196 I N Ż Y N I E R I A M A T E R I A Ł O W A ROK XXXIV

Mgr inż. Monika Słupska (m.slupska@imim.pl), dr hab. Piotr Ozga, prof. PAN, dr Zbigniew Świątek, mgr inż. Honorata Kazimierczak – Instytut Metalurgii i Inży-nierii Materiałowej im. Aleksandra Krupkowskiego Polskiej Akademii Nauk, Kraków

MONIKA SŁUPSKA, PIOTR OZGA, ZbIGNIEW ŚWIąTEK, HONORATA KAZIMIERCZAK

The development of stable baths for electrodeposition of Sn-Zn-Cu

lead free solder alloys

INTRODUCTION

Sn-Zn-Cu alloys can be attractive as a lead-free solder for electric and electronic assembly. They can be a replacement for so far used solders containing toxic lead. The maximum amount of lead in ma-terials is limited by Restriction of Hazardous Substances Directive (ROHS) by European Union from 2006. Among the lead-free sol-ders SAC (tin-silver-copper alloys) and eutectic Sn-Ag alloy are mainly used. They have good physicochemical properties, but they are difficult to obtain by electrodeposition or electroless deposi-tion methods hence pure tin is currently the main material used for deposition of solder layers [1]. Sn-Zn eutectic alloy has been con-sidered as one of the more attractive lead free solder alloys [2]. It is called a new generation alloy [3] and it can be simple obtained by electrodeposition [4÷9]. Sn-Zn eutectic alloy has good mechanical properties, low melting temperature (198.5°C) and relatively low cost. Sn-Zn alloys have been received since beginning 20th century as a corrosion protective layer (substitutes for cadmium, more ex-pensive in this time) [4÷9]. Copper, silver, indium, bismuth and several other elements can be used as a third and fourth component of alloy which improve the soldering and mechanical properties [2, 10, 11]. Small addition of Cu can improve properties of these alloys such as a flexural strength, corrosion resistance and it can reduce dezincification of solder [10÷12]. Copper improves also an electrical properties and it can reduce the amount of Zn phase in Sn-Zn eutectic which cause poor oxidation resistance [2]. Electro-deposition can be the best way to obtain Sn-Zn-Cu alloys. Knowl-edge about receiving Sn-Zn-Cu ternary alloy is limited. The elec-trodeposited Sn-Zn-Cu layers can have many application (solar cells [18], negative electrode in lithium-ion batteries [14], corro-sion resistant layer [13]), but there is no information about obtain-ing this alloys directly by electrodeposition in available literature. Few authors received Sn-Zn-Cu by means of other method (electro-less plating [13] and combinatorial, two stages method: (i) electro-deposition of a composition-spread film of binary alloy (Sn-Zn) (ii) dripped immersion plating (Sn-Zn-Cu) [14]).

The main problem with electrodeposition of Sn-Zn-Cu alloys is development of stable baths. The presence in simple electrolytic bath simultaneously Sn(II) and Cu(II) species conduct to reduc-tion of Cu(II) to metallic copper in bath by tin(II). Choice of proper complex agent can enable preparation of stable electrolytic baths by change of dominant tin(II) and copper(II) species in bath by reaction of complexation. Hence (i) reduction processes of the Cu(II) species can be inhibited (kinetic effect) or (ii) the electrochemical potential of dominant species can be changed (thermodynamic effect) [1].

The main purpose of this work was a development of the stable baths for electrodeposition of Sn-Zn-Cu on the basis of the analysis of thermodynamic models of Sn(II)-Zn(II)-Cu(II)-citrate baths and experimental investigations of baths stability. The electrodeposi-tion from several stable Sn(II)-Zn(II)-Cu(II)-citrate baths was per-formed in the final experiments for confirmation of possibility of obtain the Sn-Zn-Cu deposits.

