Electrodeposition of Sn

8/18/2019 Electrodeposition of Sn

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Electrodeposition of Sn-Zn-Cu alloys fromcitrate solutions

Abstract

The frst stage o this study involves the development o stable baths or

electrodepositing Sn-Zn-Cu alloys; these developments are based on thermodynamic

models o citrate baths and experiments exploring the stability o the baths. The eects

o the sodium citrate !complexation agent" concentration and the p# o the solution on

the stability o the baths $ere examined experimentally. The stability o the baths $as

determined through spectrophotometric analysis. Stable baths designed or the

electrodeposition o Sn-Zn-Cu alloys $ere produced in the range o dominant citrate

complexes $ith highly negative charge !the reduction process is strongly inhibited by

high activation energy". %oltammetric studies and potentiostatic deposition $ere

conducted to analyse the co-deposition o tin& 'inc& and copper& and the co-evolution o hydrogen. The eect o the solution p#& the concentration o sodium citrate and the

hydrodynamic conditions on the electrodeposition process and the composition o the

deposits $ere examined. The deposits $ere analysed using chemical !()*+" as also

grain si'e and phase analysis !)-ray diraction". The possibility o electrodepositing Sn-

Zn-Cu alloys rom citrate solutions $as confrmed. The tin content o these coatings

varied rom , to , $t./& the copper content varied rom 0.1 to 2.1 $t./& and the

'inc content reached up to approximately 03.1 $t./.

4ey$ords

• electrodeposition;• Sn-Zn-Cu alloys;

• lead-ree solders

5. 6ntroduction

Sn-Zn-Cu alloys are o great interest because they may be used industrially as lead-ree

solders& in 7esterite-based solar cells& because Cu2ZnSn!S&Se"8 is an intermediate

product during the synthesis o 7esterites& as negative electrodes in lithium-ionbatteries& and as corrosion resistant layers !combining the properties o brass and

bron'e". The composition o the alloys varies bet$een applications. SAC !tin-silver-

copper alloys" and eutectic Sn-Ag alloys have been used to replace solders containing

lead or many years. These materials have good physicochemical properties& but they

are di9cult to obtain through electrodeposition or electroless deposition methods;

thereore& tin is currently the primary material used or the electrodeposition o solder

layers :5. Sn-Zn eutectic alloys are among the most attractive lead-ree solder

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stable Sn!66"-Zn!66"-Cu!66"-citrate baths to confrm that the desired Sn-Zn-Cu deposits

could be obtained.

2. ?xperimental

The chemical compositions o the solutions used $hile studying the stability o thebaths and ta7ing electrochemical measurements are given in Table 5. The optimal

ranges or the p# and concentration o the complexation agent $ere chosen based on

the analysis presented in sub-Section 0.5. All chemicals used in this $or7 $ere

analytical grade. Beionised $ater !ultrapure& resistivity 5


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euilibrium lines in +ig. 8 or the reduction processes $ith metals!66" !Cu!66"QCu&

Sn!66"QSn& Zn!66"QZn" are calculated under the assumption that the species are reduced

to orm pure metallic phases !Cu& Sn and Zn". This assumption is accurate or the Zn-Sn

system& but the Cu-Sn-Zn system also contains intermetallic phases and solid solutions&

slightly altering its euilibrium lines.

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The compositions o the solutions given in Table 5 $ere assigned based on an analysiso the thermodynamic models. The baths $ere clear and blue ater preparation; nodeposits $ere observed in the solution. The stability o the baths $as examinedspectrophotometrically at specifc intervals. The absorbance o the baths $asmeasured rom 2 to 55 nm. The most interesting results are sho$n in+ig. 1. Theintense pea7 bet$een 5 and 11 nm is attributed to copper !66" species !blue colouro solutions". +ig. 1 a& b and c sho$ the relationship bet$een the concentration o thecomplexation agent and the stability o the bath in solutions


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deposits. Joth o these relationships are attributed to an Sn!66"-mediated Cu!66"reduction to metallic copper. 6ncreasing the p# beyond 1.31 destabilises the solutionsthrough a slo$& systematic precipitation o a $hite deposit over several $ee7s. The )-ray diraction analysis reveals that the ma@or component o this deposit is tin!66" oxy-hydroxide !Sn,U8!U#"8" !GCIBS 8,-58


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o copper!66" is strongly inhibited due to the high overpotential o the reduction rom

complexes $ith such high negative charges. Similarly& $hen the copper citrate

complexes are reduced by the tin!66" complex& the charge transer process reuires

complexes $ith negative charges to approach one another. Thereore& the activation

energy or the charge transer process is greater $hen complexes $ith a high negative

charge are present& inhibiting the reduction o Cu!66" species. Signifcant amounts o Cu!66" species occur as sulphate complexes !:Cu!SU8"n2V2n" in solutions $ith lo$

complexation agent concentrations and p# values. The reduction o these complexes

proceeds easily& particularly or neutral :CuSU8!a" species. Consuming these complexes

via reduction causes their regeneration rom the citrate complexes $ith high negative

charges. The Cu!66" reduction is slo$& but not inhibited. Yhen the sulphate complex

concentration decreases due to increases in the concentration o the complexation

agent and p# o a solution& the highly charged citrate complexes are only present in

solution& inhibiting the reduction o copper !66" and stabilising the baths over long

periods.

