مقالات آنالیز و کنترل در آبکاری

troubleshooting, testing, & analysis

CONTROL AND CHEMICAL ANALYSIS

OF PLATING SOLUTIONS

BY SUDARSHAN LAL, MECHANICSBURG, PA.

The quality of plated deposits primarily depends on factors such as currentdensity, solution composition, temperature, effective bath pH, additives concentration,speed (rpm for barrel plating or line speed for reel to reel), and solutionagitation in the tank. Apart from mechanical factors, tight control of mainingredient and additive concentrations in a plating bath is extremely importantto achieve successful plating operations. Plating solutions must be maintainedat the recommended limits, as suggested by the manufacturers. Sometimes, thelimits may not be so rigid, and bath parameters need to be optimized by a givenjob shop based on the type and layout of the plating line.The status of a given bath is dependent on the rate of its depletion due toplating operations and proper replenishment. The operator should monitorbath components frequently in order to maintain chemistries within a controlledwindow. The depletion of primary bath components mainlydependson the following factors:

1. Drag-out into rinse tanks causing loss of useful chemicals depending onmode of draining.

2. Evaporation rate of plating baths depending on temperature, air flow, andmechanical agitation in the tank.

3. Imbalance in anode and cathode efficiencies. Soluble anodes may increaseor decrease metal content due to current efficiencies. The current efficiencyissues may be ascribed to side reactions occurring at the anode and cathode.For insoluble anodes, metal replenishments are frequently required.

4. Depletion of additives due to co-deposition in electrodeposits and breakdownproducts.

5. Drastic pH changes in the bath, which may cause precipitation or turbidity.

6. Impurities introduced due to tech-grade chemical additions and leaching ofimpurities from extraneous objects that have fallen into the tanks.Wet chemical methods have been routinely employed in monitoring majorbath components. Advanced automatic instrumentation is also available foranalysis of inorganic and organic species. Plating baths are usually analyzedoffline after harvesting samples from various tanks. Metal ions are monitoredusing atomic absorption spectrophotometry (AA), inductively coupled plasma(ICP), wet titrations, colorimetry, polarography, and ion selective electrodes,depending on laboratory facilities.The analytical methods for analysis of plating solutions should be simple,direct, and operator friendly. In order to facilitate this, a standard operating procedureshould be documented, and adequate records with tank ID, date and timeof analysis, and any additions made for replenishment should be maintained, preferably using commercial software programs such as True Logic or Lab Wizard Software. Small job shops are urged to maintain paper copies for eachshift, as well as any notes from troubleshooting operations. Bulk ingredients inthe baths are easily determined mostly using titrimetric methods, which requiresimple laboratory equipment. Trace impurities in solutions may be determinedby a certified laboratory equipped with the desired instrumentation for microdeterminations

SAMPLINGSampling is an extremely important step, and the sample should be representativeof a given tank. Tanks should be identified and their levels recorded tocheck the decrease in tank volume due to evaporation, drag-out, or spillage.Ideally, sampling should be done at 10 different locations in larger tanks, and acomposite sample should be prepared. The log sheet should have entries such as:

1. Tank ID

2. Date and time of analysis

3. Analytical method used

4. Results

5. Recommended high and low limits

6. Analyst signature

After additions to the tank and adequate mixing, an analysis should be performedagain to check desired parameters. Normally, well-established platingshops have reasonably good laboratories and are well equipped to keep track oftheir chemistries. Some job shops depend on analytical support from chemicalbath suppliers, which are usually away from site, and they have key parametersanalyzed once or twice a month. The drawback of off-site analysis is that bathcomponents cannot be adjusted in a timely manner.Reagent-grade standardized solutions should be procured from a reliablesupply house. A well-trained chemist may prepare his own solutions and standardizeversus primary standards. The initial receipt date and expiration dateof these solutions should be recorded. Deionized or distilled water should usedin the analysis.Common standard solutions such as HCl, H2SO4, NaOH, Na2SO3, AgNO3, I2 K2Cr2O7, KMnO4, and KCSN are available in 0.1-N concentrations.

