1 , the role and characteristics
PCB ( Printed Circuie Board is the English abbreviation PCB ) using nickel as a precious and base metal coating on a substrate , for some single-sided PCB , also used as a surface layer . For some heavy-duty wear surface , such as switch contacts , contacts or plug is gold plated with nickel and gold as a substrate , can greatly improve the wear resistance. When used as a barrier layer of nickel can effectively prevent the diffusion of copper and other metals. Matte nickel / gold plating compositions are often used as an etch-resistant metal coating , and can adapt to the thermocompression bonding and brazing requirements , can be used as read- only nickel- ammonia type etching resist coating agent , without thermocompression also requested shiny coating PCB, usually light nickel / gold plating. Usually the thickness of the nickel plating layer not less than 2.5 micrometers, usually 4-5 micrometers.
PCB low stress nickel deposition layer, usually with a modified type of Watt nickel bath and a nickel sulfamate bath with a number of stress reducing additive to the plating .
We often say that the PCB matte nickel and nickel-plated bright nickel ( also known as low-stress nickel or semi-bright nickel ) , usually require meticulous coating uniformity , low porosity, low stress , and good ductility .
2 , nickel sulfamate ( Ni ammonia )
Amino acids are widely used as the nickel metal plating and printing plug contact hole plating film on the substrate . Internal stress of the deposited layer obtained low, high hardness , and has a very superior scalability. The one kind of agent is added to the stress in the bath , the resulting coating will be just a little stress . There are many different formulations sulfamate bath , a nickel sulfamate plating bath typical recipes in the following table . Since the stress in the coating is low, so widely used, but the poor stability of nickel sulfamate , the cost is relatively high.
3 watt nickel- modified ( S Ni )
W. Nickel modified formula, using nickel sulfate, nickel bromide or together with nickel chloride . Due to the internal stress , the selected mostly nickel bromide . It can produce a semi-bright , just a little stress, ductile coating ; and this coating is easily activated subsequent electroplating , the cost is relatively the end.
4 , the role of each component of the bath :
The nickel salt solution in the main primary salts, nickel salts is to provide the desired nickel and nickel metal ions , and plays the role of the conductive salt ─ ─ nickel sulfamate to nickel sulfate . Concentration of nickel plating with a slightly different different suppliers , nickel salt content changes allowing greater . Nickel salt content, use of higher cathodic current density , deposition rate , thick plating of nickel used at a high speed . But the concentration is too high will reduce the cathodic polarization , dispersion ability, but with the loss of a large bath . Nickel salt content is low low deposition rate , but the dispersion ability is very good, bright coating can be obtained crystalline detail .
─ ─ borate buffer is used as a buffer , so nickel plating solution PH value is maintained within a certain range. Practice has proved that the nickel plating when the PH value is too low , the cathode current efficiency will decrease ; while PH value is too high , due to the continuous precipitation of H2 , so that the liquid near the surface of the cathode layer against the PH value rapidly increases , resulting Ni (OH) 2 colloid generation , and Ni (OH) 2 mixed in the coating , and the coating increases brittleness , while Ni (OH) 2 colloids adsorbed on the electrode surface , the electrode will cause hydrogen bubble retention surface , so increase the coating porosity . Not only boric acid buffer with PH , and he can improve cathode polarization , thereby improving the performance of the bath , to reduce the current density at a high " burning " phenomenon. The presence of boric acid is also beneficial to improve the mechanical properties of the coating .
In addition to the anode activator ─ ─ sulfate type nickel plating solution using insoluble anodes , other types of soluble nickel plating anodes are used . And the nickel anode passivation process can easily energized , in order to ensure the normal dissolution of the anode , the anode certain amount of activator in the bath . Found by experiment , CI- nickel chloride is preferably an anode activator. In the nickel plating solution containing nickel chloride of nickel chloride and the conductive salt in addition to the salt as a main , but also acts as a positive electrode activator. In the nickel chloride -free or low levels of nickel plating solution, to be based on practical conditions to add a certain amount of sodium chloride. Bromide, nickel or nickel chloride is also used to remove stress agent used to keep the stress of the coating , and endowed with the coating has a semi-bright appearance.