0.2. %oltammetric studies

+ig. , sho$s the cyclic voltammograms in citrate solution !.,1 " or 'inc!66"& tin!66" and

copper!66" separately. Curve !5" depicts the voltammograms received rom a solution

containing only sodium citrate. The sharp increase in the cathodic current density or

potentials belo$ -5 % vs SC? is attributed to hydrogen!6" reduction o a dominate

protonated citrate ion& according to reaction *5 $hen the current is belo$ -5.1 % vs

SC?L

+urther& a very sharp increase o cathodic current density !belo$ -5.31 % vs SC?" occursdue to the evolution o hydrogen and is related to the decomposition o $aterL

2 # 2 U X 2 e V # 2 X 2 U # V ! * 2 "

Turn athGax on

The reduction o 'inc!66" begins at approximately -5.8 % vs SC? !+ig. ,& curve 2". Iea7

!a" indicates that the oxidation o 'inc begins at approximately -5.5 % vs SC?. The

beginning o the tin !66" reduction occurs at approximately -.< % vs SC? ! +ig. ,& curve

0"& $hile oxidation o tin starts at a potential o about -.3 % vs SC? !pea7 b". The

reduction o copper!66" starts at a potential o about -.8 % vs SC? !+ig. ,& curve 8".Uxidation o copper is invisible in the voltammogram because this process begins at

high potentials& $hich may oxidise the copper electrode !substrate in the voltammetric

study". All o the reduction processes or the metal!66" species are shited to$ard lo$er

potentials relative to the euilibrium potentials !+ig. 8"& $hich enables the reduction o

complex ions $ith high cathodic overpotentials !.0 to .1 %".

%oltammetric studies sho$ electrochemical instability or the cathodic pea7s in

negative potentials& $hich is connected $ith hydrogen evolution. The hydrogen !6"

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reduction o protonated citrate ions is inhibited by a tin layer& and by an

underpotential-deposited 'inc monolayer on the copper electrode& agreeing $ith the

higher cathodic overpotential observed or the hydrogen evolution on the tin and 'inc

suraces than on the copper surace !Tael \a] parameter is eual -5.28 % vs W#? or tin

and 'inc and -.33 to -.


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The voltammetric studies reveal that the electrodeposition o an Sn-Zn-Cu alloy is

possible $hen using citrate baths. The deposition processes or tin& 'inc and copperbegin in accordance $ith the order o their euilibrium reduction potentials !Cu!66"QCu&

Sn!66"QSn& Zn!66"QZn& +ig. 8"; thereore& this type o co-deposition is normal& according to

the Jrenner classifcations :02.

0.0. 6n>uence o the hydrodynamic conditions on electrodeposition

The eect o the *B? speed on the electrodeposition o tin rom stable citrate solutions

containing Cu!66"& Sn!66"& Zn!66" species is sho$n in +ig. a. These investigations utilised

rotation rates o 51 and ,< radQs or the cathode $hile using a number electrolyte.

The partial current density or Sn electrodeposition remains independent o the rotationrate at potentials rom -5. to approximately -5.0 % vs SC?. The limiting current density

can be observed in this range o the potential. The electrodeposition process $as also

carried out at a constant -5.0 vs SC?& $hen the rotation rate o the cathode ranged

rom 5 to ,< radQs and electrolyte number $as used to determine the limiting

current !+ig. 5a". The partial current densities obtained at -5.0 % vs SC? are

independent o the rotation rate o the cathode. Thereore& the rate o the tin!66"

reduction in this potential range is limited by the dissociation o the dominant tin!66"

citrate complex !SnCit2V" according to *0 and *8L

S n C i t 2 V X #X

_ S n # C i tV

! * 0 "

S n # C i t V X 2 e V X # X S n X # 2 ! * 8 " C i t 2 V

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The next limiting current density can be observed belo$ -5.8 % vs SC?.

