Surface Finishing - The Hong Kong Polytechnic University

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Surface Finishing - The Hong Kong Polytechnic University

IC LEARNING SERIESSurface Finishing


The Hong Kong Polytechnic UniversityIndustrial CentreIC LEARNING SERIESSurface FinishingSuitable for the following learning modules/subjects offered by the Industrial Centre:TM0206 PCB Process and Surface FinishingTM0210 Metal Surface Finishing ProcessesTM4001 Integrated Training I for ME DG StudentsTM4009 Integrated Training for ISE DG StudentIC253 Introduction to Product Prototpying and Fabrication ProcessesCCN3125 Appreciation of Manufacturing ProcessesICU328 Practical Appreciation of Electro-Mechanical Automation SystemLast updated: December 2012Copyright reserved by Industrial Centre, The Hong Kong Polytechnic University


Surface Finishing2.1.2 Abrasive blasting (Sand blasting)Abrasive blasting is a process for cleaning and matt finishing the surface byforcefully directed spray of abrasive grains either dry or suspend in a liquidagainst the surface of the article.Abrasive blasting is used for:i. To remove scale, rust, paint and soil etc.;ii. To roughen the surface for painting or other coating.iii. To remove metal `flash' or for deburring.iv. Matt finishing not only for metals but also for glasses and plastics.2.1.3 Polishing and BuffingHigh-speed rotating mops are used in polishing and buffing to produce asmooth and reflective surface. The cloth mops are mounted on thepolishing/buffing machine and coated or smeared with polishing or buffingcompounds. The sizes and shapes of the mops are different, and must be chosenproperly according to the configuration and sizes of the articles to be finished.The selection of the different kinds of abrasive or polishing compounds dependson the required surface texture to be obtained and the kind of metallic materialto be treated.The polishing/buffing operation:i. Preliminary abrasive finishing: Using fast cutting abrasive coated mop andbob;ii. Bright buffing: Using softer mop with the different sorts of the buffingcompound.Sometimes the articles which have been plated with copper, nickel, chromium,silver etc. might be buffed to provide more brightness.2.2 Chemical Surface Preparation2.2.1 Solvent CleanIt is the cleaning process by using the effective organic solvent to remove grossheavy soil, oil and grease prior to the other cleaning cycle. The parts beingcleaned is immersed or suspended into a container in which the solvent has beenvaporized. The most commonly used solvent is hydrocarbon cleaner. Adequateventilation must be provided in using organic solvents.2.2.2 Ultrasonic cleanUltrasonic clean depends on the cavitations that producing rapid formation andrapid collapse of minute bubbles in the cleaning liquid. This agitation bycountless small and intense implosion bubbles produce an effective scrabblingaction on the exposed even the hidden or recessed parts of the article beingcleaned.Page 3IC Professional Training


Surface FinishingThe main component of the ultrasonic clean device is transducer. It can convertelectrical energy to ultrasonic vibration (frequency is above 20,000 cycles persecond).The type of the transducer is piezoelectric or magnetostrictive made of nickel orits alloys or electrostrictive made of lead zirconate or barium titanate and othertype such as polarised ceramic.2.2.3 Electrolytic alkaline cleanIt is more rapid and reliable way for cleaning the surface than soak alkaline clean.In the electrolytic alkaline clean the articles are made cathodic or anodic, thegases are liberated at the surface of the article and would cause mechanicalagitation and scrubbing action to dislodge soil and contaminants on the surface.Anodic reaction:Cathodic reaction:4(OH) (-4e - ) 2H 2 O +O 2 ↑4H + (+4e - ) 2H 2 ↑Anoderod+Cathoderod-Anoderod+O2Stainless Steel Anode2H2WorkpieceFigure 1-Electrolytic cleanCathodic cleaning would evolve twice as much volume of hydrogen as theoxygen at the anode side at a given current density, so the cathodic cleaning ismore effective degreasing process than the anodic cleaning.Positive charged ions such as Cu +2 , Zn +2 , Fe +2 could be deposited and formed ametallic smut at the cathode. So, different solution should be applied when thenon-ferrous metals are to be cleaned.Hydrogen gases evolved at the cathode may penetrate into the steel or otherferrous metals.Certain ferrous metals may be susceptible to hydrogen embrittlement. For thisreason, an anodic cleaning cycle is usually applied after cathodic cleaning or,uses anodic cleaning alone in order to avoid embrittlement of the ferrous metal.Page 4IC Professional Training


Surface FinishingAnodic cleaning, the oxygen gas liberated is only half the volume of hydrogenobtained by cathodic cleaning with the same current density. Whilst it is anodic,the work will repel metallic ions. Smut formed during cathodic cycle can bedislodged during the anodic cleaning. Usually, copper and its alloys should beanodic cleaned for a short time after cathodic cleaning.Periodic reverse current is presently developed and applied in electrochemicalcleaning.A typical electro-alkaline clean process is:Solution composition:Operating condition:Current density 3 - 5 A/ dm 2Temperature 60 - 70 °CNa 2 CO 3 ……………. 30 - 40 gm /lNaOH ……………. 15 gm /lNa 3 PO 4 ……………. 10 - 20 gm /lWetting agent appropriate amountComplexing agent appropriate amountTime1-2 min.Polarity: Anodic for ferrous metal; cathode and short time anodic treatment forthe copper base metal.2.2.4 Acid dippingAcid dipping is usually follows alkaline cleaning to neutralise the alkaline andmore importantly, to remove the traces of oxide film on the surface.1 -5% HCl or H 2 SO 4 , 15 - 45 seconds2.2.5 Strike PlatingThe functions of the strike plating are:• To prevent the surface of the substrate from being attacked by theelectrolyte, which would cause poor adhesion;• To prevent the contamination of the subsequent plating bath;• To impart good adhesion on subsequent electroplating;• To activate the surface of the substrate for plating or other coatingprocess.Page 5IC Professional Training


Surface FinishingSome typical strike plating processes are:Nickel strike platingThis process is generally applied to alloy steels prior to the nickel plating.Solution composition:NiCl 2·6H20: 240 g /l; HCl: 125m/lOperating condition:Temperature: Rt.Current density: 2 - 4 A / dm 2Time:1-2 min.Non-Cyanide Copper strike plating (Refer to 4.1)3. General Principles of ElectroplatingElectroplating is one of the most widely used coating processes which, by meansof electrodeposition, a thin metallic coating is applied over the surface of thesubstrate. The purpose of the costing could be:i. Withstand oxidation or corrosion;ii. Resist to abrasion or wear ;iii. Provide an attractive appearance ;iv. Impart special mechanical, electrical, thermal or chemicalproperties to the surface of the substrate material ;v. Electroforming of moulds / dies or products.3.1 Electrochemical ReactionM n+ + ne - ↔ MH 2 O↔ 2H + + 2OH - H 2 (Cathodic) + O 2 + H 2 O (Anodic)As soon as a metal salt dissolved in water, it dissociates into electrically chargesparticles, namely, positively charge metal ions (cations), and negative acidradicals (anions). For example:CuSO 4 ↔Cu 2+ + SO 42-When direct current passes through a metal salt solution between twoconductive electrodes, the positive metal ions move towards the cathode,become discharged and deposited on the cathode.Cu 2+ +2e - CuWhen copper is used for anode; as much copper is dissolved from the anode asit is deposited on the cathode if the current efficiencies are the same.Cu Cu 2+ +2e -Page 6IC Professional Training


Surface FinishingConsumable anodes (they dissolve) are normally used in order to keep up theconcentration of metal ions in the solution to make it stable throughout theelectrolysis action. Anodes which do not dissolve are used in some situationssuch as chromium plating baths (lead alloys), or in some gold plating baths(plasticized titanium). In this later case, the `metal' concentration of the solutionis corrected by adding soluble metal compounds to the solution.3.2 The Rate of Electrodeposition3.2.1 Faraday's LawFaraday's Law is the fundamental for all electrolytic processes, which quantifiesthe amount of metal deposited during electrolysis.Faraday Law's indicates that the weight of the metal deposited in an electrolyticprocess is proportional to the current, plating time, and its chemical equivalent.(Assuming 100% current efficiency)Thus: W = I t eWhere: W =grams of metal depositedI=current (A)t=time (s)e=chemical equivalent weightIt = No. of coulombs of electricity passedFor a given coulomb, the weight of an element discharged is proportional to itschemical equivalent. One Faraday (F) liberates/deposits a gram-equivalent of anelement of atomic weight A:F = Faraday (an electricity to deposit 1 gm equivalent of metal =96500 coulomb or 26.8 ampere-hour)C = coulombn = Valance of the elementA = Atomic weightW = ItA/nFMetalAtomicWeightValenceChemicalEquivalentElectrochemicalequivalentmg/C g/AhrAg + 108 1 107.88 1.118 4.024Au 3+ 197 3 65.7 0.676 2.436Page 7IC Professional Training


