37

CHAPTER II

LITERATURE SURVEY AND SCOPE OF THE WORK

2.1 COPPER ELECTRODEPOSITION

Copper deposits find extensive applications in nickel [1] and chromium plating as

an undercoat, in printing and electronics industries [2,3]. Of the plating systems that

have been studied only a few have proved to be commercially viable. These are acid

based fluoborate and sulphate solutions, alkaline cyanide and pyrophosphate complex ion

systems. While the acid baths offer 100% cathode efficiency, the alkaline baths excel in

producing fine-grained, smooth deposits without the problem of displacement reaction

along with good throwing power.

2.1.1 Baths used for copper electrodeposition

2.1.1.1 Acid copper bath

Acid copper deposition has been referred since 1810. In 1831, Bessemer [4]

copper plated steel castings by immersion in copper sulphate solution. In 1836 the

Daniel cell was first used for elelectrodepositing copper and a technical report was

published [5]. Most of the efforts dealt with copper sulphate-sulphuric acid solutions but

nitrate [6], acetate [6] and copper chloride [7] solutions were also investigated. In recent

times, the following solutions have been evaluated; sulphate-oxalate [8] copper chloride

[9], copper chloride-sodium thiosulphate [10], iodide and chloride [11], fluoborate [12]

alkane sulfonic acid [13], sulphamic acid [14], copper2+formate with ammonium salts

[15], phosphate-sulfate [16] fluosilicate [17], fluosilicate-silicic acid [18], copper2+

glycolate, lactate, malate or tartrate [19] etc.

38

Copper sulphate and sulphuric acid are the primary constituents of the sulphate

solutions. The concentration of copper sulphate is not particularly critical although the

resisistivity of the solution is greater, when the concentration is increased [20].

G.M.Kimber.et.al. Suggested that cathode polarization increases slightly at copper

sulphate concentrations above 1M [21]. Changes in the concentration of copper sulphate

have little effect on grain size. Chloride ion, in bright and high-throw acid sulfate

solutions, reduces anode polarization [22].

An extensive list of additives used in acid copper plating, have been reported in

technical literature publication include benzothiazole [23-26], cadmium [27], casein [28],

cobalt dextrin [29], dimethyl amino derivatives [29], disulphide [30,31] 1-8-disulphonic

acid [25] disodic-3, 3-dithiobispropane sulphonate [31], 4,5,dithiooctane-1-8-disulphonic

acid [25], dithiotheritol [32], ethylene oxide [31], gelatin [33], glue [28,34], lactose

benzoyl hydrazone [35], 2-mercapetoethanol [36], molasses [28] sulphonated petroleum

[34], 0-phenanthroline [27,37], poly ethoxy ether [38], poly ethylene glycol, [39-41],poly

ethylene imine [40], polyN,N-diethyl saphranin [31], polypropylene ether [42], propylene

oxide [31], sugar [28] thiocarbamoyl-thioalkane sulphonates [43] and thio urea [44-47].

Chloride ion can also be considered as an additive. An increase in bath temperature

results in higher conductivity and reduces anode and cathode polarization. Clear evidence

has been reported of grain refinement produced by increasing the current density [48, 49].

The concentration of addition agent is controlled by many techniques such as

cyclic voltammetry stripping [50-52] cyclic pulse voltammetric stripping [53]

chromatography [33], high-performance liquid chromatography [54-56] differential pulse

polarography [57] and spectrophotometry [58-60]. Mechanical properties of copper

39

electrodeposits [61, 62] such as hardness [63, 64], tensile strength [65, 66], and stress

[67] have also been reported.

2.1.1.2 Cyanide copper bath

The first commercially successful high-efficiency solutions were introduced in

1938. It was extensively used throughout the automotive industry in subsequent years for

plating die-castings as well as steel parts. Copper cyanide solutions may be classified in

three categories, viz., strike, Rochelle and high-efficiency baths. Strike solutions are

used extensively to apply relatively thin coatings of copper as an undercoating [68].

The Taguchi method was used to evaluate a number of parameters in copper

cyanide solutions. An increase in copper concentrations and decrease in potassium

cyanide resulted in good current efficiency over a wide current density range. Hydroxide

and carbonate concentration were of secondary importance [69]. Additional information

on kinetics is also dealt with [70]. The concentration of the free cyanide increases at high

temperatures, since lower complexes are formed and free cyanide is there by liberated.

Alkali is essential for anode corrosion [71].