?lectrodeposition $as also carried out at a constant -5. % vs SC? $hen the rotation

rate o the cathode ranged rom 5 to ,< radQs at p# O 1.1 !number " to determine

the limiting partial current density o tin !+ig. 5a". The tin partial current densities are

linearly related to the suare root o the rotation rate at -5. % vs SC?& confrming that

the limiting current has a diusional nature and is connected to the direct reduction o

the dominant tin !66" citrate complex !SnCit2V

" according to reaction *1LS n C i t 2 V X 2 e V X 2 # X S n X # 2 ! * 1 " C i t 2 V

Turn athGaxon

The diusion coe9cient o the Sn!66" species $as calculated based on ^evichKs

euation& generating a value o ,.0 5V3 cm2 sV5. This value is about one order o

magnitude lo$er than or stannous ions !,.1`5V3 cm2 sV5:08"& but this diusion

coe9cient is determined mainly through the dominant citrate complex !monomeric

SnCitV2 complex". The value determined or the diusion coe9cient is closer to values

or dimeric citrate complexes !point ?N on the +ig. 55" than or monomeric citrate

complexes !point ? on the +ig. 55"; thereore& the ormula o this complex should beSn2Cit28V. This conclusion reuires additional investigation. The Sn content in the

deposits increased slightly $hen the speed o the *B? $as increased !+ig. 52".

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The relationship bet$een the partial current density and the *B? rate or 'inc

deposition is presented in+ig. b. The partial current densities are independent o the

rotation rate o the cathode& even at -5. % vs SC? !+ig. 5b". Throughout the

investigated range o potentials and disc rotation rates& the 'inc electrodeposition

process is controlled by the activation o the 'inc!66" reduction rom the dominate

citrate complex !Zn#CitV"& according to reaction *,L

Z n # C i t V X 2 e V X # X Z n X # 2 ! * , " C i t 2 V

Turn athGaxon

The amount o Zn in the deposits decreases or electrodeposition at higher rotation

rates and at more strongly negative potentials !+ig. 52" $hen the electrodeposition o

Sn and Cu is under diusion or mixed control.

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+ig. c demonstrates the relationship bet$een the partial current density and the rate

o the *B? during copper electrodeposition. T$o limiting current densities can be

observed& similar to the reduction o tin!66". The frst occurs at potentials rom -5 to

-5.0 % vs SC?. To determine the nature o the process& the electrodeposition $as carried

out at a constant 5.0 % vs SC?& $hen the rotation rate o the cathode ranged rom 5 to

,< radQs and a p# O 1.1 electrolyte !number " $as used. The partial current densitiesare independent o the rotation rate o the cathode. The rate o copper reduction is

limited by the dissociation o the dominate copper!66" citrate complex !Cu2Cit28V"&

according to *3 and *


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The electrodeposition potential strongly in>uences the partial current density o tin !+ig.

50a" or all investigated concentrations o sodium citrate. The limiting current densities

are slightly visible on the polarisation curves at lo$ citrate concentrations !.8 "; theyare more visible $hen the solution has a high sodium citrate concentration !.,1 and

.< ". The signifcant in>uence exerted by the complexation agent is apparent

bet$een .8 and .,1 . The partial current densities o tin increase $hen increasing

the sodium citrate concentration at potentials belo$ -5.8 % vs SC?; ho$ever& urther

increases in the concentration !.,1 to .< " do not aect the electrodeposition o

tin. The Sn content decreases $hen decreasing the potential !rom 2 to ,2 $t./". The

highest tin content in the deposit is reached at higher potentials !rom -5 to -5.8 % vs

SC?". The lo$est tin content is achieved at lo$ concentrations o sodium citrate& at

lo$er potentials !belo$ -5.3 % vs SC?".

The partial current densities during the electrodeposition o copper are lo$ relative to

the current densities or tin and 'inc !+ig. 50c". Changes in the sodium citrate

concentration do not aect the curves signifcantly. This eect is most apparent

bet$een -5.0 and -5. % vs SC? !bet$een .8 and .,1 Wa 0#Cit". The current

density increases $hen increasing the Wa0#Cit concentration. Similar to tin& the

dependence or copper bet$een .,1 and .< nearly disappears. The copper

content in the received deposit is sho$n in +ig. 52. At lo$ potentials& the Cu content

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remains independent o the sodium citrate concentration. The relationship bet$een the

concentrations o the complexation agent and the Cu is apparent at higher potentials

!rom -5 to -5., % vs SC?". The Cu content increases $hen the concentration o the

complexation agent is increased. The Cu content is also related to the increasing

current density in this range. The maximum Cu content is


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+ig. 58a presents the steady-state partial polarisation curves or tin. The frst limiting

current density observed or the higher potential decreases $hen increasing the p#. This limiting current is connected to reduction o the Sn#CitV citrate complex described

in reaction *8; thereore& $hen the concentration o this complex decreases $ith

increases in the p#& the limiting current is decreased. The second limiting current

density is observed at more negative potentials& but is not ully attained; thereore&

there is a minor relationship bet$een the p# o solution and the frst $ave o reduction

!presence o a reduced Sn#CitVcitrate complex". The Sn content in the deposits& $hich

is related to the p# and deposition potential& remains independent o the solution p#

!+ig. 52a and +ig. 52b" and only changes $hen decreasing the potential. The maximum

Sn content at lo$ p# values is observed rom -5 % to -5.8 % vs SC? !8-, $t./".