Surface FinishingAu 2+ 197 1 197 2.03 7.308Co 2+ 58.94 2 29.5 0.305 1.098Cd 2+ 112.41 2 56 0.58 2.088Cr 2+ 52.01 6 8.66 0.091 0.3276Cu + (CN - ) 63.54 1 63.54 0.658 2.364Cu 2+ (SO 4 2- ) 63.54 2 31.8 0.329 1.182Fe 2+ 55.85 2 31.8 0.329 1.182Ni 2+ 58.71 2 29.3 0.303 1.091Pb 2+ 207.21 2 103.5 1.071 3.86Sn 2+ 118.7 2 59.5 0.615 2.21Zn 2+ 65.38 2 32.5 0.338 1.223.2.2 Cathode Current EfficiencyIt is proportional to the total cathode current used in depositing the metal. Theportion of the current that is not used for depositing the metal is wasted forliberating hydrogen or other sub-products.Cathode Current Efficiency = (No. of coulombs depositing metal / Total No. ofcoulombs passing) X100%The current efficiency is different for different processes. Acid copper plating isabout 99%, bright nickel plating is about 95%, and chromium plating - about 10-20%.3.2.3 Current DensityCurrent density is the quantity of current distributes over the entire surface areaof the article being plated.Current density = I / SI = currentS = surface areaUnit: A / dm 2 , A / ft 2 or mA /cm 2Current density is an important factor that must be controlled in a definiteoptimum range to meet the requisite for the metal deposition.Example- To calculate the time required for plating 1 µm (micron meter)thickness of nickel on an area of 1 dm 2 . (The current density applied is 5 A / dm 2 )Volume of the 1 pm (0.0001 cm) thickness of nickel is:Page 8IC Professional Training


Surface Finishing10 x 10 x 0.0001 = 0.01 (cm 3 )Mass of nickel deposited = 0.01 x 8.907 =0.08907 g= 89.07 (mg) (Density of the nickel is 8.907g/cm 3 )The mass of nickel deposited in l ampere-hour is 1091 mg. So for depositing89.07 mg of nickel the electrical requisite is:89.07 / 1091 = 0.08164 (ampere-hour)= 4.898 (ampere-minute)The cathode current efficiency of nickel = 95%Therefore, the required ampere-minute = 4.898 x 100/95 = 5.156If the current density applied is 5.0 A/dm 2 , the plating time for depositing 1micron thickness of Nickel on 1 dm 2 is:5.156 / 5.0 = 1.03 (minute)3.3 The Rate of ElectrodepositionThe rate of electrodeposition varies with different electrolytic processes. Even ina specific plating process it also varies with current density, current efficiency,temperature, agitation, pH, etc.According to the thickness of the deposition required we must control theplating time and choose the optimum current density. However, the thickness ofthe coating calculated from the surface area, current applied and the plating timeis an average value of thickness over the whole surface. The coating thicknessvaries in different regions of the surface. This is owing to the uneven distributionof current density in the different positions of the surface.Generally, it is a minimum coating thickness on the significant area of the articleto be plated where an allowance has to be made. The plating time is alsoadjusted in order to meet the specified thickness.3.4 Electrode PotentialA piece of metal immerses in an electrolyte containing its ions will establish adefinite electrode potential.3.4.1 Nernst EquationThe electrode potential can be demonstrated by the Nernst Equation, that is:RTE=E 0 + ln a zFPage 9IC Professional Training


Surface FinishingWhere E: The electrode potential at 25 °CE 0 :Standard electrode potential of a metal in equilibrium witha solution of its ions at unit molar activity at 25 °C= molar concentration of the ions multiple activitycoefficient.The molar activity is correspondent with the molarconcentration in vary dilute solutions.R: gas constant=8.315 J/ °C mol -1 (1 cal = 4.184 J)T: absolute temperatureZ: valency of metala: the activity of the metal iron in the solution= a M /a Mn+ , At 25 0 cF: the faraday=96486 CoulombAssuming that the metal electrode is in a standard thermodynamic state (a M = 1)E= E° + 0.059/z log 10 a Mn +Change of concentration will be the prime factor affecting the electrodepotentialAssuming that activity and concentration are equivalent, we may see thatdiluting a solution shifts the electrode potential to more negative (base) valueConcentration 1M 10 -2 M 10 -7 M 10 -20 MPotential (volts) E= E° E°-0.059 E°-0.21 E°-0.59Equilibrium M/M n+ E 0 Equilibrium M/M n+ E°Cu 2+ + 2e Cu +0.34 Ni 2+ + 2e Ni -0.25H + +e1/2H 2 0.00 Cr 3+ + 3e Cr -0.71Zn 2+ + 2e Zn -0.76 Au + + e Au +1.5Page 10IC Professional Training


Surface FinishingCu + + e Cu +0.52 Pt 2+ + 2e Pt +1.23.4.2 Overvoltage and PolarisationThe Nemst Equation is limited to measure electrode potential for the processunder the reversible equilibrium i.e. the process with no gross reaction take place.But in most cases electrode reactions are irreversible and that would cause thepotential of the anode to become nobler and the cathode potential less noblethan their respective static potential calculated from the Nernst Equation.Overvoltage is measure of the degree of this irreversibility and the electrodepotential is "polarised".Thus the electrode potential can be given:1 E=Erev+YIn which, Erev is the electrode potential result of calculation from the NernstEquation. Y is the total polarisation.Xc concentration polarisationYa activation polarisationYb Ohmic polarisationKnowledge of various types of polarisation in plating systems is useful ineliminating problem. If a metal is plated at a higher concentration, polarisationdefect with dull appearance and rough crystalline structure are usually observedbecause of the short supply of the depositing ions to the cathode surface.3.4.3 The Effect of Polarisation:i. Polarisation would increase the bath voltage and decrease the current;ii. An amount of gases would be released ;iii.iv.A compound or film would be formed on the electrode;Increasing polarisation would increase the covering power of deposit andwould cause the finer deposit. Generally, Yc can be decreased by increasingtemperature, concentration and agitation of the plating bath.3.5 Distribution of DepositGenerally, metal deposited surface is thought to be covered with uniformthickness of coating. In reality, the edge, convex positions where current densityis higher, thicker coating compare to the hollow, concave positions of the articlewould be plated.Page 11IC Professional Training


Surface FinishingAnoderod+Cathoderod-Anoderod+AnodeCathodeWorkpieceMetal DepositedCurrentFigure 2-Current distributionThe factor to influence distribution of the metal deposit is complex. Factors are:The composition, concentration and other properties of the electrolyte;The configuration of the article being plated;Spacing of anodes;Rack of design;Operating conditions;Types of current applied. (i .e . pulse, reversible or straight d. c.)3.5.1 Covering and Throwing PowerCovering power: denotes the ability of an electrolyte to cover the recess area(low current density) of a component being plated.Macro-throwing power: it is a measure of ability of an electrolyte to depositmore or less uniform thickness coating over the irregular surface of the article.Micro-throwing power: it is a measure of ability of an electrolyte to deposit moreor less uniform thickness coating in minute (microscopic) irregularities of thesurface.3.5.2 To Improve the Distribution of the Metal DepositionTo achieve the uniform thickness coating over the functional areas of the articleto be plated is quite significant. Firstly, from the economy viewpoint, it isimportant to avoid wastage of metal especially the precious metal to meet theminimum required thickness of coating. Secondly, from the technical viewpoint,the thickness would affect the properties of the coating.In the electroplating process, the distribution of a metal deposit could beimproved by the use of proper devices and techniques such as:Page 12IC Professional Training


