On the synthesis Of Bi – BAsed precursOrs fOr leAd – free sOlders develOpment

Preliminary studies on the design of lead–free solders precursors by wet chemistry methods are presented. The main objective is to assess the impact of the way of hydroxide precipitates preparation on the metal elements content of the precipitates. Namely, ternary hydroxide mixtures of three systems: a. Cu(II), Bi(III), Sn(II); b. Cu(II), Bi(III), Sb(III); and c. Cu(II), Bi(III), Zn(II) were prepared, firstly, by single-element precipitation and, secondly, by co-precipitation. Thereafter, all mixtures were reduced by using hydrogen gas. Both, the initial mixtures and the reduced samples were studied by X–ray diffraction, optical and scanning electron microscopes. The chemical compositions of the precipitates were determined experimentally and their dependence on the pH was verified. It was found that alloying occurred during the reduction procedure, but in some cases the reduction was not complete (i.e. oxide phases rest in the samples). This might be a huge obstacle to use such an approach for the preparation of lead-free solders. Moreover, the materials obtained after reductions apparently are bulk alloys, thus, the preparation of small–sized metal particles would be a challenge. Another key feature to be addressed in future studies is the correlation between the chemical compositions of the parent solution and these of the corresponding precipitates.


introduction
It is well known already that the high toxicity of lead is the reason to stop the use of lead-bearing solders.Nevertheless, the demand of electronics, automotive and other industries for lead-free solders can still not be fully satisfied.Thus, studies on new leadfree solders and corresponding production techniques are under way [1].In this connection, various methods have been reported in the literature [2][3][4][5][6] which aim at producing particles of either pure metals or of metal-containing compounds (e.g.oxides, chalcogenides etc.) in solution.Certainly, a full review of these reports is out of the scope of this work.
Copper substrates are regularly used in microelectronic packaging, but copper is also frequently used as a constituent of multi-component lead-free solders.Nearly all of the modern solder materials are currently based on tin and this is expected to remain so for the foreseeable future.On the other hand, the use of bismuth in microelectronic solders is also prospective, and this is especially valid for a series of lead-free high-temperature solders that are now under development.
Taking into account these considerations, Bi-Cu-X (X=Sn, Sb and Zn) lead-free hightemperature solders were synthesized recently by Takaku et al. [7] using a gasatomization method.Nevertheless, this method requires complicated technology, and therefore it would be important to continue seeking alternative ways to synthesize nano-or micro-sized solders by methods which might be easier to employ.In the past we have acquired some experience in this field, as recently one of the authors studied the possibilities to synthesize nanoand micro-particles in the binary systems Co-Bi and Ni-Bi [8,9].The purpose of the present studies was to acquire preliminary knowledge of a wet-chemical technique for an inorganic synthesis of lead-free solder precursors.

experimental
The synthesis started initially with the precipitation of single-element hydroxides (i.e. the hydroxides of Cu(II), Bi(III), Sn(II), Sb(III), and Zn(II); see Section 2.1) to prepare ternary mixtures.In the second stage (Section 2.2), a co-precipitation of the hydroxides of three elements (i.e. the hydroxides of Bi(III) + Cu(II) + Sn(II), Bi(III) + Cu(II) + Sb(III), and Bi(III) + Cu(II) + Zn(II)) was done.Both experimental approaches were similar, but for the sake of simplicity, the two cases will be discussed consecutively.In both cases, the precipitates were reduced with pure hydrogen gas (Section 2.3).

preparation of ternary mixtures using single-element cu(ii), Bi(iii), sn(ii), sb(iii) and Zn(ii) hydroxides
Pure substances (p.a.) Bi(NO 3 ) 3 .5H 2 O, CuSO 4 .5H 2 O, SnCl 2 .2H 2 O, SbCl 3 , and Zn(NO 3 ) 2 .6H 2 O were used to prepare 1 M solutions.Thereafter, 20 cm 3 of each solution were mixed with 10 cm 3 of a 3 M aqueous NaOH solution to achieve precipitation of the pertinent single-element hydroxide.The precipitates were collected in a Büchner funnel and desiccated at a temperature of 105°C for 24 h.Ternary specimens with pre-determined metal ion contents were obtained by mixing corresponding amounts of the dry hydroxides and their compositions are listed in Table 1.The chemical compositions of the samples (concerning the metallic elements only) are plotted in Fig. 1A-C, where isothermal sections of the ternary Bi-Cu-Sn, Bi-Cu-Sb and Bi-Cu-Zn are represented.These samples were then subjected to the chemical reduction procedure by hydrogen gas as described in Section 2.3 and the products were investigated by powder X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM).

