiency. However, when the pH is too high, due to the continuous precipitation of light gas from the cathode during the electroplating process, the pH value of the coating near the cathode surface increases rapidly. When it is greater than 6, light nickel oxide colloid will be generated, resulting in the retention of hydrogen bubbles and pinholes in the coating. 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 anodic dissolution, which can improve the content of nickel salt in the electrolyte and allow the use of higher current density, so as to strengthen production. However, if the pH is too low, the temperature range for obtaining bright coating will be narrowed. Adding nickel carbonate or basic nickel carbonate increases the pH value; Add sulfamic acid or sulfuric acid to reduce the pH value. Check and adjust the pH value every four hours during operation.

c. Anode - at present, the conventional nickel plating of Multilayer PCB adopts soluble anode, and it is quite common to use titanium basket as anode to install nickel angle. The utility model has the advantages that the anode area can be made large enough without change, and the anode maintenance is relatively simple. The titanium basket shall be put into the anode bag made of polypropylene material to prevent the anode mud from falling into the plating solution. And regularly clean and check whether the eyelet is unblocked. New anode bags should be soaked in boiling water before use.

d. Purification - when the plating solution is polluted by organic matter, it should be treated with activated carbon. However, this method usually removes some stress relief agents (additives) and must be supplemented. The treatment process is as follows;

(1. Take out the anode, add impurity removal water 5ml / L, heat (60-80 ° C), air pumping (air stirring) for 2 hours.

(2) when there are many organic impurities, add 3-5ml / LR of 30% hydrogen peroxide for treatment, and stir with air for 3 hours.

(3) add 3-5g / L powdered activity under continuous stirring, continue air stirring for 2 hours, turn off stirring and stand for 4 hours, add filter aid powder, use standby tank to filter and clean the cylinder at the same time.

(4. Clean and maintain the anode, use the corrugated iron plate plated with nickel as the cathode, and drag the cylinder for 8-12 hours under the current density of 0.5-0.1 A / square decimeter (it is also often used when inorganic pollution exists in the plating solution and affects the quality)

(5) change the filter element (generally, a group of cotton cores and a group of carbon cores are used for continuous filtration in series. It can be changed periodically, which can effectively delay the large treatment time and improve the stability of the plating solution. Analyze and adjust various parameters. Add additive wetting agent to test plating.

e) Analysis - the plating solution should use the key points of the process specification specified in the process control to regularly analyze the composition of the plating solution and Hull cell test, and guide the production department to adjust the parameters of the plating solution according to the obtained parameters.

f) Stirring - like other electroplating processes, the purpose of stirring is to accelerate the mass transfer process, reduce the concentration change 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 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, resulting in the retention of hydrogen bubbles and 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 with carbon core and cotton core filtration and stirring).

g) Cathode current density - the effect of cathode current density on cathode current efficiency The deposition rate and coating quality are affected. The results show that the cathode current efficiency increases with the increase of current density in the low current density region when nickel plating is carried out in the electrolyte with low pH; In the high current density region, the cathode current efficiency has nothing to do with the current density, but when using high pH nickel electroplating solution, the cathode current efficiency has little to do with the current density.

Like other kinds of plating, the range of cathode current density selected for nickel plating should also depend on the composition of the plating solution Depending on the temperature and mixing conditions, due to the large area of multi-layer PCB, the current density in high current area and low current area is very different, and 2A / DM2 is generally suitable.

6. Fault causes and troubleshooting

a) Hemp pit: Hemp pit is the result of organic pollution. Large pits usually indicate oil contamination. If the mixing is not good, the bubbles cannot be expelled, which will form hemp pits. Wetting agent can be used to reduce its effect. We usually call small pits pinholes, and the pretreatment is poor What kind of metal do you have Boric acid content is too low Pinholes will be produced when the bath temperature is too low. Bath maintenance and process control are the key. Pinhole inhibitor shall be used as process stabilizer.

b) Rough Burr: roughness means that the solution is dirty, which can be corrected by full filtration (if the pH is too high, hydroxide precipitation is easy to form, which should be controlled. If the current density is too high, impurities will be brought in by impure anode mud and make-up water, which will produce roughness and burr in serious cases.

c) Low adhesion: if the copper coating is not fully oxidized, the coating will peel off, and the adhesion between copper and nickel is poor. If the current is interrupted, it will cause the self peeling of nickel coating at the interruption, and peeling will also occur when the temperature is too low.

d) The coating is brittle Poor weldability: when the coating is bent or worn to some extent, the coating brittleness is usually exposed. This indicates that there is organic matter or heavy metal pollution and too many additives The entrained organic matter and electroplating resist are the main sources of organic matter pollution, which must be treated with activated carbon. Insufficient addition and high pH will also affect the brittleness of the coating.

e) Coating darkening and uneven color: coating darkening and uneven color indicate metal pollution. Because copper plating is generally followed by nickel plating, the copper solution brought in is the main pollution source. It is important to minimize the copper solution on the hanger. In order to remove the metal pollution in the tank, especially the copper removal solution, the corrugated steel cathode should be used. At a current density of 2 ~ 5 A / square foot, empty plating of 5 amps per gallon of solution for one hour. Poor pretreatment Poor low plating The current density is too low The main salt concentration is too low Poor contact of electroplating power circuit will affect the coating color.

f) Coating burn: possible causes of coating burn: insufficient boric acid and low concentration of metal salt The operating temperature is too low The current density is too high PH is too high or mixing is insufficient.

g) Low deposition rate: low pH value or low current density will cause low deposition rate.

h) Blistering or peeling of coating: poor treatment before plating The intermediate power off time is too long Contamination by organic impurities Excessive current density The temperature is too low PH is too high or too low Blistering or peeling will occur when the influence of impurities is serious.

1. Anode passivation: the anode activator is insufficient, the anode area is too small, and the current density is too high.