Surface Finishingi. Stringent control of bath composition to get good throwing powerii.Use additive agents such as brightener, leveling agents, wetting agentsetc.iii.To minimise the convex and concave irregularities over the significantarea of the surfaceiv.To adjust the anode spacing and the anode-cathode distancev. To design and fabricate the proper racksvi.vii.viii.To use the conducting or insulating screen and auxiliary anodesTo increase the polarisation on the cathode in the proper processTo adjust the operating condition. To control within the optimum rangeof temperature, current density etc.ix.Pulse current is usually better than direct current.4. Copper Plating4.1 Non-Cyanide Alkaline Copper PlatingNon-cyanide alkaline copper plating is a newly developed non-toxic platingprocess that is replacing the conventional cyanide alkaline copper plating. Thisplating process fully complies with the EU Restriction of Hazardous Substances(RoHS) Directive 2002/95/EC as it does not contain Pb, Cd, Hg and Cr 6+ .Compared to the conventional plating process, this technology does not havethe toxic substances cyanide and pyrophosphate, thus eliminating the potentialdangers of cyanide in the workplace and reducing the waste treatment costs fordestroying cyanide. Moreover, die cast zinc must be strike-plated first in cyanideor other alkaline baths (e.g. copper pyrophosphate bath), but cannot be plateddirectly in acid baths; however, it can be plated successively only in a single bathwith this new technology, non-cyanide alkaline copper bath can, thereby, takethe place of both strike bath and Rochelle salt bath.Non-cyanide alkaline copper plating has the following advantages:i) Better throwing power (uniform deposit thickness)ii) Better adhesion and corrosion resistancePage 13IC Professional Training


Surface Finishingiii) Significant improved resistance to thermal shock.4.1.1 ElectrochemistryIn non-cyanide alkaline copper, the main reaction is shown as follows:4.1.2 Area of Applicationi) Pre-plating or undercoating metalsii) Wire and grounding rodsiii) Printed circuit boardsiv) Decorative coating4.1.3 Metals Platedi) Ferrous metalsii) Brassiii) Copperiv) Zincated aluminumv) Electroless nickelvi) Die cast zincCu (Complexor) → Cu 2+ + ComplexorCu 2+ → Cu4.1.4 EquipmentsTankAgitationFiltrationHeaterAnodeRigid PVC or PP madeVigorous airContinuous with 5 µm cotton filterQuartz or titaniumPhosphorus copper4.1.5 Operating ConditionsTemperature 45-60 °CpH 7.5-8.5Current Density 1-4 A/dm 2Voltage 1-6Time3-10 min.4.1.6 Solution CompositionCopper Salt350-450 ml/LBrightenerappropriate4.2 Acid Copper Plating4.2.1 ApplicationElectroforming, electro-refining, manufacture of copper powder, or the undercoating applied on the steel parts and zinc diecasting parts that have been strikeplated on the alkaline copper plating bath.Page 14IC Professional Training


Surface Finishing4.2.2 EquipmentTank (with PVC or PE lining), rectifier, electrical and heating controls, copperanodes and bags, anode and cathode bus bars, agitation, heater, filtration,ammeter and timer.4.2.3 Solution Composition and its FunctionsOne typical solution composition of the bright acid copper plating bath:CuSo 4 ·5H 2 OH 2 SO 4C 10 H 6 (SO 3 Na) 2CS (NH 2 ) 2150-250 g/L45-110 g/L1-5 g/L0.005g/LCopper sulphate CuSo 4 ·5H 2 O: It is the primary constituent of the electrolyte andfurnishing the copper ion in the solutionSulphuric acid H 2 SO 4 : Its main function is to increase the conductivityBrighteners: To improve leveling and throwing power, such as thiourea CS(NH 2 ) 2 ,molasses, dextrin, sodium salt of naphthalene disulfonic acid C 10 H 6 (SO 3 Na) 2 andso on .4.2.4 Operating ConditionsTemperature 30 - 40°CCurrent density 2 - 4 A / dm 2Voltage 2-4Agitation moderate4.2.5 Electrode ReactionIn the acid copper bath, the main reaction on the cathode side is to depositcopper; on the anode side is the dissolution of the copper anode;Cathode: Cu 2+ + 2e CuAnode: Cu - 2e - Cu 2+5. Nickel PlatingNickel is the most widely used metal in electroplating. During long exposed tomildly corrosive atmospheres nickel will tarnish - yellow colour, or turn green onsevere corrosive environment. Usually a thin layer of chromium/tin cobalt alloy isdeposited on nickel plate to prevent tarnishing. Electrodeposits of nickel possessa wide variety of properties, depending on plating bath composition andoperating conditions. It can be deposited bright, matt, satin, and black as well ashard, ductile etc.Page 15IC Professional Training


Surface Finishing5.1 Safety ConsiderationsNickel and its salts used in electroplating are non-toxic, unless the associatedanion is and unless very large amounts are ingested. Skin contact can causedermatitis (skin itch) in some individuals (The EC countries have banned the useof nickel under coat for jewellery and any other items in constant contact withskins). Vapour of nickel carbonyl Ni (CO) 4 is extremely toxic, much more so thancarbon monoxide. Long exposure to nickel containing dusts has been shown tobe carcinogenic. To the platers, therefore, the handling of nickel salts ishazardous unless proper precautions are taken.5.2 ApplicationThere are three major application areas:5.2.1 Decorative CoatingIn which the bright nickel coating is overlapped with bright chromium toproduce attractive appearance and protection against corrosion, resistance towear, and tolerance to high temperature.5.2.2 Functional FinishesWhere a matt-grey nickel deposit is applied which is generally thicker than thedecorative coating. These finishes serve to improve corrosion and wearresistance, to salvage worn or damaged parts or to modify other surfaceproperties.5.2.3 ElectroformingWhere a rather thick nickel deposit is required. The deposited layer is thanseparated from the mandrel to produce a standalone nicked component.5.3 Bright Nickel Plating5.3.1 EquipmentTank (PE or PVC lining), rectifier, voltmeter and ammeter, current regulator, nickel(nickel square or round) anodes, titanium baskets and bags, anode & cathodebusbars, agitator, filter.5.3.2 Solution Composition and Its FunctionTypical Watts bath solution composition:NiS0 4·6H 2 ONiCl 2 ·6H 2 OH 3 BO 3Wetting agentBrighteners250-350 g/l45-60 g/l30-40 g/lappropriateappropriatePage 16IC Professional Training


Surface FinishingNickel Sulphate NiS0 4·6H 2 O - is the main source of nickel ion. Increasing thenickel content would permit the use of higher current density.Nickel Chloride NiCl 2 ·6H 2 O - it would improve anode dissolution and increaseconductivity; however it would increase the stress of the deposit.Boric Acid H 3 BO 3 - Act as buffing agent to maintain the pH level, help to producesmoother, more ductile deposit.Anti-pitting or Wetting Agent - Used to improve surface uniformity of depositmainly by reduce pitting which caused by hydrogen bubbles that cling to thesurface of the cathode during plating. One kind of wetting agent is sodium laurylsulphate CH 3 (CH 2 ) 11 SO 3 Na, 0.01 - 0.5 g/L.5.3.3BrightenersMost of the bright nickel baths utilise two classes of brighteners:Class I BrightenerUsed alone could produce certain degree of brightness but unable to producelustrous deposit as plating continues. This kind of brighteners includingnaphthalene disulfonic (or trisulfonic) acid, benzene sulphonamide andsulphonamide, saccharine,etc. Relatively high concentrations (15g/1 naphthalenepolysuphonic acid type of compound) may be used without harmful effects.Class I brighteners do not have a marked effects on the physical properties ofdeposits as class 11 brighteners, which can only be used at low concentrations.Often the combined effects of class I and class II used together are much greaterthan the sum of the effects of each used separately. When used together withclass II brighteners, they can effectively use less class 11 brightener to give asuperior brightness and to reduce the deleterious effect on mechanicalproperties caused by class II type brighteners.The characteristics of class I brighteners are:Presence of sulfon group or groups (=C-SO 2 ); Presence of unsaturated carbonlinkage in close proximity to the sulfon group.The brightener can be absorbed on the cathode surface via the unsaturatedlinkage. The absorption may occur on the growth site, edge points of crystalsand dislocations.Class II BrightenerIt is used in combination with the class I compounds to obtain lustrous or fullbright deposit. It never used alone for brightening due to high tensile stressbuild up in the coating and other deleterious effects.Commonly, this kind of brighteners is organic compounds containingPage 17IC Professional Training