Additive - the main ingredient of additives are stress -relieving agent , stress relief agent is added to improve the cathodic polarization of the bath , the coating reduces the internal stress , the stress to suppress the variation with the concentration of stress in the coating can be the inner sheets stress changes to compressive stress . Conventional additives include: naphthalenesulfonic acid , p-toluenesulfonamide , saccharin and the like. Compared with no stress agent to nickel plating bath by adding to the stress agent will get even meticulous and has a semi- glossy coating. To stress agent is usually by one hour to add the ampere (now a combination of special additives including anti- GM pinhole , etc.) .
Wetting agent - in the electroplating process , hydrogen is inevitably deposited on the cathode , the hydrogen reduces the precipitation of only the cathode current efficiency, and because the hydrogen bubbles on the electrode surface retention , the coating will appear pinholes. The porosity of the nickel plating layer is relatively high , in order to reduce or prevent the generation of pinholes , the bath should be added to small amounts of wetting agents such as sodium lauryl sulfate , sodium diethyl -hexyl , n- octyl base such as sodium , which is an anionic surface active substance can be adsorbed on the cathode surface , the interfacial tension between the lower electrode and the solution , the hydrogen gas bubbles in the wetting contact angle of the electrode decreases, so that the bubble easily away from the electrode surface , the coating prevents or reduces the generation of pinholes .
5 , bath maintenance
a) Temperature - type nickel process , the temperature of the bath used is different . Effect of temperature on the nickel plating process is complicated . At higher temperatures the nickel plating solution, the nickel plating to obtain a low stress , scalability, stress coating to achieve a stable temperature of 50 degrees C, plus cause . General operating temperature is maintained at 55 - 60 degrees C. If the temperature is too high , the nickel salt solution will occur , and the resulting colloidal nickel hydroxide colloidal retention of hydrogen bubbles , pinholes causing the coating , while also reducing the cathode polarization . Therefore, the operating temperature is very strict and should be controlled within the specified range , the actual work is based on the value of the optimal temperature control provider , using the temperature controller to maintain the stability of the operating temperature .
b) PH value - practical results show that the performance of the coating and plating electrolyte performance greatly affect the PH value of nickel electrolyte. In PH ≤ 2 strongly acidic plating solution , no deposition of nickel , but the precipitation of light gas. Usually PCB nickel electrolyte is maintained at a value between PH 3-4 . PH value is higher nickel plating solution having a high strength and high dispersion cathodic current efficiency. However, when PH is too high, since the cathode of the electroplating process continuously precipitated light gas , PH value near the surface of the cathode coating increases rapidly when more than 6, the light will generate colloidal nickel oxide , causing retention of hydrogen bubbles , so that coating pinholes . Nickel hydroxide inclusions in the coating , the coating will increase brittleness . PH lower nickel plating solution , preferably anodic dissolution can increase the nickel content of the electrolyte salt , allows the use of higher current density and thus enhance production . PH is too low , however , will allow access to bright coating temperature range narrows . Nickel carbonate or basic nickel carbonate , PH value increases ; adding amino acid or sulfuric acid , reduce the PH value , adjusted once every four hours to check the PH value in the work process .
c) anode - can currently be seen in the conventional PCB soluble anodes are made of nickel , titanium anode baskets as the nickel corner has built quite common . The advantage is that the anode area can not be made sufficiently large change , the anode is relatively simple maintenance . Titanium basket woven polypropylene material should be loaded into the anode bag to prevent falling into the anode mud bath . And should be regularly cleaned and checked the hole is smooth . The new anode bag before use , should be immersed in boiling water.
d) Purification - organic pollution exists when the bath , you should use activated carbon treatment . However, this method is usually to remove a portion of the stress agent ( additive ) must be supplemented . Its treatment process as follows ;
( 1 ) Remove the anode , add water impurity 5ml / l, heated ( 60-80 ° C) pump ( air stirring) for 2 hours.
( 2 ) organic impurities for a long time , to join the 3-5ml/lr of 30% hydrogen peroxide treatment, gas and stirred for 3 hours.
( 3) 3-5g / l powdered active was added under constant stirring and continue stirring for 2 hours the gas , allowed to stand for 4 hours with stirring off , the filter aid added to the powder filter while using the reserve tank clean tank.