ANALYTICAL TECHNIQUESIt is important to review the flow sheet of a given plating process (rack, barrel,reel to reel) and understand the sequence of the operation. This guides theoperator to prepare for a safe start by analyzing the plating baths needed inthe process. Most job shops use traditional wet methods.The commercial use of instrumental techniques is limited due to complexoperations, maintenance, frequent calibrations, personnel training, and theexpense of initial investment. The workload of a plating plant can justify theuse of any instrumental technique, such as AA, ICP, chromatographic techniques,or any dedicated electrochemical method. The following outline maybe helpful in understanding their relationship.

a) Photometric methods:

1. Spectrophotometry

2. Colorimetry (measures color intensity at a given wavelength)

3. Turbidimetry (measurement of radiation passing through suspension)

4. Nephelometry (measurement of radiations scattered from suspension)

All methods are based upon the absorption of light.

b) Spectrophotometric methods are based on emission of light. For example:

1. Flame photometry

2. Flame spectroscopy

3. Mass spectrometry (MS)

4. X-ray fluorescence (XRF)

c) Electroanalytical methods involving electric current and potential.

1. Electrogravimetric

2. Conductimetric

3. Potentiometric

4. Coulometric

5. Polarography

6. Amperometric

d) Chromatography is a method of separation of components.

1. Gas chromatography (GC) is suitable for volatile compounds by separationon a specific column and speciation by GC-MS.

2. Column, thin layer, and paper chromatography are seldom used in platingbath analysis.

3. High-performance liquid chromatography (HPLC) involves separation ofcomponents using suitable columns and eluents. The separated speciescan be identified by MS.

4. Ion chromatography (IC) deals with separation of anions or cations usingspecific columns and detection achieved by UV-Vis, refractive index, andvariations in conductivity. The separations occur due to the differencein ionic mobilities and differential distribution coefficients of variouscomponents.Solvents conforming to spectroscopy and chromatography grades should be used to eliminate background noise.

TITRIMETRIC METHODSThe active component of a plating bath sample is stiochiometrically titrated witha standard solution of a required titrant. The end point of a titration may bevisually determined with a color change of the indicator or by an electrometricmethod. Indicators are auxiliary reagents added to samples and aid in end-pointdetermination. For low concentrations, volumetric methods yield inaccurateresults due to obscured end points. For accurate determinations, the methodshould be selective and free from interferences, with crisp end points.The details of volumetric titrations are simple, and supply houses provideprocedures for analysis. For accuracy and precision, standardized reagent-gradesolutions in conjunction with A-grade pipettes and burettes should be used.Automatic titrators (Metrohm, Fisher) and digital burettes are gaining popularityfor their reproducibility and accuracy. The role of chemical interferencesshould be considered for a given multi-component bath.

GRAVIMETRIC METHODSGravimetric methods involve the separation of the desired component fromother constituents by chemical precipitation, isolation, washing, and weighingafter drying. These methods are time consuming, but for precious metalsthe gravimetric method is considered a referee method. Some metals (Cu, Ag)are determined by electrodepositing on pre-weighed platinum cathodes. Thegravimetric methods are employed for chloride, sulfate, carbonate, phosphate,and certain metals.

INSTRUMENTAL TECHNIQUES

In wet chemical methods, the chemical property of the component is utilized inits determination, whereas instrumental methods utilize the physical propertyof the component. The analyst should weigh the cost, degree of precision, andaccuracy for a given instrumental method.Plating solutions can be analyzedusing the following instrumental methods:

1. Spectroscopic methods: A given substance is analyzed by the measurementof emitted light from the excited atoms by radiant energy,AC, or DC arc.Each element has characteristic wavelengths depending on its electronicconfiguration. A distinct set of wavelengthsare generated and separatedby a monochromator, and intensities of various wavelengths are measuredby a spectrograph or photoelectric detectors (spectrophotometry).Spectroscopic methods are unique and specific and are employed for tracequantitative analysis. The accuracy of spectrographic methods is not veryhigh, with limit of detection at about 3%. Sensitivities are much smaller forhigh-energy elements, such as zinc, than for elements of low energy, suchas sodium.