preparation of ternary mixtures by co-precipitation of {cu(ii)+ Bi(iii)+X} hydroxides {X= sn(ii), sb(iii) or Zn(ii)}
Ternary aqueous solutions with predetermined metal contents were prepared first.For this purpose, weighed amounts of the pure salts were dissolved in 20 cm 3 water together.The chemical compositions of the ternary solutions (referred to the metal content) are shown in Table 2 (see Fig. 1A-C as well).
Co-precipitation was done by adding 10 cm 3 of a 3M NaOH solution to 20 cm 3 of each ternary solution.The pH value of the solution was measured after the precipitation by means of a pH-meter (HANNA).It is well known that the hydroxides Cu(OH) 2 , Bi(OH) 3 , Zn(OH) 2 , Sb(OH) 3 , and Sn(OH) 2 precipitate in different pH intervals [10], i.e. 5.5-10.0;2.0-7.0;7.0-8.8;0.0-6.0 and 2.0-3.4, in the same order.As a consequence, the chemical compositions of the precipitated hydroxide mixtures might not correspond exactly to the chemical compositions of the pertinent three-element aqueous solutions.
In order to test quantitatively for possible differences, three solutions (Table 2, Nos. 18, 22, 24, i.e. one of each system) were chosen for a more detailed investigation.Thus, both the precipitated hydroxides as well as the remaining aqueous solutions were subjected to quantitative chemical analyses.These particular compositions (e.g.Nos.18,22,24) were chosen in agreement with the ternary phase diagrams of the respective systems (Bi-Cu-Sn, Bi-Cu-Sb or Bi-Cu-Zn) assessed by Takaku et al. [7]  the liquid miscibility gaps appearing in the respective ternary phase diagrams (Fig. 1A-C).In such a way, a natural composite structure of the solder materials might be formed at solidification, under the condition of the existence of a (stable or metastable) miscibility gap.It should be mentioned that recently Manasijevic et al. [11] investigated the Bi-Cu-Sb phase diagram experimentally and performed a CALPHAD-type optimization.In their study, no liquid miscibility gap was discovered; however, they did not especially look for it.Therefore its existence is not ruled out entirely for the present investigation.Any further discussion on this topic would be out of the scope of this work.
The three-component co-precipitates were treated in the same way as described in Section 2.1.The dried masses were around 5g in each case.Thereafter, reduction by hydrogen gas was performed.The obtained products were again characterized by powder XRD, OM, and SEM.

reduction
After some preliminary experiments, a standardized reduction procedure (as described in Table 3) was established.As one can see, it recommends a preliminary oxidation heating, i.e. converting the hydroxides to oxides of the corresponding elements, before the reduction step.This is necessary in order to remove the hydrogen contained in the hydroxides so that no water vapors are formed during the reduction.

chemical analyses
The chemical compositions of the precipitated three-element hydroxides had to be determined experimentally since the solubility product (i.e., the solubility) of every hydroxide depends on the pH value in a different way.Quantitative analyses of the Bi and Cu content in the precipitates were done by volumetric and gravimetric methods, respectively.For the determination of Bi, around 0.03 g of the dry precipitate mixture were dissolved in concentrated nitric acid (0.7 cm 3 ).Water was added to adjust the volume to 10 cm 3 .The solution was titrated by EDTA (complexon III) using xylenol orange as indicator.Five parallel analyses were performed each time in order to obtain a statistically reliable mean value.For the determination of copper, samples of around 0.58 g of the hydroxide mixture were weighed, dissolved in nitric acid (2:1) and subjected to a standard electrochemical deposition procedure.The each content of  the corresponding third element (Zn, Sb, or Sn) was obtained from the corresponding difference in mass.

samples characterization
The samples were characterized by powder X-ray diffraction (XRD) on a Bruker D8 powder diffractometer equipped with a high speed LynxEye one dimensional silicon strip detector and a Cu X-ray tube.The patterns were analyzed using the 'Topas 3' software (Bruker AXS, Karlsruhe, Germany).Crystal structure data for phase identification were taken from Pearson's Handbook of Intermetallic Phases [12].
A representative number of samples were also investigated by scanning electron microscopy using a JEOL JSM-6460 apparatus with EDAX analyzer.