Surface Finishingunsaturated group such as:Formaldehyde HCHO, coumarin C 8 H 6 OCO, olefinic - C = C - group, ethylenecyanohydrin HOCH 2 CH 2 CN etc.A sample of brightener in bright nickel plating:Naphthalene 1, 3, 6 - trisodium sulphonate - 4 g /l (brightener of the 1 st class)2 - butyne -1, 4 - diol - 0.2 g / l (brightener of the 2 nd class)5.3.4 LevelerLeveling agents has the ability to hide surface defects and is commonly used toproduce semi-bright deposits. Coumarin is the best known. These agents canproduce finer grain deposit hence the brightness but cannot produce a fullbright deposit. Leveling is the ability to deposit thicker coating in recesses thanon the flat part of the surface. Scratches are preferentially filled when levelers areadded.5.3.5 Wetting agentNo brightening system is operational without wetting agent to produce pit-freedeposit. Unsuitable wetting agents can result in loss of brightness, lost ofleveling and even cause pitting. The ability to lower surface tension is by nomeans the only criterion of a wetting agent.5.3.6 Operating ConditionTemperatureRoom Temperature 50-60 0 CCurrent density 0.5-10A/dm 2pH range 3-5.5AgitationModerate to vigorous5.4 Maintenance of Nickel Plating Solution5.4.1 pH ValueDuring operation, the pH value of the nickel plating solution would tend to rise,so, in order to adjust the pH value, 3 – 5% of dilute H 2 SO 4 is generally added at atime with stirring.To increase the pH value of solution nickel carbonate or nickel hydroxide arepreferred than sodium hydroxide.5.4.2 Solution CompositionPage 18IC Professional Training


Surface FinishingThe solution constituents should be maintained at the given concentration byregular checking and addition of chemicals.5.4.3 Contamination and CleaningThe solution must always be kept clean. Cleaning of articles before plating iscritical to prevent introducing contaminants into plating baths. The metallicimpurities such as iron, lead, copper, zinc etc. would degrade the qualities andappearance of deposit.Organic impurities would result in stress or brittleness, pitting or poor adhesionon the deposit.5.4.4 Removal of Organic impurities and Metallic ContaminantsTo remove the breakdown of organic additives, the solution is transferred to anauxiliary treatment tank and then activated carbon in the concentration of 1-3g/L is added. The solution is kept with vigorous agitation about 1-2 hours at 50 -60 °C, allows the carbon to settle then filtered back to the plating bath.Or the solution is transferred to the treatment tank. Nickel carbonate or nickelhydroxide is added until the pH value reaches 5.2. Then 0.5-1 ml/L of H 2 O 2 isadded, the solution is agitated and kept at 60 °C about 2 hours. After settling thesolution is filtered back to the plating bath.Dissolved metallic impurities are eliminated by electrolysis carrying out at a lowcurrent density 0.01 - 0.2 A/dm 2 using a corrugated or mesh cathode sheet forseveral hours or overnight under agitation and plating temperature. After thesolution has been purified the addition of chemicals, brighteners, wetting agentshould be conducted.If necessary to remove all the organic substances, e.g. When brighteners aredisintegrated, the following treatment may be adopted after activated carbontreatment:5.4.5 Treatment with Hydrogen Peroxide and Activated carbonOxidation with 30% H 2 O 2 in this treatment the pH value of solution should beraised to 6.The addition of 0.5 - 3 ml/L of H 2 O 2 to the solution should be agitated for morethan 30 minutes.Second carbon treatment and allow solution to settle and filter.Low current density treatment and filtering until the solution is free from foreignparticles.5.5 The Thickness of the Nickel DepositPage 19IC Professional Training


Surface Finishing6.4 Operating ConditionsTemperature 50-60 °CpH 8.5-9.0Current Density 0.1-1.0 A/dm 2Time1.5 min.6.5 Solution CompositionStannous PyrophosphateCobalt SulphateBrightener14-20 g/L.10-20 g/L.appropriate6.6 Tin-Cobalt Plating ProcessNickel Plating → Water Rinse → Tin-Cobalt Alloy Plating → Water Rinse →Post treatment → Water Rinse → Drying7. Anodising and Dyeing of AluminiumThe basic reaction in anodising is the conversion of the aluminium surface toaluminium oxide while the part is made anode in an electrolytic bath. Anodicoxidation of aluminium provides a hard, protective and decorative aluminiumoxide film. The process is used for:Decorative finishing - The micro-porous oxide film formed provides wearresistantand protective finish. It can also be dyed to give a wide range ofattractive colours.Hard finish - A thicker (hence harder) oxide film can be formed for architecturaland engineering applications. If sufficiently thick coating (over 30pm) is formed,it become electrically insulating of up to 300V.7 .1 ClassificationAccording to the types of electrolytes adopted, anodising processes can beclassified into different classes, from which different properties of oxide films areproduced.Electrolyte Operating condition Properties of oxide filmSulphuric acid 10-15% 1-2 A/dm 2 , 12-18VTransparent to grey colour ratherhard and low ductilityChromic acid 10% 0 .3 A/dm 2 , 45-56 V Light to dark grey colour, moreductilePage 21IC Professional Training


Surface FinishingOxalic acid 1-2 A/dm 2 , 65 V Creamy colourPhosphoric acid 1 A/dm 2 , 30-60 V Transparent to bluish colourThe processes using sulphuric acid or chromic acid are the most widely appliedmethods commercially:A. Sulphuric acid process provides a hard and transparent film, and can beeasily dyed to various colours but the different alloys would result invariation of opaqueness. Pure aluminium or aluminium-magnesium alloyswould produce colourless and bright oxide film, aluminium-silicon-copperalloys of low contents of alloy elements may produce undesirable opaqueto dull grey colours. Aluminium alloys with high silicon, copper, magnesiumetc. are not suitable for anodising.B. Chromic acid process would produce darker colour of oxide film, and it ismore ductile to permit bending and folding. It is widely used for anodisingaircraft parts particularly where entrapping areas from blind holds andoverlaps are present. For the reason that entrapped chromic acid will not, assulphuric acid will do attack the aluminium.7.2 Sulphuric Acid Anodising Process7.2.1 PrincipleThe parts to be anodised are made anodic during treatment, and aluminiumoxide would be formed. The main reactions include electrolysis of water andformation of oxide film.2H 2 O2H 2 +O 24Al+3O 2 2Al 2 O 3AlAl 3+Al 2 O 3 +6H + 2Al 3+ +3H 2 OCellPore→ absorb dyestuffOxide Film(Built-up)50%10-30µmOxide Film(Penetrated)50%AluminiumFigure 3-Structure of oxide filmPage 22IC Professional Training


Surface FinishingThe oxide film is formed on the anode, but on the other side the sulphuric acidhas a solvent action on the aluminium oxide and aluminium metal. It corrodesthe film so that a porous structure of oxide is formed. The size of the pore or cellof the oxide film depends on the solution concentration, temperature andcurrent density. Using a more concentrated acid and higher temperature wouldgive a larger size pore suitable for dyeing. Conversely, using a lower temperaturewould give smaller size pores and harder film suitable for architectural andengineering usage. When dyeing, the porous structure of oxide film wouldabsorb the dyestuff to produce the desired colour.In order to close up the porous oxide coating the articles must go through asealing process; generally, hot water or hot cobalt-nickel acetate solution arecommonly used as sealing agents. After sealed in the hot or boiling water theamorphous alumina (Al 2 O 3 ) of the cell is converted to boehmite (Al 2 O 3· H 2 O) ofgreater volume; therefore, the smaller pore after sealing will be closed up.7.2.2EquipmentTank (PVC lining), rectifier and electrical controller, thermostat, chiller orrefrigerator, anode and cathode bars, lead cathodes, continuous filtration andagitation.7.2.3 Solution Composition and Operating ConditionSolution composition: H 2 SO 4 10-15%H 2 C 2 O 4· 2H 2 O 4 2-5%Current density 0.5-2.5A/dm 2Temperature15-20 o CTo obtain a thicker (harder and high wear) film, the temperature should becontrolled in a lower range.For semi-hard anodising, temperature at 10 - 15 °C; for hard anodising,temperature at +5°C to -5°C.7.2.4 Thickness SpecificationFor the most decorative application, the oxide film thickness is about 5 -10µm;and for architectural used the oxide film should not be less than 25 µm; for hardanodising the oxide film thickness range is 25-125 µm.7.2.5 Rate of Oxide FormationNormally a current density of 1 .5 A/dm 2 and in a period of 2.4 minutes wouldproduce 1micron thickness of oxide film.The thickness of the oxide film for a given electrolyte can be calculated by theformula:σ = KltPage 23IC Professional Training