2. Flame photometry (FP): A liquid sample is atomized at constant air pressureand aspirated into a flame (1,800–3,100 K) as fine mist. At high temperaturesthe solvent evaporates, forming a solid, which then vaporizes, dissociatingthe atoms into a ground state. The valence electrons of the groundstate are excited by the flame energy to their higher energy level and fallback to the ground state. The intensities of emitted spectrum lines aredetermined by spectrograph or spectrophotometer. The flame photometeris calibrated with standards of a known matrix and concentrations. Theintensity of a given spectral line is compared and quantified against thestandard. Solutes and solvents affect the signal intensity causing inaccuracyin results. Elements with adjacent wavelengths interfere. FP is mainlyused for analysis of: Al, B, Cr, Co, Cu, In, Fe, Pb, Li, Mg, Ni, Pd, Pt, K, Rh,Ru, Ag, Na, Sr, Sn, and Zn.

3. Emission spectrometry (ES): ES involves exciting a cast metal or a solutionby electric discharge by AC or DC in a graphite cavity. Graphite electrodesare favored due to the least spectral interference and a temperature of4,000–6,000 K. At these higher temperatures, elements emit a higher numberof spectral line characteristics of each element. The method is mostlyqualitative, and density of a given line provides semi-quantitative results.ES is sparsely used, mainly for trace analysis.

4. X-ray fluorescence (XRF): XRF is based on the excitation of samples byan X-ray source of high energy, resulting in the emission of fluorescenceradiation. The concentration of elements being determined is proportionalto the intensity of its characteristic wavelength. XRF is a non-destructivetechnique and can be applied in measuring the constituents of platingbaths, such as Cd, Cr, Co, Au, Ni, Ag, Sn, and Zn. The method is less sensitivecompared with ES, and for XRF, proper calibration standards withsimilar matrices should be employed. Applications have been developed bySpectra-Asoma Instruments. Coating thickness and its composition canalso be determined using XRF after calibrations.

5. Mass spectrometry (MS): This technique utilizes gas or vapors derived fromliquids or solids that are bombarded by a beam of electrons in an ionizationchamber, causing ionization and resulting in the rupture of chemicalbonds. Charged moieties are formed and may contain elements, molecules,and fragments and are separated by electric and magnetic fields based onmass to charge (m/e) ratio. Based on the mass spectrum, best possible fitfrom the software library is suggested. MS is only applicable to substancesthat have sufficient vapor pressure and is good for compounds with aboiling point <450¼C. This technique is used in speciation and molecularweight determination.

6. Inductively coupled plasma (ICP): A liquid sample is aspirated in a streamof argon gas and ionized by an applied radio-frequency field. The field isinductively coupled to the atomized gas by a coil surrounding a quartztorch that supports and encloses the plasma. The sample aerosol is heatedin plasma, molecules become almost completely dissociated, and then,atoms emit light at their characteristics frequencies. A high temperature of7,000 K of argon plasma produces efficient atomic emissions and provideslow detection limits for many elements. ICP allows simultaneous analysisof many elements in a short time with sensitivity to parts per billion (ppb)levels. A comparison of both techniques is given in Table 1.ICP instrumentation is not practical for small plating operations due to theinitial cost and its prohibitive operating expenses.

7. Photometric methods: Photometry is based on the absorption of UV light(200–400 nm) or visible radiant energy (400–1,000 nm) by species in solution.The absorption is proportional to the concentration of absorbing speciesin solution and valid up to 2% concentration. Colorimetric methodsinvolve comparing the color produced by a standard containing a knownquantity. Errors in this method may be due to turbidity, sensitivity to theeye, color blindness, dilutions, photometer filters, chemical interferences,and temperature variations.

8. Atomic absorption (AA): Metals in plating baths and wastewater effluentsare readily determined by AA spectrophotometry. Optimum ranges, detectionlimits, and sensitivities of metals vary with different instruments. Thesolution is directly aspirated in the flame (air-acetylene or nitrous oxideacetylene),which absorbs radiations from a hollow cathode lamp of a givenmetal. The difference between flame photometry and AA is that flame photometrymeasures the amount of emitted light, whereas AA measures theabsorbed light. Graphite furnace-AA is utilized for ppb levels of metal ions.