Additives for cyanide copper system include compounds having active sulphur

group and/or containing metalloids such as selenium or tellurium. Other reported

addition agents include organic amines or their reaction products with active sulphur

containing compounds. An extensive listing of additive used in acid and cyanide copper

prior to 1959 are available in patents [72-85]. Polarisation phenomena in copper cyanide

solution have been extensively investigated [86]. Copper plated from cyanide solution in

highly embritiling to high strength steel substrates because of the low current efficiency

of the solution and the easy entry of hydrogen into steel in cyanide solutions. Cyanide

40

solutions are finding least importance nowadays because of their toxicity, waste disposal

and effluent treatment problems. They are being replaced by non-cyanide solutions.

2.1.1.3 Copper pyrophosphate bath

A number of desirable features have been observed with copper pyrophosphate

bath. It is non-corrosive, non-toxic, with good throwing power and produces smooth to

bright copper deposits with 100 % cathode efficiency. It is commonly used in the

electronic industry for plating of PCB, due to its good throwing power. Though the

cyanide baths are having very good throwing power they cannot be used in the electronic

industry because cyanide is harmful to the plastic laminated boards.

. Organic and inorganic additives such as mercaptothiadiazoles [87-94]

pyrimidnes [95, 96] glycerol, triethanol amine, sodium sulfite [97], naphthalene

disulphonic acid [98], gelatin, bakers yeast, casein, glycol [99] sodium selelnite [96] have

been used as brighteners. The influence of pH, temperature, agitation, and current

density on throwing power and efficiency has been dealt with by B.F.Rothschild [100].

Tam and coworkers [101,102] used cyclic voltammetry stripping technique to study the

effects of all constituents of cathodic and anodic reactions. Structure of deposit [103],

porosity, adhesion [104], physico-mechanical properties of coatings and their application

[105] has also been reported for pyrophosphate bath. Pyrophosphate solutions are

considerably more expensive to make up than high-throw acid copper formulations.

Waste pyrophosphate solutions are difficult to treat because of ammonia in the solutions

and also the complexes that are formed [106]. Hence, these solutions have been almost

entirely replaced by acid copper, except for some military and special applications.

41

2.1.1.4 Copper-EDTA bath

In the case of electrodepositon of copper from EDTA bath, complexation of

copper and effect of ultrasonic agitation on initial process of copper deposition was

studied [107-110]. The properties of copper deposit obtained from EDTA bath was

reported [111]. The influence of dipyridal ion on kinetics of cathodic reaction of copper

from EDTA solution has been studied [112].

2.1.1.5 Alkaline non-cyanide copper bath

Alkaline non-cyanide plating solutions have found increasing popularity since the

mid 1980s because of environmental issue. It is having very good throwing power than

the high-throw acid formulations used in PCB boards. Alkaline copper plating baths

based on tripolyphosphate [113], phosphate [114], ethylenediamine [115-117],

polyethylene polyamine [118], triethanolamine [119], glycerolate [120] alkaline glycerol

[121] have been reported. Disadvantages of alkaline non-cyanide bath (i) include higher

operating costs, (ii) difficulty during plating of Zinc die-castings, (iii) greater sensitivity

to impurities and (iv) more difficult chemistry for controlling purposes [122].

2.1.1.6 Copper methane sulphonate bath

Deposits from sulphonate bath are similar to those from sulphate bath but are

useful for high current density operations. The copper methane sulphonate was obtained

by dissolving copper carbonate in pure methane sulphonic acid. However, the literature

available on this bath is scarce, not much work has been carried out on this bath.

2.1.2 Application of Copper electrodeposits from various bath

Electrodeposition of copper from acid solution is extensively used for

electroforming, electrorefining and electroplating. Acid copper sulphate solutions are

42

widely used for plating of printed wiring boards and for semiconductor interconnect

technology. Electroformed copper articles include band instruments, heat exchangers,

reflectors and a variety of articles for military and aerospace applications [123]. Zinc

die-castings are plated with approximately 15 micron of leveling acid copper [124] before

nickel and chromium plating, to eliminate buffing before plating. Metal powders

produced by deposition in acid solutions are used for making sintered compacts and

pigments [125]. IBM researchers have successfully implemented copper electroplating

technology for the fabrication of chip interconnecting structures [126,127].

Electroforming with acid copper is used as one of the primary methods of fabricating the

outer shells of regenerative cooled thrust chambers for advanced design rockets and other

applications. High-speed copper plating has been used for commercial applications such

as electroforming of flexible printed circuits [128], in mold plating of polymeric

materials [129].