Surface Finishingσ The thickness in µmKAnodising coefficient, 0.25 -0 .36, usually 0.3 is used.It decreases when the concentration and temperature is increased.I Current density A/dm 2TTime in minutesWhatever if a dark colour is required in the subsequent dyeing process anodisingtime should be longer, generally 60 minutes is required.7.2.6 Dyeing of Anodised Oxide FilmDifferent colour dyestuffs are available commercially. When dyeing, immerse thearticle into the aqueous dyestuff solution.Dyestuff solution concentration 1 - 5 g/1Temperature 30 - 40 °CpH 5 - 7Time1- 5 minutesAgitationmoderate7.2.7 Sealing anodic coatingAfter dyeing, the pores in the film must be sealed to block the dyestuff fromrunning out and to prevent attack by contaminates and corrosive substance inthe atmosphere.Sealing usually involves subjecting the anodic coating to a hot aqueous solution,which causes hydration of the coating:Al 2 O 3 + H 2 O Al 2 O 3 ·H 2 OThe resistance of anodic coating to staining and corrosion is affected by theabsorption of foreign substance into the coating. Therefore, sealing treatmentsusually are employed immediately after anodising. Improperly sealed coatingscan account for pitting attack, colour change, and mottling.The most widely used sealant employs water although only high-purity water isrecommended. Sealing temperature is important and should be 98-100 °C.Sealing time should not be shorter than 10 min for thin (2.5 µm) decorativecoating and can be as long as 60 min for thicker (25 µm) coatings7.3 SafetyElectrical hazardsAs the voltage used in anodising tends to be higher, care must be taken duringmanual handling. Interlocking switches, warning devices and fast acting overloaddevices and leakage breakers are to be installed.Chemical hazardsPage 24IC Professional Training


Surface FinishingSulphuric acid (H 2 SO 4 ) is highly corrosive. Thus an apron, rubber gloves and faceshield should be worn when handling the sulphuric acid.8. Electroforming8.1 IntroductionElectrodeposition is to deposit an adherent metallic coating on an article byelectrochemical means. Electroforming is the production or reproduction ofarticles by electrodeposition upon a mandrel or mould that is subsequentlyseparated from the deposit.Electroforming is a rapidly expanding technology owns much of the highprecision obtainable on intricate and complex shaped articles that could bedifficult if not possible by other means. For example, the stamper forphonograph-records, electrotype printing plates, moulds for CDs and laser discsare all made by electroforming.Many of the principal pieces of equipment and techniques employed inelectroforming, e.g. mandrels metalizing or passivation techniques, electrolytictanks, filters, pumps and sources and control of direct current or pulse current,are the same as used in electroplating.As a production process electroforming has the following advantages:i) Simplicityii) Fidelityiii) Reproducibility for any type of surfaceiv) Ease of replication8.1.1 Typical properties of electroformed metalsMetalMech.propertiesBathMech.PropertieselectroformedRemarksCopper34000 psi45%E45VHNSulphate20000-68000 psi15--40%E40-85VHN Internalstress400-2000 psiBath composition,additional agentsand operationalconditions couldinfluence propertiesSulphate +Phenosulphonicacid69000-90000 psi1-20%E80-180VHNSulphate +molasses andthiourea80000 psi3%E, 170VHNPage 25IC Professional Training


Surface FinishingNickel43600psi30%E220 VHNWatts50000-80000 psi10-35%E100 -125VHNinternal stress16000-30000 psisulphate +chloride (1 :1)75000 psi20%E 30VHNsulfamate60000-130000 psi4%E200-550VHNinternal stress500-8000 psi8.1.2 ApplicationsTypical applications are as follows:1) Manufacturing of duplicating plates :Electrotype, phonograph record masters, embossing plates, graining plates.2) Thin-walled sections :Foils and sheets, hypodermic needles, fine mesh screens, seamless tubing.3) Precision componentsMoulds and dies, denture moulds, paint-spray masks.4) Parts difficult or impossible to make otherwiseRadar wave guides, linear electron accelerator structures, surface roughnessgauges, reflectors, musical instrument components .8.1.3 Advantages1) Varying the bath composition and operational conditions may vary themechanical properties of an electrodeposited metal. In many instancesproperties can only be obtained by electrodeposition.2) Parts can be produced in quantity with very high dimensional accuracy, beinglimited only by the accuracy in machining the mandrel. This can be illustratedin the electroforming of record stampers, since faithful reproduction of therecorded sound depends on precision modulations in the sound track itself,which is only one thousandth of an inch.3) Any desired surface textures could be reproduced. For instance, in amicrogroove phonograph record, the smallest modulation to reproduce forhigh fidelity is about half a millionth of an inch. Also, machine surfaces of anystandard; highly polished (mirror finishes), the graining of leather, the textureof skin etc.4) There is virtually no limit to the size of the object that can be electroformed.i.e. hypodermic needles, foil of 0.0001 inch thick, 400 mesh screen or bulkyobjects of complicated geometry weighing several hundred pounds.8.1.4 DisadvantagesPage 26IC Professional Training


Surface FinishingCost is relatively high:1) The deposition rate in electroforming is relatively slow, often being measuredin days.2) There are some limitations in design. Sharp angles, deep and narrow recesses,sudden changes in cross sections or wall thickness etc. should be avoided.3) Because of the excellent reproducibility, scratches and imperfections in theelectroformed master will appear in the products.8.1.5 Electroforming with Copper/NickelCopper is widely used. It is relatively inexpensive, deposition rate is high and theelectroformed component is low in internal stress. Nickel can be deposited overa wide range of properties than any other metal and the effects of bathcomposition, additives and operational conditions are more fully studied.8.1.6 Stress in depositsIt is a well-known fact that electrodeposits generally are highly stressed. Theinternal stresses may cause tight grip of the mandrel, distortion of theelectroformed article, bucking, blistering. During the process the stress level maycause the deposit to curl up at the edges or to exfoliate.Copper deposit has a lower stress when compared with nickel. Depending onbath type and operational conditions, some of the nickel deposits have stressesso high that rupture may occur during the process. Thus, internal stress is one ofthe main concerns in electroforming and for nickel; a bath that results of lowerstresses is desired (i.e. containing stress reducing agents).8.2 Nickel ElectroformingNickel Sulphamate and Watts Nickel are commonly used:Watts Nickel (g/L)Nickel Sulphamate (g/L)NiSO 4·6H 2O 225-300Ni(SO 3NH 2) 2 315-450NiCl 2·6H 2O 37.5-52.5 0-22.5H 2BO 3 30-45 20-45Temperature, o C 44-66 32-60Agitation Air or mechanical Air or mechanicalCurrent density, A/dm 2 3-11 0.5-32pH 3-4.2 3.5-4.5Page 27IC Professional Training