EXPERIMENTAL

The predominance area diagrams were made on the basis of ther-modynamic models. The diagrams show complex forms and de-posits in solution containing 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4 depending on concentration of sodium citrate (complex agent) and pH of the solution. The optimal range of pH and con-centration of complex agent was chosen on the basis of the analy-sis of the predominance area diagrams and potential-pH diagrams. The chemical composition of the solutions which were used in investigations of the baths stability are given in Table 1. The con-tent of deposits in solutions was examined after one month. Based on the above results, the optimal baths parameters were chosen to eletrodeposition of Sn-Zn-Cu alloys. The chemical composition of the solutions studied during electrochemical measurements is given in Table 2. The solution pH was adjusted by the addition of sulphuric acid or sodium hydroxide. All electrochemical measure-ments were carried out in a 200 cm3 cell, in a system with rotat-ing disc electrode to ensure constant and controlled hydrodynamic conditions. The measurements were performed in a three electrode cell, by means of potentiostat PARSTAT 2273. The working elec-trodes were a low carbon steel which were chemically polished using a solution of oxalic acid and 30% hydrogen peroxide (perhy-drol). A tin was used as a counter electrode. The working electrode potentials were referred to the saturated calomel electrode (SCE). The chemical composition of electrodeposits was characterized by the Micro X-ray Fluorescence (µ-XRF) analysis. X-ray structural studies were performed using polycrystalline diffractometer Phil-ips X’Pert Pro (using CuKα radiation) Analysis phase composi-tion of the layers was carried out using crystallographic databases PDF4 [15].

RESULTS AND DISCUSSION

The main problem with electrodeposition Sn-Zn-Cu ternary alloy is preparation of a stable electrolyte bath. It must be homogenous with tin(II), zinc(II) and copper(II) in active forms of electroactive aque-ous species. The electrolytic bath should be stable for at least one month. The citrate electrolytes are very attractive due to the fact that they are environmentally friendly. Sodium citrate forms electroac-tive complexes with Sn(II), Zn(II) and Cu(II). The thermodynamic models are very useful for preparation of stable baths. The main sources of stability constants data are databases such as those by Sillen and Martel [16] and the IUPAC stability Constants Database (SC-Database) [17].

Predominance area diagrams for Sn(II), Zn(II) and Cu(II) spe-cies in solution 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4 de-pending on concentration of complex agent (sodium citrate) and pH of the solution are shown in Figures 1÷3.

Analysis of the received results shows optimal range of sodium citrate concentration and pH solutions. Range of pH for copper(II) with active complex forms is from pH = 4 to 6, for tin(II) is from pH = 4 to 7 and for zinc(II) is from pH = 3.5 to 5.5. Optimal con-centration of sodium citrate in the solution is from 0.3 to 1 mol/dm3. The dominant electroactive complex forms in selected range of pH

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Table 1. The compositions of used electrolytic baths and the contents of precipitates in bath prepared on the basis of thermodynamic models after a period of one month from the preparation of the bathTabela 1. Skład badanych kąpieli elektrolitycznych oraz zawartości osa-dów w kąpielach opracowanych na podstawie modeli termodynamicz-nych po okresie 1 miesiąca od sporządzenia kąpieli

No.

Electrolyte composition +0.5 g/dm3 PEG + 0.05 g/dm3 SDS + 6.6 g/dm3 WX

SnSO

4m

ol/d

m3

ZnS

O4

mol

/dm

3

CuS

O4

mol

/dm

3

Na 3H

Cit

mol

/dm

3

pH

Dep

osit

colo

ur

Sn6O

4(OH

) 4% Cu %

1 0.05 0.25 0.02 0.65 4.5 red – 76.92 0.075 0.25 0.02 0.65 4.5 red – 115.63 0.1 0.25 0.02 0.65 4.5 red – 104.54 0.05 0.25 0.02 0.65 5 red – 107.45 0.075 0.25 0.02 0.65 5 red – 108.36 0.1 0.25 0.02 0.65 5 red – 95.97 0.05 0.25 0.02 0.65 5.5 red – 75.18 0.075 0.25 0.02 0.65 5.5 red – 93.49 0.1 0.25 0.02 0.65 5.5 red – 80

10 0.05 0.25 0.02 0.4 4.5 red – 76.111 0.075 0.25 0.02 0.4 4.5 red – 9412 0.1 0.25 0.02 0.4 4.5 white 7.1 –13 0.05 0.25 0.02 0.4 5 red – 56.414 0.075 0.25 0.02 0.4 5 red – 81.115 0.1 0.25 0.02 0.4 5 white 6.1 –16 0.05 0.25 0.02 0.4 5.5 red – 70.817 0.075 0.25 0.02 0.4 5.5 red – 62.418 0.1 0.25 0.02 0.4 5.5 white 6.1 –19 0.1 0.25 0.02 0.4 6 white 2.9 –20 0.1 0.25 0.02 0.4 6.5 white 8.721 0.1 0.25 0.01 0.4 5 – – –22 0.1 0.25 0.01 0.4 5.5 – – –PEG-3000 polyethylene glycol 3000, SDS – dodecyl sulphate sodium salt,