Deposits obtained from cyanide baths are used as undercoats for other deposits (i)

to protect basis metal or promote adhesion, (ii) for surface improvement in buffing, (iii)

to improve soldering operations (iv) to improve lubricity and (v) on printing rolls. A

high-speed cyanide copper-plating process is used to produce heavy coatings of copper

on steel wire for electrical application [130].

The production of electroformed objects such as wave-guides, paint spray masks,

helical antennae [131], high strength deposits are produced from pyrophosphate bath.

Other applications include, plating on zinc die castings before nickel and chromium

plating on steel for selective hardening operations such as nitriding and carburizing [132],

and in roll plating [133]. Pyrophosphate solutions are also used for plating through holes

43

on PCB, although in most cases this application has been replaced by high throw-acid

copper sulfate formulations.

2.2 LEAD ELECTRODEPOSITION

Lead has high hydrogen over potential. The values for the hydrogen over

potential are dependant on the structure and surface of the electrode, as varying between

0.84 V for 99.8 % pure lead [134] to about 1.2 V for electrolytically deposited coatings

[135]. Electrodeposited lead has good ductility and higher hardness then metallurgical

lead. In addition, lead plating is superior to lead dipping since objects can be stamped

and machined without distortion and there is a minimum amount of stress present in

electrodeposited lead coating. The principle uses of electrodeposited lead are for the

protection of accumulator clips, and terminals horticultural spraying equipment, fire

extinguisher bodies, and small items of chemical plant that are subjected to corrosion by

sulfuric acid. Electrolytic deposition of lead has been investigated using acid alkaline and

neutral solutions. An important perquisite for such a process is a high solubility of the

lead salts to be used and stability of the anion against hydrolysis in the electrolyte.

2.2.1 Electrolytes used for lead electrodeposition

2.2.1.1 Per chlorate Electrolyte

Per chloric acid electrolytes are very suitable for electrodeposition of lead because

of the high solubility of lead is such electrolyte and the high electrical conductivity of the

solution. However such electrolytes are not widely used in practice, partly because of the

high price of the acid and its hazardous nature which can lead to the high risk of

explosion, if wood or paper becomes saturated with it [136,137].

44

2.2.1.2 Amido sulphonate Electrolyte

Electrodeposition of lead from amido sulphonic acid, based electrolytes is also

reported in literature [138,139]. It is used for electrolytic refining of raw lead. One

problem with the use of this electrolyte is the tendency for sulphamate anion to get

hydrolyzed to ammonia and sulfate [140], which cause the precipitation of lead sulfate

forming huge amount of sludge.

2.2.1.3 Fluorosilicate Electrolyte

It was developed for electrolytic refining of lead by a process known as the Betts

process. It is suitable for production of raw lead. It is more stable and economical than

sulphate electrolyte. The kinetics of lead deposition in fluorosilicate electrolyte has been

widely studied [141-143], although the fluorosilicate solution is difficult to prepare and is

somewhat unstable.

2.2.1.4 Fluoborate Electrolyte

The fluoborate electrolyte is the most common process for the electrodepositon of

lead [144]. It is stable lead salt, which is very soluble in water. Concentrated solutions

are used for barrel plating and the continuous plating of mild steel sheet. A number of

addition agents, mostly organic compounds, have been used to prevent treeing in the

deposit, arrest crystal growth and improve appearance. Extensive experimentation was

carried out by Graham and Pinketron [145,146] to select suitable additive for the lead

deposition. The kinetics of lead deposition was studied by Thomas.M.Tam [147].

In the fluoborate electrolytes temperature will increase the limiting current density

and lead to coarser grain structure to a greater or less extent, depending on the grain

refiner used in the electrolyte [148,149]. Graham and Pinketron [150] investigated the

45

influence of cathode rotation on the limiting current density. The properties of

electrodeposited lead from fluoborate electrolytes such as hardness, eleongation, and

tensile strength are given in the literature [151]. The density and electrical resistivity

have been measured [152]. The corrosion resistance of the electrodeposited lead coatings

has been investigated by Graham.et.al [153] in atmospheric exposure tests in several

industrial, rural and marine climates.

2.2.1.5 Sulphamate and acetate Electrolytes

Alternatives to electrodepositing lead from non-fluoborate systems are

sulphamate and acetate. Certain basic parameters for lead sulphamate have been

established [171]. Sulphamate electrolytes are used to obtain a compact lead deposit of

required thickness [154]. The properties of the solution and the conditions for

electrodeposition of lead from sulphamate solutions were studied using polarization

behaviour of the system [155]. The main disadvantage of sulphamate bath, the

hydrolysis of sulphamate resulting in an insoluble lead sulfate made it unsuitable for

continuous process.