Surface FinishingTensile strength MPa 345-485 415-625%Elongation 15-25 10-25Watts Nickel (g/L)Nickel Sulphamate (g/L)VPN 100g Load 130-200 170-230Internal Stress 125-185 0-55As can be seen nickel sulphamate is preferred formulation for electroforming asit can be deposited at higher rate with much reduced internal stresses in theelectroformed items. Record stampers, plastics moulds, intricate shaped filters,meshes; screens etc. which cannot be produced in any other way are oftenformed with nickel sulphamate formulations.8.3 Electroforming in Nickel Sulphamate BathNickel Sulphamate based plating solutions are employed wherever high speed,low stress, heavy nickel plating is required. The solutions are used for salvage ofwore out or damaged parts and, for the production of gramophone masters andstampers, moulds for plastics and rubber, filters, meshes and screens for thechemical, food, electronic and aircraft industries and a whole range of articlesthat cannot be produced in any other way.In the printing industry nickel plating is employed for the facing of stereo-typesand copper half tone plates in order to provide a hard wear-resistant surface andthus permit a very considerable increase in the number of impressions.Electro-types-duplicates of printing plates are now generally electroformed innickel rather than copper.8.3.1 Solution CompositionNickel Sulphamate Concentrate sp. gr. 1.47 -- 500 ml/lBoric acid ------------------------------------------ 30g/lNickel Chloride ----------------------------------- 15g/lWhen used for electroforming at high current densities it may be advisable toadd a wetting agent to prevent pitting. Anti-pit liquid is recommended for thispurpose. An initial addition of up to 0.5 ml/L should be made, further smallquantities being added from time to time as required.8.3.2 Solution PreparationFill the tank 1/4 full with warm deionized waterAdd the measured amount Nickel sulphamate and then heat up to 55-66 °CAdd the required amount of boric acid and nickel chlorideAdjusting pH to 4.2 using sulphamic acid or nickel carbonateBefore use: Filter through with activated carbon and Electroplating at 0.5 A/dm 2Page 28IC Professional Training


Surface Finishingfor a period calculated using 10 ampere-hour/Liter.8.3.3 Operating ConditionCathode Current Density2 to 16 amp/dm 2. For the nickel facing of silvered plastics use 1 amp/dm 2 for 15minutes increasing to 5.5 amp/dm 2 for minutes.For nickel facing printing stereos, 2.2 to 6.5 amp/dm 2 for 10 to 15 minutes.Higher current densities of up to 16 amp/dm 2 may be employed for theproduction of heavy deposits, e.g. for electroforming.pH Value3.9 to 4.2. Unlike other nickel solutions the pH is lowered by means of sulphamicacid.TemperatureThe temperature at 40°C (100°F) for the plating of plastic mandrels or lowmelting point alloys. 55 to 60°C (130 to 140°F) for general purpose nickel platingand electroforming. Do not let the temperature rise above 75°C (167°F) orhydrolysis of the nickel sulphamate will occur.AgitationVigorous air agitation is essential, especially when plating at high currentdensities.8.3.4 Solution MaintenanceThe density of the nickel sulphamate solution should be 30°Be (52° Tw). If thedensity falls an addition of Nickel Sulphamate Concentrate should be made tothe extent of 9 ml/L for each1° Tw deficiency in density, 16 ml/L for each 1° Be deficiency.PurificationWhere the Nickel Sulphamate solution is used for the production of electroforms,it is essential to have continuous filtration through activated carbon andperiodically plating-out at low current density. This will maintain the internalstress at low value and prevent distortion of the electroformed parts afterremoval from the mandrels.Pumping the solution from the bath using a filtering pump (with activatedcarbon filter) into low current density treatment bath at a higher level than theplating bath. The solution then returns via an overflow pipe. This treatment bathmay be 10 or 20 per cent the capacity of the main bath and the total volume ofPage 29IC Professional Training


Surface Finishingsolution should be circulated though it 2 to 5 time per hour. It should be platedout at a current density of between 0.1 - 0.5 amp/dm 2 . The cathodes should beremoved regularly and cleaned.8.3.5 Nickel AnodesNickel anodes for electroplating are available in the form of the bar of ovalsection, which is made by casting, rolling, or extruding carbon-containing ordepolarised nickel. Carbon bearing anodes are re-commended for use in `bright'solutions operated at pH below 5.0.Depolarised anodes may be used in `dull' solutions operated over the wholerange of pHUsed commercially. Nickel anodes are also made by fixing hooks or clamps tostrips of electrolytic cathode sheets, or by putting pieces sheared from suchsheets, or nickel pellets, into titanium mesh basket. A new form of nickel anodehaving particularly good dissolution characteristics is available in pieces 1 in. x 1in. x 1/2 in. (2 .5 cm x 2.5 cm x 1.2 cm). All anodes require the use of bags toprevent the normal anode sludge from entering the solution. These bags, arenormally of woven polypropylene, TERYLENE*, or cotton twill.8.3.6 Rate of ElectrodepositionThe deposition rate of Nickel in Sulphamate Solution is approximately listed:Temperature o C 35 40 45 50 60C.D. A/dm 2 1.1 2.7 4.3 8.1 17.8Plating rate (Approx.) µm/h 12 31 50 94 2968.3.7 Mandrel Preparation and SeparationAll mandrel surfaces must be treated with a separating film to prevent strongadherence of the electrodeposits. This film must be thin enough and sufficientlyconductive to allow formation of a sound, faithful replica of the surface.8.3.8 Stainless Steel or Electrodeposited Chromium MandrelsSuch mandrels are prepared easily for electroforming. A suggested procedurefollows:a. If heavily coated with grease or oil, degrease in chlorinated solvent.b. Rinse in water.c. Anodic treatment in a (45g/1) solution of trisodium phosphate, opened at140 to 180°F (60 to 82°C) for 1 minute at 5.4 amps per sq. dm.d. Rinse in water at 100°F (38°C). If surface is not uniformly wetted, repeatstep "c".Page 30IC Professional Training


Surface Finishing8.3.9 Plastics MandrelsPlastic mandrel must be made conductive before electroforming by theapplication of a silver, nickel, or copper film. This also the way for electroplatingof plastic.A suggested procedure for preparing non-metallic materials to receive the silverfilm is as follows:a) Remove grease from film, finger marks, etc. by using mild cleaners. Inlarge-scale production operations, as in electrotype manufacture usingvinyl moulds, scrubbing with a soft brush and magnesium oxide slurry isan effective aid to the cleaning operation.b) Rinse thoroughly with running water: spraying is preferred.c) Sensitising. This treatment is probably the most important and criticalstep in silvering any non-conductive surface for electrodeposition. Thesensitising solution is sprayed or dipped until a continuous film isobtained. A typical sensitising solution has the following composition:Stannous chloride ----------------------- 2.5 to l0g/lHydrochloric acid (35%) -------------------- 40 cc/ld) Remove excess sensitising solution and rinsing thoroughly with runningwater prior to the application of the silver film.e) Apply silver film by the chemical reduction of silver nitrate immediatelyafter sensitising and rinsing. The silver and reducing solutions generallyare applied simultaneously as a spray by means of a double-nozzle gun,which mixes the reducing solution with the silver nitrate. Among thenumerous solutions used for this purpose, the commonly applied one isdetailed below:Silver Solution:Silver nitrate ----------------- 55 to 60 g /lAmmonium hydroxide (29%) ------------------------- 60 to 75 cc /lReducing solution:Dextrose ---------------------- 100 g /lFormaldehyde (40% by vol) ----------------- 65 cc /lA concentrated ammonium hydroxide is added slowly to the silver nitratesolution, a precipitate is formed, which slowly dissolves when excess amount ofammonium hydroxide is added. The excess free ammonia is essential to keep thesolution ammoniac and prevent silver from precipitating from the solution. Anumber of proprietary silver solutions and reducing solutions are available.9. Plating on PlasticsThe use of plastics materials have been increased many folds during the pastdecades. Plastics have replaced substantial amounts of metals. In particular, highstrength and rigid plastics are commercially viable to replace zinc-based die-Page 31IC Professional Training


Surface Finishingcastings for industrial, office, and domestic applications. Plastics are lightweight,excellent corrosion resistance and in many cases may offer greater flexibility indesign. Metallic appearance is more attractive and much desired by customers,and in some cases it is necessary to protect the plastics from degradation byultra-violet light.To electroplate on any non-metallic object, the first step is to make the surfaceconductive; when this is accomplished, the subsequent steps in electroplating onthe object dose not differ substantially for any other types of electroplatingprocesses.9.1 Pre-plating cycleProcess Description OperatingConditionAgitationAcidicDegreasingThis is a mould release process which used toremove dirt, including dust, stains on thesubstrate before any other treatment.Cleaner:40-60ml/LH 2SO 4:40-60ml/LTemp. :25-40 o CTime:0.5 - 5min.MechanicalChemicalRougheningApplicable ABS. PP etc. To obtain adhesion forsubsequent metal deposition by chemicalreaction. Chemical etching can be replaced bymechanical means.For good coating adhesion, under etchingand over etching should be avoided. Underetching may lead to skip plating and overetching may result in surface degradationcausing poor adhesion and blistering. Thereare many proprietaries etching agentsavailable. The etching solution is make up ofchromic agents.Chromic acid(CrO 3): 75g/lH 2SO 4:250ml/lTemp.:30 – 60 o CTime:3 – 10minMechanicalDuring etchingCr 6+ + 3e - Cr 3+Etching away the Butadiene roughens thesurface of ABS (Acrylenitrile – Butadiene –Styrene. This gives a porous surface that isable to retain the catalytic substances duringSensitising.Page 32IC Professional Training