WX-Na2WO4 ∙2H2O, Cit – C6H4O7

Table 2. Chemical composition of electrolytic bath used to electrodepo-sition of Sn-Zn-Cu alloysTabela 2. Skład kąpieli do elektroosadzania stopów Sn-Zn-CuElectrolyte composition + 0.5 g/dm3 PEG + 0.05 g/dm3 SDS + 6.6 g/dm3 WX

No. SnSO4mol/dm3

ZnSO4mol/dm3

CuSO4mol/dm3

Na3HCitmol/dm3 pH

1 0.1 0.25 0.02 0.4 5.52 0.1 0.25 0.01 0.4 5.5

are: Cu2HCit23– for Cu(II) (Fig. 1), SnCit2– for Sn(II) (Fig. 2) and

ZnHCit– for Zn(II) (Fig. 3). Dependence between potential and pH for solution 0.02 M

CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4, 0.4 M Na3HCit is shown in Figure 4. Dark grey is an area where there is a possibility of be-ginning reduction of a Cu(II). In this area Cu(II) can be reduced by Sn(II) hence the bath is not stable in this area (area denoted by Cu(s)). Below is area, denoted by Cu(s) + Sn(s), where Cu(II) can be reduced simultanously with Sn(II) to Cu-Sn deposit. The bottom brightest area shows a range where three Cu(II), Sn(II) and Zn(II) can be reduced together. This is theoretical optimal range of pa-rameters for electrodeposition of Sn-Zn-Cu ternary alloys. Because of the overpotential a real potential range for electrodeposition of Sn-Zn-Cu alloys is moved to lower potential.

Fig.1 Predominance area diagram for Cu(II) species (Solution: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4).

Rys.1 Diagram form dominujących Cu(II) (Roztwór: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4 ).

Fig. 1. Predominance area diagram for Cu(II) species (solution: 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4)Rys. 1. Diagram form dominujących Cu(II) (roztwór: 0,02 M CuSO4, 0,1 M SnSO4, 0,25 M ZnSO4)

Fig.2 Predominance area diagram for Sn(II) species (Solution: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4).

Rys.2 Diagram form dominujących Sn(II) (Roztwór: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4).

Fig. 2. Predominance area diagram for Sn(II) species (solution: 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4)Rys. 2. Diagram form dominujących Sn(II) (roztwór: 0,02 M CuSO4, 0,1 M SnSO4, 0,25 M ZnSO4)

The electrolyte baths were made in order to test their stability on the basis of the thermodynamic models analysis. The concentrations of sodium citrate, tin(II) sulphate, copper(II) sulphate in the baths were changed. The compositions and pH of the investigated elec-trolytic baths are given in Table 1. Only two electrolytic baths were stable during all investigated period. In other cases red or white precipitate appeared in the solution. The phase analysis on the basis of X-ray diffraction was carried out to identify obtained precipi-tates. Figure 5 and 6 show the received results. “Red” precipitates were precipitates of copper powder when “white” precipitates were precipitates of a tin(II) hydroksyoxide Sn6O4(OH)4.

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Two electrolytes with different concentration of copper sulphate were used for examination of electrodeposition of Sn-Zn-Cu alloys. The electrolyte baths compositions are shown in Table 2. The po-tential of a working electrode was changed from –1.0 to –2.0 V vs SCE. The effect of RDE speed on the electrodeposition of ternary system was also investigated. The rotation rate of the cathode was 15 and 68 rad/s. Process of electrodeposition of alloy proceeded in accordance with normal codeposition behaviour according to Brenner [19]. Elements were deposited in accordance with their equilibrium potentials. The composition of the received deposits is shown in Figure 7 and 8.

When potential is more negative than –1.0 V vs SCE the amount of copper in alloys decreases and content of tin increases. The con-

tent of zinc distincly increases for potential more electronegative than 1.2 V vs SCE. Above changes are in accordance with E-pH diagram (Fig. 4) and equilibrium potentials of deposited metals. A lower concentration of copper sulfate in a solution reduces the content of copper in received deposits of Sn-Zn-Cu alloys. The con-tent of tin and copper in alloy increases with increase of RDE rota-tion in solution (1) because deposition of these metals is controlled

Fig.3 Predominance area diagram for Zn(II) species (Solution: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4).