Electrodepositon of lead from acetate electrolyte in the absence and in the

presence of chloride ions was investigated [156]. A highly adherent and smooth deposit

of lead has been obtained on steel substrate from acetate electrolytes containing hexa

decyl pyridinium bromide and sodium dodecyl sulphate [157].

2.2.1.6 Alkaline Electrolytes

The two most usual alkaline electrolytes used for the deposition of lead are the

pyrophosphate and plum bite process [158]. In the alkaline solution the lead ion is present

in a complexed form. The electrodepositon has therefore a higher over potential

46

compared to acidic solution. Hence it can be operated without inhibitors and show a

high-throwing power. It is less corrosive against the plants and equipment. The main

disadvantages are process-engineering problems. Hence no commercial use has been

made of this bath.

2.2.2 Application of lead electrodeposits

Lead is commonly used in the chemical industry, lead battery technology, piping,

roofing solders etc. mainly because it undergoes limited corrosion in many media [159]

such as sulphuric acid, chromic acid, and phosphoric acid. The surface of lead coating is

covered by a tightly adhering layer of lead sulphate, which protects the underlying metal

against any further attack by the sulphuric acid. Lead plating on aluminium and its alloys

may find considerable importance in the lead acid battery industry where plated

aluminium sheet can be used in place of conventional copper sheet. This results in a

considerable economic savings [160].

Some new application for lead deposition has been developed in the field of

electronics and component manufacture in recent years. In PCB manufacture a layer of

lead is deposited to act as etch resist. Bright tin or matt tin-lead coatings are reflowed

and used as metal resists [161]. In comparison to tin or tin-lead alloys, lead has the

advantage that no intermetallic layers can be formed. Deposition of thin undercoating of

pure lead under a tin-lead layer on reflowed PCB’s improves their solderability. The use

of electrodeposited lead layers for application in the field of fabrication of electrochromic

devices and semiconductor devices is described in the literature [162,163].

47

2.3 TIN ELECTRODEPOSITION

Tin is useful for its resistance to corrosion and tarnish, for its nontoxic nature,

and for its solderability, softness and ductility; it owes most of its acts as a coating to one

or more of these characteristics. Tin is employed as a coating on refrigerator evaporators,

dairy, and other food-handling equipment, washing machine parts, hardware, radio and

electronic components, electrical lugs and connectors, copper wire for tele

communications and piston rings. There are four basic choice of electrolytic plating

processes that can be used to deposit tin viz., acid fluoborate, acid sulfate, acid

sulphonate, neutral gluconate and alkaline stannate.

2.3.1 Electrolyte used for tin electrodeposition

2.3.1.1 Fluoborate electrolyte

It is one of the acid-electroplating baths usually used in high-speed plating [164-

166]. Tin fluoborate is highly soluble; therefore it permits the use of high current

densities. Deposition in the absence of organic addition agents gives mostly porous,

coarse and poorly adherent coatings. Formation of needle, whiskers and dendrites cause

short circuit in electronic and related industries [167,168]. Organic additives most

commonly used are peptone [169], gelatin [170], beta naphthol [171], catechol and

hydroquinone [172], the last two are generally used as anti oxidants. The mechanisms of

inhibition for Sn (IV) formation by antioxidants and mechanism of additives has been

summarized in the literature [173-175]. With the above-mentioned additives smooth-fine

grained deposits obtained [176]. Rob schetty studied the influence of several variables of

the electroplating chemistry on tin whiskers growth [177]. The major disadvantages of

48

fluoborate system are the environmental concerns for the fluoride and the high host of

waste treatment. It is also the most corrosive of all acidic tin electroplating solutions.

2.3.1.2 Sulphate/sulphuric acid electrolyte

Sn (II) sulphate electrolytes are gradually replacing the fluoborate electrolyte.

The characteristics of this chemistry are its high cathode and anode current efficiency

under normal operating conditions. It is important to avoid adding too much sulphuric

acid since concentration exceeding 400 g/l decreases the conductivity of the solution

[178]. Deposits ranging from matt, semi-bright to bright can be obtained from sulphate

bath using addition agents [179]. The effect of non-ionic addition agents on

electrodeposition of tin from sulphate has been studied [180]. Solderability of bright tin

deposit has been studied [181,182]. Various organic additives such as surface-active

agents [183-186], aromatic carbonyl compounds [187,188] and amine- aldehyde reaction

products [189] are used in the plating solution to modify the deposit properties and the

kinetics of tin deposition. Tin electrodeposition process has been studied from sulphate

bath using electrochemical and morphological (nano-micrometric) techniques [190]. In

bright tin-plating, chloride concentration is kept as low as possible. Higher chloride

concentration causes a decrease in brightness of the deposit [191].