( 4 ) cleaning and maintenance anode hanging back , with nickel plated corrugated iron as the cathode , carried out at 0.5-0.1 A / dm current density drag cylinder 8-12 hours (when the presence of inorganic pollution bath when quality is also often used )
( 5 ) replaced the filter ( usually with a cotton core set of continuous carbon filter core series , according to a periodic change will be effective extension of large processing time and improve the stability of the bath ) , adjust the parameters analyzed , wetting additives agent trial plating .
e) Analysis - plating process control points should be provided for process planning , regular analysis of the bath components and Hull cell test , adjust the parameters of the bath parameters guiding light of the productive sector .
f) stirring - nickel plating process and the other process, the purpose is to accelerate the stirring of the mass transfer process , in order to reduce the concentration change and improve the current density limit allowed . Stirring the bath there is a very important role, is to reduce or prevent the nickel plating pinholes . Because the plating process, an ion plating cathode poor near the surface , a large number of hydrogen precipitated PH value rise produce colloidal nickel hydroxide , hydrogen bubbles caused by stagnation of pinholes . Strengthen the plating solution left stirring , the above phenomenon can be eliminated . Common air cathode Mobile and forced circulation ( carbon core combined with cotton core filter ) stirring.
g) a cathode current density - cathodic current density on the cathode current efficiency , the deposition rate and coating quality are affected. The test results show that when compared with the end use PH nickel electrolyte , at low current density area , while the cathode current efficiency increases with increasing current density ; at high current density , regardless of the cathode current efficiency and current density , and when using the higher the nickel plating solution PH little to the cathode current efficiency and current density .
Like other types of plating , nickel -plated cathode current density range selected should be as components of the plating solution , the temperature and stirring conditions, due to the large size PCB puzzle , the current density and the high current region of the low current region vary greatly , the general 2A/dm2 appropriate.
6, the cause of the malfunction and troubleshooting
a) Hang Ma : Ma Hang is the result of organic pollution . Ma Hang usually indicates major oil pollution. Stir bad , you can not expel air bubbles out , which will form Ma Hang . You can use a wetting agent to reduce its impact , we usually called pitting small pinholes , poor pre-processing , metal and even quality , too little boric acid content , bath temperature is too low will produce pinholes, bath maintenance and process control is critical , anti- pinhole agent used as a stabilizer to additional processes .
b) rough burr : rough dirty solution to explain fully the filter can be corrected (PH too easy to form
Hydroxide precipitate should be controlled ) . Current density is too high, the anode mud and additional impurities into the water impure , will have serious rough and glitches.
c) the binding force is low: If the copper oxide coating layer is not sufficiently removed , the coating will peel adhesion phenomenon between the copper and nickel poor. If the current is also interrupted flaking , it will be interrupted , resulting in peeling of the nickel plating itself , the temperature is too severe .
d) the coating brittle, poor weldability : When the coating is bent or subjected to some degree by the wear of the coating will usually reveal brittle. This indicates the presence of organic compounds or heavy metals even mass pollution, excessive additives , entrained organics and plating resist , is the main source of organic pollution , must be treated with activated charcoal , add inadequate economic and PH is too high will affect the coating brittle .
e) coating dark and uneven color : dark coating and color unevenness , it shows there is metal contamination. Because the nickel-plated copper are generally the first , the copper into the solution is the main source of contamination. It is important , we should stick to hang with the copper solution is reduced to a minimum. In order to remove metal contamination tank , especially to copper solution should be used corrugated steel cathode in the 2 to 5 amps / ft of current density , the solution space plating 5 amps per gallon an hour. Adverse pre-treatment , poor low plating current density is too small , the main concentration is too low, the power supply circuit plating bad will affect the coating color.
Coating may cause burns :: f) coating burns acid deficiency , low salt concentration , the temperature is too low , the current density is too high , PH high or inadequate stirring .
g) the deposition rate is low : PH value is low or low current density will cause the deposition rate is low.
h) coating blistering or peeling : bad treatment before plating , the middle off for too long, organic impurity contamination , the current density is too large , the temperature is too low , PH is too high or too low , will have a serious impact blistering time of impurities or peeling phenomenon .
I) anode passivation : anode activator deficiency , small size anode current density is too high.