experimental results and discussion
White (for Sn(II), Sb(III), Zn(II)) or blue (for Cu(II)) colored precipitates were obtained after adding the alkaline solution.
In the case of bismuth, a light-yellow precipitate was obtained, probably due to the spontaneous dehydration of Bi(OH) 3 to BiO(OH).In addition, it was also observed that Sn(OH) 2 changed color from white to grey during filtration and even to dark-grey after heating at 105°C.A similar phenomenon was also observed during desiccation heating of Cu(OH) 2 .This could be explained by a gradual dehydration of the corresponding hydroxides and the formation of SnO or CuO [13], respectively.Nevertheless, these processes do not influence the ratio of the metallic elements that was determined experimentally.
Concerning the experiments described in Section 2.2, white precipitates were observed during the dissolution of the Bi(III)-, Sn(II)and Sb(III) salts.This phenomenon is easily understood, taking into account the above mentioned pH intervals of precipitation.It could be avoided for Sn(II)-and Sb(III)containing samples by using concentrated hydrochloric acid due to formation of soluble chloride complexes, ([SnCl 4 ] 2-and [SbCl 4 ] -, respectively).
The following observations concern some effects due to the sequence of mixing.They were observed when co-precipitations were done.Firstly, when Sn(II) solution was added to the Bi(III) containing beaker, then the color of the common precipitate became yellow (possibly due to the formation of BiO(OH)).Another explanation might be that Sn(II) acts as reducing agent to Bi(III), reducing it to Bi(0).Indeed, the standard potentials of the redox couples Sn(II)/Sn(IV) and Bi(0)/Bi(III) have values of +0.151 V and +0.308 V, respectively [13], and the overall redox potential of the reaction 3Sn(II)+2Bi(III)=3Sn(IV)+2Bi(0) is actually positive.Anyhow, the experiments have shown that the sequence of adding the components had to be changed to bismuth-copper-tin (solutions).Finally, the measured quantities of aqueous NaOH solutions were added in portions until the pH value became about 8 to 9. Such a pH is needed to assure the precipitation of Cu(OH) 2 .
When co-precipitating Bi(III)-, Cu(II)-, Sb(III) hydroxides, it was found that one should mix Bi-and Cu-solutions first in order to achieve proper results.The reason is that if Sb(OH) 3 precipitates first (this occurs at pH=0), it sticks to the glass wall, and large weight losses could occur.
Six of the co-precipitated three-element samples (i.e. two of each system studied) were subjected to the reduction process (Nos.14, 18, 19, 21, 24, 25, Table 2), applying the procedure described in Table 3.Some challenges concerning the reduction were encountered.Namely, the reaction under hydrogen flow had to be performed at relatively high temperature (i.e.430°C, see Table 3) in order to obtain effective reduction.Higher temperatures were not available due to limitations by the reactor's materials.Moreover, it is known that some of the elements exhibit lower melting points (e.g.Zn at 419.6°C, Bi at 271.3°C, Sn at 232.0°C [14]).This seemed to pose no problems except for zinc-containing specimens where overheating used to occur in the reactor, made of thermal glass.In such a case, the sample became partially liquid, sticking firmly to the reactor wall and breaking it on cooling.That is why the reduction of such risky samples was performed at 390°C.Even then, the appearance of a liquid phase sticking to the reactor walls was frequently observed.This was obviously caused by additional heating by the heat deliberated in the alloying reactions between liquid Zn and solid Cu (Cu-Zn intermetallic phases exhibit large negative enthalpies of formation [15]).
A short literature review of the Sn, Bi, Zn, Sb, and Cu oxides and hydroxides thermal stabilities is given in Table 4.It is evident that the chosen desiccation temperature of 105°C is low enough in order not to activate decomposition of the pertinent hydroxides.

The circles (,) represent the samples composition (up to Table 1), the triangles (7) -chemical compositions of the ternary solutions (referred to the metal content) as shown in Table 2. Solidus and liquidus lines are not represented in Figs
After reduction various samples were investigated by SEM.It was found that the particles, consisting initially of single elements, had obviously reacted with each other forming microcrystals of the corresponding alloys.As an example, a SEM micrograph of sample No. 6 (Bi-Cu-Sb) is represented in Fig. 2. Three points of the sample were analyzed in order to obtain the local chemical compositions, and the results are listed in Table 5.One ought to be aware that the Bi-Sb alloys exhibit complete  White.No literature data about the decomposition temperature.K S = 6.3x10 -27 ; S=7.9x10 -11 mol.l -1 [16] solubility both in liquid and in solid state while in the Cu-Sn system a number of intermediate phases are formed [14].The results indicate that metastable alloys (supersaturated in copper) might have formed during the reduction process.Most of the reduced samples were checked for their phase composition by powder XRD.Figs.3A-C   in Sb).In the Bi-Cu-Sn samples that had been obtained by co-precipitation and reduction were still observed considerable contents of oxides, with the highest content of Bi 2 O 3 in sample 18 (61.6%) which was also the Bi-richest of all.It is interesting to note that the two Bi-Cu-Sb samples, obtained by co-precipitation and reduction, consisted apparently only of (Bi, Sb) and Cu 2 Sb as was to be expected from the phase diagram [10].It is of worth noting that the latter two ternary systems are subject of further studies [19,20] thus confirming the actuality of the present work.
All samples containing Zn (Nos.11, 12, 24, and 25) contained also very high amounts of oxides (Bi 2 O 3 and ZnO), practically independent of their method of preparation.
The chemical compositions of the solid samples (hydroxides) were determined by chemical analyses (see Paragraph 2.4).Experimental data of the pH values of the residual solutions (i.e. after precipitation) of samples 18, 22 and 24, respectively, with stoichiometric quantities of NaOH are shown in Figs. 4 A-C, combined with results of the samples chemical compositions.The contents of the corresponding third element (Zn, Sn or Sb) are assessed from the mass difference.As one can see, the Bi and Cu contents of the precipitates do not vary significantly with pH for the systems Figs.4A-C.Experimental pH of the parental solutions (after precipitation with stoichiometric quantity of NaOH) versus chemical compositions of samples 18 (Bi-Cu-Sn system) , 22 (Bi-Cu-Sb system) and 24 (Bi-Cu-Zn system), respectively.The symbols ()and (8) flow of 100 cm 3 .min - ; increase of the temperature up to 300°C b) Initial 120 min Oxygen flow of 100 cm 3 .min - , 300°C c) Transitional 10 min Oxygen flow of 100 cm 3 .min - , increase of the temperature from 300 to 400°C d) Main 120 min Oxygen flow of 100 cm 3 .min - gas flow of 100 cm 3 .min - , 430°C b) Main 240 min Hydrogen flow of 100 cm 3 .min -