2.3.1.3 Phenol sulphonic acid (PSA) electrolyte/Halogen electrolyte

These two baths are mostly used to plate tin in the continuous steel strip plating

industry. An attractive feature of these two processes is their ability to operate at very

high current densities, which is very important in the steel industry. The PSA process

used with either soluble or insoluble anode, where as halogen process utilizes with

soluble anode only. S.Hirano et.al [192] studied the effect of current density, Sn (II)

49

concentration and flow rate on tin plate characteristics. Yung-Herng Yari et.al. [193]

compared the electrodeposition of tin between halogen and sulphonate bath. Sludge

formation in a major problem in halogen bath where typically more than 20 % of tin goes

to sludge. Another problem is related to the waste treatment of the sludge because of

ferro-ferric cyanide hazardous waste. Compared with halogen process PSA process

produces much less sludge, due to its anti-oxidizing power. A draw back of PSA process

is the toxicity of the phenolic group, which is released during electrodeposition.

2.3.1.4 Alkaline tin plating electrolyte

Alkaline stannate process is based on either sodium or potassium stannate. For

high speed plating applications the potassium stannate is used because of its very high

solubility. Anode efficiencies in the range of 75 to 95 % and cathode efficiencies in the

rage of 80 to 90 % are typical for the alkaline process [194,195]. It has superior throwing

power. It does not require the use of organic addition agent but must operate at elevated

temperature. In this bath the critical need is the proper control of the anode. If the tin

anodes are not controlled during deposition rough, porous deposits will result. Figure 2.1

shows the characteristics of tin as anode in an alkaline solution. Starting at A and

increasing the anode current density along the line AB and voltage increases slowly: The

anode is dissolving as stannite and is grey in appearance. It may be covered with a gray-

black smut, which rubs off easily. Plating in this range is entirely unsatisfactory.

When the current density reaches the point C, the voltage rises suddenly along the

line CD and anode acquires a yellow-green film and begins to dissolve as stannate. If, the

current density is maintained at C or increased further along the curve DE, the film turns

black and the anode become passive, oxygen is evolved and behaves as insoluble anode.

50

Having reached point D on the curve, if the current density is decreased, the current-

voltage curve will be found not to retrace itself; instead it follows the dashed line DF.

The yellow-green film on the anode persists, and dissolves in the desirable stannate form.

The portion DF is therefore the proper operating range, which can be reached only by

first impressing on the anodes the critical current density C and then decreasing the

current density to desired value. If the current density is further decreased, the curve

follows the line FG, and the anode film becomes lighter at about G the film is lost

entirely, and the tin is dissolving as stannite.

2.3.2 Application of tin electrodeposits from various bath

Tin deposits are widely used in the production of food processing and shipping

equipment and containers pump parts and automotive pistons, copper and steel wire for

telecommunication and in plating electrical components and printed wiring boards. Tin

is appealing to the food processing industry because tin is a non-toxic, ductile and rather

than corrosion resistant metal. Thicker deposits up to 30 micron are generally required in

food processing equipment and shipping containers.

Although tin is used primarily for corrosion resistance to sulfur containing

atmosphere it is often used where a solderable ductile finish is required on small chassis

parts, brackets and on contact areas subjected to hydrogen sulfide contamination and to a

minor extent for decorative purposes. Tin has excellent anti-scuffing properties, and is

widely used in engineering to reduce fretting corrosion and to combat the effects of

abrasion. Lubricants adhere well to tin and tin coatings can thus help in reducing friction

between sliding components. Thick deposits [50 to 250 micron] are electroplated on

pump parts and piston rings.

51

Tin is widely used in electronic industry because of its ability to protect the base

metal from oxidation. Tin thus preserves solderability of the base material, a critical

requirement when plating for electronic applications. Electrodeposited tin needle like

structure have applications as novel cathodes in the field of vacuum microelectronics.

Figure 2.1 Anodic behaviour of tin in alkaline solution

52

2.4. Electrodepositon of lead and tin from Methane Sulphonic Acid bath (MSA)

In the 1940s proell [196] recognized the utility of alkane sulphonic acids for

electroplating applications. Alkane sulphonic acids, having between one and five carbon

atoms in the alkyl group form water soluble salt of various metals (mesylates) [197].