The steady-state partial polarisation curves or Zn !66" are sho$n in +ig. 58b. The curves

remain independent o the solution p#. The Zn reduction begins belo$ -5.2 %. Jet$een

-5.2 % and -2 % vs SC?& the Zn content remains independent o the solution p# and

increases $hen increasing the solution p# over a narro$ range at potentials belo$ -2 %

vs SC?.

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The steady-state partial polarisation curves or copper are similar to those o tin !+ig.

58c". The solution p# is $ea7ly related to the limiting current density. Yhen increasing

the solution p#& the current density decreases. This result is related to the

decreased concentration at increased p#; thereore& the limiting current

related to *< decreases. The Cu content in the deposit remains independent o the

solution p# !+ig. 52" and decreases $hen the electrodeposition potential decreases. The Cu content pea7s at the highest potentials !< $t./".

+ig. 58d presents the steady-state partial polarisation curves or hydrogen& $hich agree

$ith the results obtained previously. Jelo$ -5., % vs SC?& the reduction o hydrogen

increases signifcantly& according to reactions *5 and *2. The solution p# appears

independent o the current density. The current e9ciency decreases $hen decreasing

the electrodeposition potential. This process is the most e9cient at high potentials&

$hile remaining independent o the solution p#.

0.,. Ihase structure and grain si'e

The initial investigations $ere perormed or phase structure and grain si'e

determination by )-ray diraction ! Table 2". The -tin phase !tetragonal& space group

685Qamd !585"& IB+ 8-,30" is the main phase in all deposits. The second phase

Cu,Sn1 !hexagonal& space group I,0Qmmc !58"& IB+ 83-5131" is present in deposits

$ith lo$ 'inc content. The Cu1Zn


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Thermodynamic analysis o the model o citrate baths reveals that the optimal

conditions or the electrodeposition o Sn-Zn-Cu alloys rom citrate solutions reuire a

p# bet$een 8 and ,& but the citrate baths generated under these conditions may be

unstable due to the reduction o Cu!66" by Sn!66" species.

E

The development o a stable electrolyte citrate bath or the electrodeposition o

SnZnCu alloys is possible at p# 1 to 1.31 $hen the citrate ion concentration

exceeds .8 . The reduction o copper !66" species by negatively charged tin!66"

complexes is inhibited because the activation energy o the charge transer

process is high. #igh overpotentials are also observed or the reduction o

citrate copper!66" complexes during voltammetric investigations.

E

The voltammetric studies reveal the possibility o electrodepositing Sn-Zn-Cu

alloys rom citrate baths. The deposition processes or tin!66"& 'inc!66" and

copper!66" agree $ith their standard potentials& $hich is normal co-depositionaccording to the Jrenner classifcation. The 'inc!66" reduction $as strongly

inhibited during its co-reduction $ith tin!66".

E

A mechanism or the co-reduction o the tin!66"& 'inc!66" and copper!66" citrate

complexes& as $ell as the protonated citrate species& $as proposed to explain

the in>uence o the potential& hydrodynamic conditions& p# and concentration o

the complexation agent on the composition o the Sn-Zn-Cu deposit and current

e9ciency.

E

Hnli7e the electrodeposition o tin and copper& the electrodeposition o 'inc is

under activation control due to the reduction o an electroactive citrate 'inc!66"

complex !Zn#CitV" $ith the investigated range o electrode potentials; thereore&

changes in the hydrodynamic conditions !increase o *B? rate" decrease the

'inc content.

E

6ncreasing the citrate concentration in the bath induces the release o citrate

ions during the co-reduction o tin!66" citrate complex conduct to !i" decrease the

concentration o the electroactive 'inc!66" citrate complex !Zn#Cit V" near the

electrode surace and to !ii" increase the concentration o the non-electroactive

'inc!66" citrate complexes $ith high negative charges !Zn2Cit28V and Zn#2Cit28V&

$hich have a high overpotential or Zn!66" reduction".

E

The changes in p# bet$een 1. and 1.31 have little in>uence on the

concentration o the electroactive 'inc!66" citrate complex. #o$ever& these

changes in p# decrease the concentration o complexes $ith lo$er

overpotentials. Conseuently& the frst plateau in the limiting current or Cu!66"


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and Sn!66" decreases& and the 'inc content increases slightly $hen the p# is

increased.

E

The possibility o electrodepositing Sn-Zn-Cu alloys rom citrate solutions $as

confrmed via potentiostatic deposition. The tin content o the coatings varied

rom , to , $t./& the copper content varied rom 0.1 to 2.1 $t./& and the

'inc content approached 03.1 $t./.

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