ANALYTICAL METHODS FOR ADDITIVES AND SURFACTANTSPlating baths, in addition to electrolytes and metal salts, also contain certainproprietary organic chemicals, such as wetting agents, grain refiners, and brighteners.Additives play a significant role in controlling the material properties ofdeposits. Additive concentrations may be determined by cyclic stripping voltammetry(CSV), polarography, spectrophotometry, HPLC, and IC. Voltammetrictechniques are employed to study solution composition (oxidizable or reducible)and valence states using the current–potential relationship in an electrochemicalcell. Current–time response of a microelectrode provides useful informationabout a system. The common electrodes used include dropping mercuryelectrodes (DME), hanging mercury drop electrodes (HMDE), thin-film mercuryelectrode, and solid electrodes, such as gold, platinum, and glassy carbon.Additives are indirectly determined by CSV by studying their influence on depositionof metal from the plating bath using a rotating platinum disk electrode.13Anionic, cationic, and non-ionic surfactants may be present in plating baths.After extraction in hexane or another suitable solvent, color is developed using aspecific reagent. The intensity of absorbed light at a given wavelength providesan estimate of surfactant concentration. Anionic surfactants are measured asmethylene blue active substances (MBAS), but the method is not specific and isprone to interferences. Non-ionic surfactants are measured as cobalt thiocyanateactive substances (CTAS). The methods are not specific and may be used as anempirical correlation.Brighteners may be empirically estimated by plating brass panels for 1 or 2minutes at 2 or 5 amperes in a hull cell. Plated panels indicate the useful currentdensity range for bright deposits, and additions to plating tanks are madebased on these results.

OPERATIONS CARRIED OUT IN COMMON PLATING PROCESSES

Substrate preparation using chemical etchants: In order to achieve a robust deposit,it is extremely important to prepare the substrate to be free of soils, scales, rust,and oily residues. The parts may be mechanically polished, buffed with varyingdegrees of abrasive compounds, belt polished, undergo simple tumbling, and/or impact blasting in the presence of various types of media. It is imperative toremove any residual contaminants from these operations prior to starting theplating sequence.For removing shop soils on parts, the workpieces are soaked in organic solventssuch as petroleum naptha, stirred to loosen residues, de-scaled, pickled,and de-smutted. For de-scaling, usually 20–30% aqueous solutions of HCl orH2SO4 in the presence of proprietary inhibitors are employed, and their strengthcan be monitored by titrations. The potency of these solutions should be frequentlymonitored depending on the surface area processed each day. Obstinateand aged scales are loosened by immersion in hot alkaline potassium permanganatesolutions. The concentration and type of de-scaling chemistries shouldbe selected for their compatibility with substrates.

Alkaline cleaners: Alkaline cleaning solutions usually contain sodium hydroxide,sodium carbonate, trisodium phosphate, sodium metasilicates, and proprietarysurfactants as wetting agents. Total alkalinity is monitored for such solutions.Ultrasonic cleaning may be achieved by introducing high-frequency soundwaves (20–80 kHz) causing cavitation on parts, thus removing soils. Anodic andcathodic cleaning involves the liberation of hydrogen or oxygen, which promotesscrubbing action on the parts, causing the following reactions to occur:At anode: 4OH– t 2H

2O + O2+ 4e–At cathode: 4H2O + 4e– t 4OH– + 2H2

Periodic reverse (PR) electro-cleaning is also employed in special cases.Table 2 presents a summary of the methods for analysis of soak cleaners andelectro-cleaners for efficient cleaning and maintenance. For semi-transparentsolutions, a suitable range refractometer may be employed for estimation oftotal detergent concentration.