Proell indicated that it is possible to plate many metals from alkane sulphonate baths

including cadmium, lead nickel, silver, zinc and tin.

Although the usefulness of methane sulphonic acid systems was known for

several decades, it only gained importance during the early 1980’s and became a

desirable electrolyte for tin and lead plating [198]. MSA chemistry shows clear

advantages over fluoborate, sulphate, halogen and phenol sulphonic acid chemistry. It is

less corrosive than fluoborate and sulphate chemistries and it is less costly than

fluoborate, phenol sulphonic acid and halogen chemistries for effluent treatment and

disposal. It is environmentally more friendly [199]. The ‘green character of MSA

especially as compared to fluoboric acid and fluorosilicic acid is derived from several

factors. In general MSA is less toxic than fluoboric and fluorosilicic acid. Fluoboric acid

and fluorosilicic acids have lachrymatory properties and both acids evolve HF. MSA

solutions do not, under normal conditions, evolve any dangerous volatile chemicals. In

general, the low toxicity of MSA, especially when compared to HF complex acids, makes

it a safe electrolyte to handle. MSA is considered readily biodegradable ultimately

forming sulfate and carbon dioxide. In fact, MSA is considered to be a natural product

and MSA is part of natural sulfur cycle [200,201].

53

An additional attractive feature of MSA chemistry is its ability to access very high

current densities [202]. The throwing and covering power of this chemistry are adequate

to applications such as PWB and connectors with restricted areas [203]. Insoluble anodes

can be used in MSA based plating baths.

The use of MSA for deposition of lead was first mentioned in a patent by Proell in

1950 [204]. Nobel Fred [205] described tin and lead plating baths that remained stable

over a broad pH range. The design of acid electrolytes for the electroplating of lead and

tin is dominated by solubility, performance and environmental considerations. Two

important modern technologies depend integrally on the aqueous electrochemistry of

lead: these are the electrodeposition of tin-lead solder in the electronic industry and the

electrodeposition of lead on aluminium, which has been developed for bus bars of

accumulator lead acid batteries. Only certain acid electrolytes will allow a high aqueous

solubility of lead (II) ions. These electrolytes include Methane sulfonic acid, fluoboric

acid, fluorosilicic acid, nitric acid, perchloric acid, acetic acid and dithionic acid. The

aqueous saturation solubilities of the relevant lead salts are shown in the Table 2.1

The methane sulphonate ions form less stable complexes than fluoborate,

fluorosilicate and associated fluoride ions [206]. The complexing nature of the

fluoborate, fluorosilicate and fluoride anions make it difficult to efficiently remove metal

ions (e.g. Pb2+) from effluent streams. Basically, the residual level of lead left in an

effluent after a caustic precipitation operation is directly related to the complexing ability

of the anions present in the effluent. The residual level of Pb(II) left in solution after

optimized caustic precipitation of lead from MSA(aq) based Sn/Pb plating solution is less

than 1 ppm. While the residual level of soluble Pb (II) left in solution after caustic

54

Table 2.1 Saturation solubility of some lead salts

Lead salts Aqueous saturation solubility

Methane sulphonate

Fluoborate

Fluorosilicate

Nitrate

Per chlorate

Chlorate

Acetate

Dithionate

143 g /100 g water (25oC)

50 wt % (20 oC)

69 wt % (20oC)

37 wt % (25oC)

226 g / 100 g water (25oC)

225 g /100g water (25oC)

55 g/100 g water (25oC)

115 g/100 g water (25oC)

55

precipitation of lead from HBF4 (aq) based solutions is typically more than 10 times

higher than the former [207]. In total, the lower toxicity and greater ease of waste

treatment of MSA based solutions makes MSA the ‘green’ choice for commercial

electrolytes, especially those containing lead. The suppression of lead electrodepositon by

organic compounds in MSA bath has been studied [170].