Rys.3 Diagram form dominujących Zn(II) (Roztwór: 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4 ).

Fig. 3. Predominance area diagram for Zn(II) species (solution: 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4)Rys. 3. Diagram form dominujących Zn(II) (roztwór: 0,02 M CuSO4, 0,1 M SnSO4, 0,25 M ZnSO4)

Fig.4 Potential E-pH equilibrium diagram for the solution 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4, 0.4 M Na3HCit

Rys.4 Zależność E-pH dla roztworu o składzie 0.02M CuSO4, 0.1M SnSO4, 0.25 M ZnSO4, 0.4 M Na3HCit

Fig. 4. Potential E-pH equilibrium diagram for the solution 0.02 M CuSO4, 0.1 M SnSO4, 0.25 M ZnSO4, 0.4 M Na3HCitRys. 4. Zależność E-pH dla roztworu o składzie 0,02 M CuSO4, 0,1 M SnSO4, 0,25 M ZnSO4, 0,4 M Na3HCit

Fig.5 XRD pattern from a deposit denoted as the„red” (Table1).

Rys.5 Dyfraktogram osadu wytrąconego z kąpieli (osad oznaczony jako „czerwony”, Tablica 1).

Fig. 5. XRD pattern from a deposit denoted as the „red” (Tab. 1)Rys. 5. Dyfraktogram osadu wytrąconego z kąpieli (osad oznaczony jako „czerwony”, abela 1)

Fig.6 XRD pattern from a deposit denoted as the „white” (Table 1).

Rys.6 Dyfraktogram osadu wytrąconego z kąpieli (osad oznaczony jako ,,biały”, Tablica 1).

Fig. 6. XRD pattern from a deposit denoted as the „white” (Tab. 1).Rys. 6. Dyfraktogram osadu wytrąconego z kąpieli (osad oznaczony jako ,,biały”, tabela 1)

Fig.7 The dependence of potential on the composition of deposited coatings for solution 0.1 M SnSO4, 0.25 M ZnSO4, 0.02 M CuSO4, 0.4 M Na3HCit, pH=5.5.

Rys.7 Wpływ potencjału na skład osadzanych powłok dla roztworu 0.1 M SnSO4, 0.25 M ZnSO4, 0.02 M CuSO4, 0.4 M Na3HCit, pH=5.5.

-2,0 -1,8 -1,6 -1,4 -1,2 -1,0

0

10

20

30

40

50

60

70

80

90

Sn,Zn,Cu

con

tent[%

]

E[V]vs.SCE

% Cu 15 rad/s % Zn 15 rad/s % Zn 68 rad/s % Cu 68 rad/s % Sn 15 rad/s % Sn 68 rad/s

Fig. 7. The dependence of potential on the composition of deposited coatings for solution 0.1 M SnSO4, 0.25 M ZnSO4, 0.02 M CuSO4, 0.4 M Na3HCit, pH = 5.5Rys. 7. Wpływ potencjału na skład osadzanych powłok dla roztworu 0,1 M SnSO4, 0,25 M ZnSO4, 0,02 M CuSO4, 0,4 M Na3HCit, pH = 5,5

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by transport of citrate complexes of tin(II) and copper(II) to elec-trode surface. The content of zinc is greater in the layers received at the rotation rate of 68 rad/s from solution (2) (Fig. 8) because in this range of potential (below –1.4 V vs SCE) deposition of zinc is also controlled by transport of citrate complexes of zinc(II). The most similar content of tin and zinc to SnZn9 eutectic alloys were obtained during the deposition at the potential –1.7 V vs SCE.

X-ray structural studies were carried out for electrolytic layers Zn-Sn-Cu with high content of tin (from 56 to 90 wt %). Other alloying elements in layers change, respectively, for zinc from 2 to 12.4 wt % and for copper from 5 to 14.3 wt %. Figure 9 shows a part of the diffraction pattern for Sn-Zn-Cu layer with composi-tion: 76.8 wt % Sn, 12.4 wt % Zn and 10.8 wt % Cu. Phase analysis of this layer revealed the presence of two phases (Fig. 9). The first phase, dominant in all studied layers is tin (β-Sn), crystallizing in the tetragonal system (space group I41/amd (141)), the second is Cu5Zn8 – phase crystallizing in the regular system (space group I3m (217); PDF# 04-007-1117).