Nickel plating baths: Several nickel plating baths—such as matte, semi-bright, andbright—usually containing nickel sulfate, nickel chloride, nickel sulfamate, boricacid, and proprietary additive packages (comprising carriers, wetting agents orsurfactants, brighteners, auxiliary brighteners, grain refiners) have been formulated.All sulfate, all chloride, high sulfate nickel, and black nickel bathsare formulated for engineering applications. The primary parameters—such asnickel, boric acid, and pH—are routinely monitored (Table 3). The estimation ofadditives, carriers, and wetting agents are treated separately. Electroless nickelbaths for low, medium, and high phosphorus content are monitored for pH, Ni,hypophosphite, and orthophosphite.There are a series of binary nickel alloy baths with other metals such as P,Co, Cr, Mo, Mn, Pd, S, W, Au, Cu, Ge, In, Ru, Ag, Tl, and ternaries like Co-Ni-P,Co-Ni-Fe, and Fe-Cr-Ni. Standard analytical techniques are used for bath control.

Gold plating baths: Several types of gold plating baths have been formulatedbased on end uses, such as decorative, soft gold, nickel- and cobalt- hardenedgold for industrial electronics, and miscellaneous applications involving repairsand electroforming. Low-speed and high-speed gold baths have been designedfor use in rack, barrel, and reel-to-reel applications, namely:

1. Alkaline gold cyanide for gold and gold alloys

2. Neutral gold cyanide for high-purity gold plating

3. Acid gold cyanide for bright, hard gold and its alloys

4. Non-cyanide gold with sulfite, thiosulfate

5. Electroless goldNon-cyanide gold and electroless gold baths are used for special applications.The analytical methods for gold baths are summarized in Table 4.

Silver plating baths: Both cyanide and non-cyanide silver baths are used in theelectroplating industry. Parameters such as silver metal, free cyanide, pH, andbrighteners are routinely measured by common analytical methods. Electrolesssilver baths are employed in special applications.

Palladium, palladium-nickel, and Pd-cobalt baths: Palladium , Pd-Ni, and Pd-Co(80/20 alloy composition) baths containing sulfate and chloride at acid andalkaline pH have been employed in engineering finishes. The analysis methodsare summarized in Table 5.

Tin and tin-lead baths: Tin-lead baths—(with electrolytes fluoboric acid, sulfuricacid, alkyl sulfonic acid, and methane sulfonic acid (MSA)—have been employedin the industry in conjunction with proprietary brighteners. With the advent ofnew technologies and RoHS compliance requirements, several new tin chemistrieshave been developed. Presently, MSA-based chemistries are more popularthan sulfate-based chemistries, especially the proprietary additive packagesclaiming whisker resistance in highly reliable electronic applications. Immersiontin baths are used in special applications where low tin thickness is required.Tin and lead metal concentrations are conveniently determined by AA, ICP, andtitrimetric methods, whereas additives are estimated using hull cell, HPLC, IC,or spectrophotometric methods (Table 6).

Copper baths: Several electrolytic copper baths, namely acid copper (sulfuricacid, fluoboric acid), cyanide copper, alkaline non-cyanide, and pyrophosphatecopper are available. Copper metal may be determined by AA, ICP, and titrations.Pyrophosphates and orthophosphates are determined by EDTA titrations.Additives consist of organic chemicals, such as carriers, levelers, and brighteners,which are estimated using chromatographic methods, IC, CVS, and CPVS foraged baths. Electroless copper baths containing Cu, EDTA, formaldehyde, andproprietary additives are used for dedicated applications. Copper is analyzedby AA, ICP, and titrations. Formaldehyde may be measured by titration andcolorimetric procedures.

Zinc and zinc alloy baths: Zinc baths with cyanide, non-cyanide, and acid zinc areformulated for rack and barrel applications. Zn-Ni, Zn-Fe, Zn-Co, and Zn-Cuare also employed. Metals are analyzed using AA, titrations, or colorimetricmethods. Brighteners are estimated using bent cathodes, jiggle cell, and otherapplicable instrumental methods.

Chrome plating: Chromic acid in the presence of sulfuric acid and additives isemployed in high-speed chrome applications. Trivalent and hexavalent chromiumare determined by ion chromatography and titrations. A hydrometer mayalso be employed for chromium estimation after correcting for sulfate content.