In tin electrodeposition the oxidation of stannous to stannic is very less in

MSA based plating formulation due to its less oxidizing power compared to sulphuric

acid [208]. The effect of process variables like agitation, current density and temperature

on plating efficiency and coating morphology were examined in high-speed tin

electrodepositing process based on MSA electrolyte [209]. The electrochemical

dissolution behavior of tin in MSA solution was also studied [210]. The kinetics of tin

electrodeposition in presence of organic surfactants [211] and the role of glycol type

additives in electrochemical and morphological changes [212] were studied from MSA

based plating formulations. C.Lizzul Rinne et.al reported that electrodeposition of tin

needle like structure can be produced from MSA bath using ethoxylated surfactant and

Pb (II) salts [213]. The uses of gallic acid with the multiple rolls of antioxidant, grain

refiner, selective precipitation and selective chelator in MSA based tin-plating bath have

been patented by Genon Michael.D et.al. The initiation growths of tin immersion coating

on copper substrate from MSA bath proceed by the formation of tin nano-crystals [214].

Effects of sulphate [215] and chloride [216,217] ions are studied in acid methane

sulphonate electrolytes.

56

MSA has largely replaced fluoboric acid as the electrolyte of choice for

the elelectrodeposition of tin, lead on electronic devices. These changes have been driven

by the superior functional properties and environmental advantages of MSA [218].

2.5 Application of Methane sulphonic acid (MSA) in various processes

MSA offers all the qualities sought by today’s increasingly demanding chemical

industry and finds application in several areas.

Metallurgy

MSA has been described for a variety of metal stripping processes. It has been

used to design selective electrochemical stripping procedures that only dissolve one metal

while they leave the other unaffected [219, 220]. MSA can be used to regenerate cation

exchange resins. The acid can be recovered by electro winning to remove the dissolved

metal.

The feasibility of MSA for metal stripping, ion exchanger regeneration and

electro winning processes makes it a powerful tool for metal recycling. MSA triggers

innovation in aluminium surface treatment. It has been used in a two-step process for

anodizing and colouring of aluminium [221]. Moreover, it can be used for electro

polishing of aluminium.

Batteries

Based on the table of electrode potentials, the cerium ion is expected to be a good

candidate for a high-energy electrolyte in a battery. Still, the low solubility of the cerium

sulfate has limited its use in this application. MSA is used advantageously. The

construction of a high-energy density zinc/cerium battery has been made possible through

the identification of MSA as an electrolyte for this system.

57

The familiar lead/acid battery operates with sulfuric acid and the source of lead

(II) is insoluble lead sulfate. The electrode reactions involve the conversion of one solid

phase to another, thus reducing the performance of the battery. Using MSA allows the

concept of a battery with a lead (II) soluble in the electrolyte, using carbon electrodes and

no cell separator [222,223]. The soluble-lead (II) battery has the potential to serve as a

large storage battery in electricity networks.

Advanced materials

MSA is particularly attractive for surface modification reactions and solid-state

reactions because it is at the same time a nonoxidizing and a strong acid. Moreover, it

forms highly soluble salts. It is the combination of these properties that allows the use of

MSA in innovative processes leading to high-performance materials.

Alumina-based, sprayable cement can be partly neutralized with MSA [224].

Rganophilic clays have been made by partly neutralizing clay with MSA [225].

Crystalline aluminosilicates such as zeolites may be readily dealuminated-using MSA to

provide catalysts or catalyst precursors. A high degree of aluminum removal with

minimal loss of crystallinity is possible even when an organic template is not present in

the aluminosilicate [226].

MSA offers great potential for new polymer-based materials. Methane sulfonic

acid has been shown to improve the strength of reinforcement fibers for composites

[227]. For this purpose, both carbon fibers and polyaramide fibers can be treated with

methane sulfonic acid. Polyaramide fibers may also be directly spun from methane

sulfonic acid. Aromatic high-performance Polymers such as polyether ketones are soluble

in methane sulphonic acid [228] and may be processed from such solutions. This opens a

58

pathway to the use of these polymers in applications requiring thin cast layers. MSA

allows post-cross linking of vinyl polymers in macro porous polymeric adsorbents [229].

MSA is a technical standard as catalyst in the synthesis of formaldehyde resins

and other condensation resins. MSA has been discussed as a catalyst for the synthesis of

silicones [230]. Cataphoretic dip coats based on epoxyamine-isocyanate resins may be

neutralized with methane sulphonic acid. MSA is useful in lead-free cataphoretic dip

coats where it serves as a counterion for cations such as vanadium, manganese, iron, zinc,

zirconium, silver, tin, lanthanum, cerium or bismuth without impairing their solubility,

like other acids would do [231].

Methane sulphonic acid has been used to modify electrically conductive polymers

and to prepare composites [232] or to cast films or fibers of them [233]. Optical light-

emitting devices (OLED’s) are based on thin luminescent polymer films. Such films have

been spin-coated from MSA solution [234].