Fig. 2 .
Fig.2.Micrograph of sample No.6 (belonging to the Bi-Cu-Sb system).The points where SEM analyses were done are indicated.
show powder patterns of three reduced Bi-Cu-Sb samples as examples (No. 8, 10, and 21, respectively).It can be seen that two of them (Fig. 3A and 3C) were obviously metallic with two separate phases present, i.e. (Bi, Sb) solid solution plus Cu 2 Sb (in basic agreement with the phase diagram by Manasijevic et al [10]), whereas sample 10 (with the highest Sb content) showed also a high content of Sb 2 O 3 (Fig. 3C) which had obviously developed in the process of preparation, possibly due to an insufficient reduction step.All XRD results are collected in Tables 6 A-C where the compositions are estimated from the calculated diffraction line intensities.It is obvious that in the first series of samples in the systems Bi-Cu-Sn and Bi-Cu-Sb (Nos.1-10) which had been prepared by a reduction of a mixture of the individual oxides the pure elements were still present, and only part had reacted to form intermetallic compounds.Sometimes, there are also rather high oxide contents in the samples probably due to insufficient reduction, with the highest oxide contents in sample 5 with 64.3 % Bi 2 O 3 (this sample was also richest in Bi) and sample 10 with 50.9 % Sb 2 O 3 (again this sample was richest Figs.4A-C.Experimental pH of the parental solutions (after precipitation with stoichiometric quantity of NaOH) versus chemical compositions of samples 18 (Bi-Cu-Sn system) , 22 (Bi-Cu-Sb system) and 24 (Bi-Cu-Zn system), respectively.The symbols ()and(8) show the mol fractions (in the precipitate) of Bi and Cu, in the same order.The contents (o) of the third element (i.e.Sn, Sb or Zn, respectively) are assessed.Figs.5A-C.Experimental pH of the parental solutions versus chemical compositions of samples 18, 22 and 24, respectively.On the left side are exhibited cases precipitated with a deficit of 20 % NaOH, and on the right sidewith an excess of 20 % NaOH.In the middle are represented results of the samples precipitated with stoichiometric quantity of NaOH.Line 1 and line 2 correspond to the overall bismuth and copper content, respectively, obtained in all samples.

Table 2 .
Chemical compositions (referred to the metal content) of the ternary solutions used for co-precipitation; samples used for subsequent reduction are marked.

Table 3 .
Optimized procedure for oxidation and reduction of the specimens a a These experiments were performed (under hire) in the Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences.

Table 4 .
Review of some physico-chemical properties of Sn, Bi, Zn, Sb, and Cu oxides and hydroxides, Ref. -literature source, K S -solubility product, S -solubility

Table 5 .
Results of SEM analyses with the reduced sample No. 6

Table 6 A
. Composition of the reduced Bi-Cu-Sn samples assessed by powder XRD.The numbers given in the Table are percentages of the calculated powder pattern intensity

Table 6C .
Composition of the reduced Bi-Cu-Zn samples assessed by powder XRD.The numbers given in the Table are percentages of the calculated powder pattern intensity

Table 6B .
Composition of the reduced Bi-Cu-Sb samples assessed by powder XRD.The numbers given in the Table are percentages of the calculated powder pattern intensity