Surface FinishingThe etched surface is neutralized in ansolution, it is rinsed and followed by acid dipNeutralizationWhen used after the rinses following chromicacid etch, the Neutralizer reduces residualhexavalent chromium and prepares theetched plastic surface for catalysis.HCl: 8-12% byvolumeNeutralizer: 2- 3% byvolume Temp.:48.9-60 o CTime: 2 - 4minMechanicalSensitising(Catalyst predip)This is the absorption of a reducing agent onthe surface, a kind of colloidal substance, tofacilitate the adsorption of a catalyst in thenext step of treatment. The commonly usedsensitizer, or reducing agent. Is stannouschloride.SnCl 2 + H 2O Sn(OH)CI + H + + Cl - ------- (1)and SnCl 2 + H 2O --> Sn(OH) 2 + 2H + + 2Cl---- (2) Sn(OH)Cl in (1) and Sn(OH) 2 in (2)combine to produce a slightly water solublecolloid of Sn(OH)Cl·Sn(OH) 2 at the roughenedsurfaceSnCl 2: 20 g/lHCl: 40ml/lpH: 0.5-1.9Temp.:20 - 36 o CTime: 1- 5minMechanicalCatalytictreatment(nucleation)To precipitate a precious metal element onthe surface. Palladium being the mostcommon. The precipitate acts as the catalystfor subsequent electroless copper, silver ornickel deposition.Prior to use,mix sol. A andB and dilute200 times withdistilled water.MechanicalSolution A : Solution B :pH: 3.5-4.5PdCl 2 1.0g/L Na 2SnO 3· H 2O75g/LTemp.:20 - 35 o CSnCl 2· 2H 2O 2.5g/L SnCl 3· 3H 2O7g/LTime: 1-5min.Dissolve in 100ml of HClDissolve in200ml of HClDistilled water 200mlPd 2+ + Sn 2+ Sn 4+ +Pd↓ Sn 2+ +H 2O Sn(OH) 2↓ Sn 2+ +H 2O Sn(OH) 4↓ SnCl 2 +H 2O Sn(OH)Cl↓Page 33IC Professional Training


Surface FinishingA colloidal [Pdm .n Sn 2+ .2(n-x)Cl]. 2xCl - is formedfrom the above reaction.The values of m, n and xare affected byconcentrations of Pd, Snand HCl.AcceleratorAfter rinse with distilled water, the article isimmersed into a solution (accelerator). Wherethe outer layer of the tin ion colloid etc. isdissolved exposing the Pd particles at thesurface.Sn (OH) 2 +HCI Sn 2+ + 2Cl - +H 2OHCl/H 2O = 1:1Temp.:20-30 o CTime: 1- 3min.MechanicalElectrolessNickelDepositionElectroless Nickel provides an adherentconductive coating on plastic substrates thatserves as a protection for the Electrolesscopper layer.Nickel:3.2-3.6 g/lHypophosphit:20-25g/LpH: 8.7-9.1Temp.: 26-32 o CTime: 5-8 min.MechanicalOrElectrolessCopperDepositionCu 2SO 4 15g/lNaOH 10-15g/lKNaO 4H 4O 6 30g/lNa 2CO 3 10g/lPrior to use add HCHO (35%) 10-25 ml/L; Pdat the surface act as the catalyst.[Cu(0 4H 4O 6) 3] 4- + HCHO Cu + (0 4H 4O 6) 2-+CO 2+H 2OpH: 11-12.5Temp.:15-35 o CTime:10-20 min.Mechanical/Air9.2 Plate through holes (PTH) of print circuit board(PCB)9.2.1 PCB materials:Epoxy-impregnated fiberglass cloth, (e.g. G-10, FR-4) is normally used for doublesided and multi-layer computer and industrial electronics application.9.2.2 PTH of PCB:The purpose of an electroless copper plating process is to render enoughconductivity to the drilled through-holes and vias of a circuit broad toelectroplate those surfaces. Once plated, the hole wall surfaces then permitelectrical connection form one side of the circuit board to the other side, and/orform layer to layer in a multilayer board.Page 34IC Professional Training


Surface FinishingFigure 4-Plate through holes9.2.3 Process of PTHProcedure Description OperationalConditionDesmear Cleans and activates the hole-wall. Temp.: 60 – 80 o CTime: 2-10 min.AgitationMechanicalNeutralisationRemove manganese residues andenhance coverage on hole-wallTemp.: 50 – 55 o CTime: 4-6 min.MechanicalSensitising &CatalytictreatmentThis is the absorption of a reducingagent on the surface, a kind of colloidalsubstance, to facilitate the adsorptionof a catalyst in the next step oftreatment. Palladium being the mostcommon. The precipitate acts as thecatalyst for subsequent electrolesscopper.Temp.: 20 - 36 o CTime: 1- 5min.MechanicalAccelerationThe outer layer of the tin ion colloidetc. is dissolved exposing the Pdparticles at the surface.Temp.: 20 – 30 o CTime: 1- 3 min.MechanicalElectrolessdepositionElectroless copper provides anadherent conductive copper layer onhole-wall.pH: 11-12.5Temp.: 15-35 o CTime: 10-20 min.Mechanical/AirAcid copperelectroplatingIncrease thickness of the copper layerTemp.: 20-25 o CTime: 10-20 min.Mechanical/AirPost-treatmentAntitarnish of copper layer is formedand conduce following processTemp.: 20-25 o CTime: 0.5-1 minMechanical9.2.4 Troubleshooting of PTHPage 35IC Professional Training


Surface FinishingProblemsPartial voids refers to areas onthe hole wall that are notcovered by copperCauseHole wall damaged in drillingNot enough desmear and hole cleaningUnsuitable operating conditions of electroless copperbathsPoor rinsingHole wall pull awayNot enough desmearExcessive conditioningUnsuitable operating conditions of electroless copperbathsInner connective defectNot enough accelerationRinse too longUnsuitable operating conditions of electroless copperbaths9.3 SafetyChemical hazardsChromic acid and sulphuric acid are highly corrosive and hexavalent chromiumcompounds are toxic and carcinogenic. For this reason, chromic acid oxidation isnot used on an industrial scale. Thus an apron, rubber gloves and face shieldshould be worn when handling the acid solutions.The standard first aid treatment for acid spills on the skin is irrigation with largequantities of water. Contaminated clothing is removed immediately and theunderlying skin washed thoroughly.10. Electrophoretic Coating ProcessElectrophoretic coating process is a clear or coloured paint (lacquer) applied overmetals by electrodeposition. The parts are firstly cleaned and rinsed in deionizedwater, and then transferred to the aqueous electrophoretic paint emulsion bath;a d. c. current is allowed to pass through the emulsion and the parts.The electrostatic charged lacquer particles are attracted to the surfaces of theparts under the influence of the electric current. The paint particles adhere to thesurfaces and build up to a certain thickness and become electrically insulatingPage 36IC Professional Training