Ionic liquids based on quaternary ammonium salts and methane sulphonate are

discussed as non-toxic solvents for chemical synthesis [235] and as lithium-conducting

electrolytes [236] for lithium batteries. The removal of MSA from a reaction mixture as a

liquid salt allows easy working-up of the reaction mixture by phase separation [237].

59

2.6 SCOPE OF THE PRESENT WORK

Methane sulphonic acid (MSA) based electrolytes are being actively investigated

as environmentally superior alternatives to the currently used system such as fluoborate

and sulphate. The primary advantages for MSA based plating process are (i) less toxicity

and biodegrability (ii) higher metal ion solubility (iii) Wider operating window (iv) the

ease of treating MSA effluents (v) higher conductivity and (vi) possible to use insoluble

anode. These salient features of MSA electrolytes makes it as a green choice for

electrodeposition of silver, nickel, chromium, cadmium, zinc, copper, lead, and tin

metals. Among these metals copper, lead and tin has numerous application in electronics

and engineering fields. Hence the objective of this study is adopting MSA electrolyte for

copper, lead and tin electrodepositon process.

Copper is commonly electroplated from acid sulphate and cyanide baths. The

cyanide baths is commonly used in the commercial plating, but it has its drawbacks, viz.,

(i) its poisonous nature,(ii) costly effluent treatment method . Normally the acid sulphate

bath has 100 % cathode efficiency. Deposition of copper from methane sulphonate bath is

similar to those from sulphate bath, but it is useful for high current density plating

operation. The methane sulphonic acid baths have high conductivity, and are less

corrosive than sulphate bath. It can be operated at room temperature. However,

literature survey shows that, not much work has been carried out on this bath. Hence

attempt has been made to carry out copper deposition using MSA bath and the results are

compared with sulphate system.

Since a long time fluoborate electroplating baths have been used to commercially

60

plate tin and lead. Alternatives to electroplating from non-fluoborate systems have been

sought such as fluorosilicate, gluconate, sulphamate and sulfate. Sulphamate lead bath,

due to its hydrolysis resulted in the formation of sulfate ions which precipated and made

the bath commercially unsuitable. The less conventional fluorosilicate, gluconate and

pyrophosphate baths are employed mainly for lead, tin, plating. They are however, more

expensive than the fluoborate based electrolytes. Fluoborate baths, though most widely

used, have disadvantages such as high corrosivity owing to oxidation of Sn2+ to Sn4+ in

tin baths and environmental contamination due to fluoride ion. Nowadays the methane

sulphonic acid bath is quickly replacing the fluoborate bath mainly because of

environmental concern. The deposits obtained from MSA baths are highly adherent, and

less porous, which are important factors in the tin plate and electronic industries.

However, literature survey shows that less work has been done on the electrodeposition

of lead, and tin using MSA bath. Hence attempt has also been made to carry out lead, tin

electro deposition using MSA baths and all the results are compared with fluoborate bath.

To start with, Hull-cell studies have been carried out to fix the maximum

operating current density for the deposition of copper on brass from the MSA and

sulphate bath. The effect of various additives such as Gelatin (GE), Poly ethylene glycol

(PEG), Octyl phenoxy poly ethoxy ethanol (Triton-X-100), peptone on the process of

lead and tin electrodeposition on steel from MSA bath has also been studied.

The current efficiency and throwing power of MSA bath have been measured for

copper, lead, and tin deposition, which is compared with sulphate and fluoborate system

respectively. The polarizations behavior of copper, lead and tin electrodeposition from

MSA bath has also been studied.

61

Cyclic voltammetry has been used to study the reaction mechanism of copper, tin,

lead deposition from MSA bath, sulphate (for copper) and fluoborate (for lead, tin) bath.

The corrosion resistance of the deposits obtained from MSA bath has been studied by

potentiodynamic polarization method, and electrochemical impedance method, and

compared with deposits obtained from sulphate and fluoborate bath. The corrosion

behaviour of copper, tin and lead, in MSA was studied by self-corrosion method. The

samples were subjected to various tests to asses properties of deposits such as, adhesion,

porosity, hardness and solderability (for tin deposition). The surface morphology of the

deposits has been obtained from SEM photomicrograph. The phase orientation of the

deposits has been ascertained using XRD.

Based on the results of all these studies, a comparative study has been made on

the electrodepositon of copper, tin and lead from MSA electrolytes with other

commercially adopted electrolytes. The study indicated the superiority of MSA bath

compared to sulphate bath for copper deposition and fluoborate bath for lead and tin

deposition.

62

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