Iron and iron alloys: Iron alloys such as Fe-C, Fe-B, Fe-P, Fe-Cr-Ni, Fe-Co, Fe-Zn,and Fe-Ni are used in special applications. Metals can be determined by AA orICP. Iron (II) is estimated by titrations, spectrophotometry, and Fe(III) reducedto Fe(II) for titrations.Imbalance in plating processes is manifested in solutions due to varyinganode or cathode efficiencies, depending on the mechanism. For example, athigh anode efficiencies (soluble anodes), a high pH is encountered, resultingin higher metal ion concentrations. On a micro-scale, high variations in pH areobserved in the immediate vicinity of the electrodes. Conversely, in the case ofhigh cathode efficiency, pH is lowered. In cases of high deposition rates, quickdepletion of metal ions in the bath necessitates frequent addition of metal salts.In the case of gold and silver baths, high-free cyanide may be due to low anodeefficiency and low-free cyanide may trigger high anode efficiency.

CONCLUSIONS

Metals may be determined by wet titrations, UV-Vis, colorimetry, AA, ICP, andXRF. Acids and alkalis are determined by acid–base and electrometric titrations.Additives are quantitatively analyzed by chromatographic methods (GC,HPLC, IC), CVS, and hull cells. Surface tension (Sensadyne), solution baume,and refractive index may be used, where applicable, for indirect estimation of aparticular parameter. The difficulty is that no single instrument can analyze allthe components. A plating facility should develop a logical approach to controlsolution composition and follow the schedule.Poor control of bath constituents is a major problem and can lead todecreased reliability of parts. Substandard, poor-quality parts produced usingan imbalanced process leads to customer complaints and rework, negativelyimpacting the reputation of the plating shop. The primary reason is due tolimited analytical capabilities for the analysis of additives (which are usuallya mixture of two or more chemicals). For troubleshooting, adequate resourcesare very important, and these analytical methods are the backbone of successfulplating operations. The analysis of baths should not be discounted, and jobshops should conduct frequent analyses of baths to ensure robust SPC processcontrol.

Author’s note: This article provides an overview of popular analytical methods used in processcontrol but does not claim its completeness. Special applications may require amendedmethods or simple tests, such as specific gravity, baume, refractive index, conductivity, etc.,to suit a shop’s needs in day-to-day operations. Additionally, the author does not endorseany commercial products mentioned in this paper. Plating shops should evaluate baths,analytical techniques, and ancillary procedures for best results in their operations.

NOTES

1. Modern Electroplating. 4th ed. Schlesinger, M., Paunovic, M., eds. John Wiley& Sons, Inc., 2000.

2. Dini, J.W. Electrodeposition. Park Ridge, NJ: Noyes Publication, 1993.

3. Gold Plating Technology. Goldie, W., Reid, F.H., eds. Ayr, Scotland:Electrochemical Publications, 1974.

4. Electroless Plating: Fundamentals and Applications. Haydu, J. B., Mallory,G.O., eds. American Electroplaters and Surface Finishers Society, 1990.

5. Brenner, A. Electrodeposition of Alloys. Vols. I & II. New York, NY:Academic Press, 1963.

6. Electroplating Engineering Handbook. 4th ed. Durney, L.J., ed. New York,NY: Chapman & Hall, 1984.

7. Jordan, M. The Electrodeposition of Tin and its Alloys. Saulgau/Wurtt,Germany: Eugen G. Leuze, 1995.

8. Langford, K.E., Parker, J.E. Analysis of Electroplating and Related Solutions.Teddington, UK: Robert Draper Ltd.,1971.

9. Irvine, T.H. Chemical Analysis of Electroplating Solutions. New York, NY:Chemical Publishing Co., 1970.

10. Standard Methods for Examination of Water and Waste Water. 18th ed.Washington, DC: American Public Health Association, 1992.

11. American Society for Testing Materials. Standard practice for the use ofcopper and nickel electroplating solutions for electroforming. In: AnnualBook of ASTM Standards. Philadelphia, PA: ASTM, 1993:B503–69.

12. Metal Finishing Guidebook and Directory. New York, NY: Elsevier, 2008.

13. Instruments based on this technique are marketed by ECI Technology andTechnic, Inc. for automatic analysis of acid copper bath. Metrohm offersauto-titrators for potentiometric determination of copper, sulfuric acid,and chloride and estimation of brighteners using cyclic voltammetry.