Increasing the content of copper and tin in layer (respectively to 14.3 wt % and 78 wt %) while reducing the content of zinc (to 7.7 wt %) leads to a crystallization of η-Cu6Sn5 phase (hexagonal system; space group P63/mmc (194); PDF# 00-047-1575) and de-crease of content of Cu5Zn8 phase. The layers are three-phases: β-Sn, η-Cu6Sn5 i Cu5Zn8 in this range of chemical composition.

The layer with the highest content of Sn and the lowest content of Zn (90 wt % Sn with 8 wt % Cu and 2 wt % Zn) recorded only diffraction lines derived from two phases: β-Sn i η-Cu6Sn5. In case of layer with the lowest content of Sn and the highest content of Zn (56 wt % Sn with 38 wt % Zn and 6 wt % Cu) there are two phases present: (i) dominant phase β-Sn and (ii) new phase crystallizes in a hexagonal lattice of ε-CuZn5 type.

According to the phase diagram of Cu-Sn system [20] the elec-trodeposits in the room temperature should contain phase η′-Cu6Sn5 crystallizes in monoclinic system (space group C2/c (15), PDF#04-007-2658), but not high temperature hexagonal phase η-Cu6Sn5 (type η-Cu6.26Sn5, space group P63/mmc (194), PDF# 00-047-1575). However, electrodeposition process proceeds in presence of Zn at-oms, which can be substituted in positions of tin atoms [21]. This mechanism can conduct to stabilization of high temperature hex-agonal phase η-Cu6Sn5 [22], hence hexagonal η-Cu6(Sn,Zn)5 phase can be present in Zn-Sn-Cu electrodeposits. The rest of obtained results of the phase analysis of electrolytic layers Zn-Sn-Cu is in a good agreement with the thermodynamic data of the ternary phase Sn-Zn-Cu in the tested range of the chemical composition.

SUMMARY

– The citrate electrolytic baths for electrodeposition of Sn-Zn-Cu ternary alloys were based on the analysis of thermodynamic models of the Zn(II)-Sn(II)-Cu(II)-Cit-SO4-H2O system. Op-timal pH range is between 4 and 6. Optimal concentration of complex agent (sodium citrate) is within the range from 0.3 to 1 mol/dm3.

– The experimental investigations of baths from optimal range of pH and for optimal concentration of complex agent determined the stable baths with no precipitates.

– Electrodeposition of Sn-Zn-Cu alloy layers from prepared aque-ous citrate baths is possible in wide range of alloy compositions.

– The received results of phase analysis of the electrodeposits are in a good agreement with thermodynamic data for Sn-Zn-Cu system except presence of hexagonal η-Cu6(Sn, Zn)5 phase in-stead monoclinic η′-Cu6Sn5.

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Fig.8 The dependence of potential on the composition of deposited coatings for solution 0.1 M SnSO4, 0.25 M ZnSO4, 0.01 M CuSO4, 0.4 M Na3HCit, pH=5.5.

Rys.8 Wpływ potencjału na skład osadzanych powłok dla roztworu 0.1 M SnSO4, 0.25 M ZnSO4, 0.01 M CuSO4, 0.4 M Na3HCit, pH=5.5.

-2,0 -1,8 -1,6 -1,4 -1,2 -1,0

0

10

20

30

40

50

60

70

80

90

100

Sn,Zn,Cu

content[%

]

E[V]vs.SCE

% Sn 68 rad/s % Zn 68 rad/s % Cu 15 rad/s % Sn 15 rad/s % Zn 15 rad/s % Cu 68 rad/s

Fig. 8. The dependence of the potential on the composition of depos-ited coatings for solution 0.1 M SnSO4, 0.25 M ZnSO4, 0.01 M CuSO4, 0.4 M Na3HCit, pH = 5.5Rys. 8. Wpływ potencjału na skład osadzanych powłok dla roztworu 0,1 M SnSO4, 0,25 M ZnSO4, 0,01 M CuSO4, 0,4 M Na3HCit, pH = 5,5

Fig.9 XRD pattern from a Sn-Zn-Cu deposit.

Rys.9 Dyfraktogram rentgenowski osadu Zn-Sn-Cu.

Fig. 9. XRD pattern from a Sn-Zn-Cu depositRys. 9. Dyfraktogram rentgenowski osadu Zn-Sn-Cu

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