Surface Finishingthus no further addition of particles is possible, resulting in a perfectly levelcoating even in the recessed areas of complex shaped articles . The parts arethen removed from the bath and baked in oven.10 .1 PrincipleIn electroplating, metal ions are attracted to the cathode and becomedischarged.M + + ne - MIn electrophoresis charged particles attracted to move to the electrode ofopposite sign under electrostatic force where no discharge takes place but theparticles accumulate. The particles (sols or colloids or others) have a net surfacecharge due to absorption of ions from solution, so large particles can migrateunder a few adsorbed ions. The rate of deposition is many times greater thanFaraday's law can predict. The particles are essentially covalent and becomecharged in the presence of absorbed ions. The particles may be positively ornegatively charged depending on the absorbed ions.For example, Fe 3 O 4 can be positively charged or negatively charge in thepresence of the trace ions:[Fe 3 O 4 ·xH 2 O]Fe 3+ and 3Cl - or [Fe 3 O 4 ·xH20]OH- and Na +As the film is deposited, Endo-osmosis effectively forces out the liquid phase(water and minor additions which are present e.g., organic solvents and othersact as suspending medium), to leave a film that may approach 95% solids. As thefilm thickness increases, its electrical resistance also increases, until it virtuallystifles further deposition. The film thickness is normally 5 - 30 µm for consumerproducts. The deposition process is progressively shifted to less accessible areas.The deposited film contains little soluble ingredients except paint solids, is acompact and adherent coating which, even not subjected to curing by stoveheating, will stand reasonable handling, although this should be avoided.10.2 AdvantagesIn comparison with other painting methods, the electrophoretic coating has thefollowing advantages:1. A uniform film is deposited on all parts of the article with excellent coverageon recess, sharp edges with absence of runs and sags from holes andoverlaps.2. No shielding effect, boxes and complex shaped articles can be full covered.3. No wastage of paint.Nowadays, water based polymers are most commonly used, it offers certainadvantages over solvent based system, particularly with respect to harmful vapor,fire risk, costs, ease of cleaning and maintaining, readily applied to wet surfaces.Page 37IC Professional Training


Surface FinishingThe initial applied current is relatively high; it quickly drops to a low value once asteady state of deposition is established. The film grow faster, becoming moreelectrical resistive.After electrophoresis, the coating is heated to dry out traces of suspension andcure the particles to form a mechanically strong film, which can sustain flexing.10.3 PretreatmentIn electrophoresis or any other surface finishing process, the ultimate successlays on the care taken in prior cleaning. The adhesion of coating to metal hasbeen extensively studied and the complexity of the bonding mechanism is stillnot fully understood. Furthermore, the study of boundary conditions shown thatpresence of traces of gaseous matters or other impurities during the coatingprocess can greatly impair adhesion.The most commonly used surface preparation for painting is mechanicalroughening which provides some degree of mechanical keying.Chemical treatment is also used to produce an etched micro-roughen surface orto produce a micro-porous film which act as a primer for subsequent painting,typical examples are phosphating and chromating. Prior to any chemical orelectrochemical treatment, the metal surface must be clean and free from grease,scale and rust. As mention above the commonly use process is Ultrasoniccleaning. The sequences of cleaning operation are detailed as follows.10.4 Coating ProcessUltrasonicCleanRinsingElectrolyticcleaningRinsingDrag outElectrophoreticcoating at 18-50VdyeingD.I. waterrinsing×3Acid dipD.I. waterrinsingRinse aidDrying & Stoveheating at150°C 40min.The article is attached to one electrode (anode or cathode) and another is a nonconsumableelectrode made of stainless or any suitable metal, a D.C. current isapplied ( The following figure shown the basic set up of the system) for a periodfrom l - 3 minutes, after rinsing in a still tank is washed with deionized water.Page 38IC Professional Training


Surface FinishingAnoderod+Cathoderod-Anoderod+S.S AnodeCathodeWorkpieceFigure 5-Electrophoretic coating bathOperating ConditionsVoltageandCurrent20 -50 VWhen operating at constant voltage, the initial currentdensity is higher, as the process a proceeds, current fallsquickly owing to the growth of the insulating coating.Depending on the coating thickness required.Time15- 150 SecDepending on coating system, after a certain period, anoptimum coating thickness is attained, due to the increasedresistance of the coating, the current is decreased to a pointthat electrolysis effectively ceased.Temp. 20-25 °CIn a continuous production, it is essential to maintain aconstant temperature. At higher temperatures conductivity,pH, and viscosity may be affected, dissociation of thecolloidal or sol may occur.AgitationContinuousKeep the solution stirred to maintain uniform viscosity andprevent settlement of the paint particles. A gentle solutionmovement is sufficient.pH 4.2Higher pH may arise from the traces of alkali cleaningsolution, causes precipitation. Lower pH may due to drag inacids. If the low pH is due to normal acid built up, nopermanent harm is done to the bath.Filtrationand ultra-FiltrationContinuous at60 -70 litersper hourTo remove solid impurities by filtration. AndFiltration per to remove metal ions cations and anions(chlorides, sulphates, alkalis etc.) and, some of the water byultra-filtering with special ion exchange cartridge. Theemulsions are retained during ultra-filtration.Page 39IC Professional Training


Surface Finishing10.5 SafetyElectrical hazardsAs the voltage used in electrophoresis tends to be higher, care must be takenduring manual handling. Interlocking switches, warning devices and fast actingoverload devices and leakage breakers are to be installed.FumesAdequate ventilation should be provided to ensure that all toxic fumes arereadily extracted. Tank additions may include ammonium hydroxide, amine, andsolvent, the vapour generated are harmful if inhaled.Fire riskThe electrophoresic tank depending on the system may contain inflammablematerials.11. Electroless Nickel DepositionElectroless nickel deposition (EN) is used to deposit nickel without the use of anelectric current. Electroless nickel-phosphorus coating consists of an alloy ofnickel and phosphorus. The process relies on the presence of a reducing agent,for example hydrated sodium hypophosphite (NaPO 2 H 2·H 2 O) which reacts withthe metal ions to deposit metal. The alloys with different percentage ofphosphorus, ranging from 2-5 (low phosphorus) to up to 11-16 (highphosphorus) are possible. The metallurgical properties of alloys depend on thepercentage of phosphorus.EN has several advantages versus electroplating. Free from flux-density andpower supply issues, it provides an even deposit regardless of workpiecegeometry, and with the proper pre-plate catalyst, can deposit on non-conductivesurfaces.Advantages include:1. Does not use electrical power.2. Even coating on parts surface can be achieved.3. The process can plate recesses and blind holes with stable thickness.4. High wear resistance and excellent corrosion resistance5. Easily soldered with a highly active acid flex6. Matte, Semi Bright or Bright finishes can be obtained.Disadvantages include:Page 40IC Professional Training


Surface Finishing1. Lifespan of chemicals is limited.2. Waste treatment cost is high due to the speedy chemical renewal.Each type of electroless nickel also has particular advantages depending on theapplication and type of nickel alloyApplicationsIt is commonly used in engineering coating applications where wear resistance,hardness and corrosion protection are required. Applications include oil fieldvalves, rotors, drive shafts, paper handling equipment, fuel rails, and opticalsurfaces for diamond turning, electrical/mechanical tools and office equipment. Itis also commonly used as a coating in electronics PCB manufacturing, typicallywith an overlay of gold to prevent corrosion. Coatings of 25 to 100 micrometrescan be applied and machined back to final dimensions. Its uniform depositionprofile mean it can be applied to complex components not readily suited toother hard wearing coatings like hard chromium.Its use in the automotive industry for wear resistance has increased significantly,however it is important to recognize that only End of Life Vehicles Directive orRoHS compliant process types (free from heavy metal stabilizers) may be usedfor these applications.Bath composition and operating conditionsNickel5.5-6.5g/lSodium Hypophosphite 25-35/lpH 4.2-5.2Temperature85-95°CAgitationmild airDeposition rate 15-25 micron meters/hrPage 41IC Professional Training


Surface FinishingReferences1. “Electrodeposition -The Materials Science of Coatings and Substrates”.Jack W Dini, 1993, Noyes Publications.2. “Surface Finishing Systems”. George J. Rudzki, American Society ForMetals Finishing Publications.3. “AESF Third International Pulse Plating Symposium” .October 28-29.4. “Electroforming”. Peter Spiro,1971, Robert Draper5. “Electroplating Engineering Handbook”. Lawrence J. Durnet, 1984, NewYork.6. ELECTROPLATING –Fundamentals of Surface Finishing. Frederick A.Lowenheim.7. PRINTED CIRCUIT BOARD MATERIALS HANDBOOK. Martin W. Jawitz.8. Electroless copper and nickel-phosphorus plating processing,characterisation and modelling. W. Sha, X. Wu and K.G. Keong.9. COATING AND SURFACE TREATMEN0T SYSTEMS FOR METALS. J Edwards.Page 42IC Professional Training

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