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Chem. Ind. Chem. Eng. Q. 21 (2) 311−317 (2015) CI&CEQ

311

JAU-KAI WANG JIR-MING CHAR

Department of Applied Chemistry & Material Science, Fooyin

University, Ta-Liao Hsiang, Kaohsiung City, Taiwan, R.O.C.

SCIENTIFIC PAPER

UDC 544.6:546.59:62

DOI 10.2298/ CICEQ131216030W

OPTIMIZATION STUDY ON HARDNESS OF GOLD FILM THROUGH SUPERCRITICAL ELECTROPLATING PROCESS BY RESPONSE SURFACE METHODOLOGY

Article Highlights • The nanometer size gold film was developed by supercritical electroplating • The hardness of deposited gold film can be for industrial application of soft and hard

gold • A theoretical approach was examined by statistical experimental method Abstract

A non-cyanide gold bath has been used to deposit gold film on a brass substrate through electroplating process using supercritical carbon dioxide emulsion. The hardness of the deposited gold film was considered as a response variable to optimize the process parameters of electroplating oper-ation by statistical experimental methods. Effects of current density, pressure temperature, and chemical composition of the solution were investigated to select the optimal operation factors. Scanning electron microscopy and micro--hardness testing were applied to determine the characteristics of the metallic film. The screening of significant variables was examined by a 25-1 fractional factorial design with V resolution method. The experimental results showed that the significant variables affecting the deposition of gold film were current density, pressure and temperature. Based on Box-Behnken design and res-ponse surface methodology (RSM), a regression model was built by fitting the experimental results with a polynomial equation. The optimal operating vari-able conditions can be searched at a specified hardness for industrial hard and soft gold application ranged from 83.8 to 157.7 HV.

Keywords: electroplating; gold; optimization; statistical experimental method.

Due to its remarkable characteristics in terms of chemical and electrical properties, electroplated gold classified into soft gold and hard gold has been widely used in the electronics industry [1]. Hard gold is used on electrical connectors and contacts requiring resist-ance to mechanical wear as well as low electrical contact resistance. On the other hand, the gold applied for bump must be sufficiently soft so that it is easily deformable to accommodate small variations in thickness [2]. However, a common problem exists during electroplating, which is that the electric current

Correspondence: J.-K. Wang Department of Applied Chemistry & Material Science, Fooyin University, 151 Chin-Hsueh Rd, Ta-Liao Hsiang, Kaohsiung City, 831 Taiwan, R.O.C. E-mail: jkw.jkw@msa.hinet.net Paper received: 16 December, 2013 Paper revised: 27 July, 2014 Paper accepted: 28 August, 2014

also causes the dissociation of water in addition to the electrolysis of metal ions, resulting in hydrogen to be released at the cathode. The formation of hydrogen may create several defects of deposited gold film owing to the pinholes effect. This is an important problem for industrial application that has recently been investigated by other researchers [3-4].

Supercritical carbon dioxide (Sc-CO2) has rec-eived much attention as an alternative to harmful organic solvents used for extraction, separation, reac-tion, and for many other processes. The low viscosity, high diffusivity and zero surface tension of Sc-CO2 has been exploited in a variety of impregnation pro-cesses [5]. Recently, plating technology with Sc-CO2 has attracted special attention because Sc-CO2, in particular, can transport the solute into fine nano-meter-space of the materials and clean even integ-

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rated circuits without shrinking or causing other harm due to the interface tension that exists between liquids and gases. Electroplated films obtained in emulsions composed of Sc-CO2 and electroplating solutions have a uniformity and hardness, superior to those of films obtained using conventional electro-plating methods [6]. While the several advantages of this new technique in plating have been well des-cribed, many studies have attempted to explain the mechanism by which electroplating within Sc-CO2 creates such excellent films [7]. However, in gold-con-sumed industrial processes, the operation cost of electroplating system is of critical importance because expensive gold chemicals affect bath formulation and volumetric productivity. A combination of variables generating a certain optimum response can be ident-ified through factorial design and the use of regres-sion methodology [8]. This pattern is designed by using statistical methods to yield the most information by a minimum number of experiments.

In this work, a non-cyanide system for gold elec-troplating was designed to investigate whether nano-meter-scaled gold film could be achieved using super-critical carbon dioxide processing. Furthermore, the surface response method combined with Box-Behn-ken design was applied to deal with the preparatory conditions of supercritical electroplating process in order to obtain suitable hardness of metal films. The aim of this paper was also to elucidate a simple model of electroplating conditions to control the hard-ness of gold film by supercritical electroplating pro-cess through statistical experimental method.

MATERIALS AND METHODS

Materials

The electroplating solution, which is usually referred to as a non-cyanide gold plating bath, con-sisted of sodium gold sulfite (Na3Au[SO3]2, 0.08 mol/L), ammonium sulfite ([NH4]2SO3, 0.5mol/L), diso-dium dihdrogenethylenediamine tetraacetate, (0.01 mol/L) and potassium oxalate, (K2C2O4, 0.01 mol/L). All chemical reagents used in this work with a min-imum purity of 99.9% were purchased from Toshin Yuka Kogyo Co., Ltd. Carbon dioxide with a minimum purity of 99.9% was purchased from Yun Shan Co. Ltd. A non-ionic block copolymer - poly(ethylene oxide)-poly(propylene oxide) (HO(CH2CH2O)100CH3

(CHCH2O)30 (CH2CH2O)100H, 0.001 mol/L), PEOPPO was obtained from Serva AG, Heidelberg, Germany and employed as a surfactant in our experiments. The anode was a 99.99% purity platinum plate with a size of 20×20 mm2 and the cathode was a brass substrate of the same size. The brass substrate, which was composed of 65.4% of Cu and 34.6% of Zn, had a Vickers hardness value of 112.5 HV. Before the plat-ing reaction, both the anode and the brass substrate were degreased by dipping successively in a 10 wt.% NaOH and a 20 wt.% HCl and rinsing in de-ionized water.

Experimental apparatus

A high-pressure experimental apparatus were fabricated by ourselves and its outline was shown in Figure 1, was used for electroplating. The tempera-ture variation of each run was observed to be less than 1.0 °C. The maximum working temperature and

Figure 1. Schematic representation of the apparatus used for a batch electroplating reaction with emulsion of a CO2, surfactant and the

electroplating solution. The parts of the apparatus are labeled as follows: a) CO2 cylinder; b) cooler; c) high pressure pump; d) temperature-controlled air bath; e) reactor with magnetic stirrer; f) trap; g) programmable power supply; h) gas meter;

BPR: back-pressure regulator; PI: pressure indicator; TI: temperature indicator; V: valve.

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the maximum pressure were 90 °C and 250 bar, respectively. The integrated electroplating cell that had a volume of 200 ml was a stainless steel 316 vessel in a temperature-controlled air bath with an agitator. Both the anode and the cathode were attached using platinum wires to the reactor and were connected to a programmable power-supply; model YPP15030, manufactured by Yamamoto-ms Co., Ltd. A typical electroplating reaction was performed in a constantly agitated ternary system of Sc-CO2, the electroplating solution and a surfactant. The 100 ml gold electroplating solution and the surfactant both were put in a high-pressure cell. CO2 was introduced to the high-pressure cell using a pump and pres-surized to a predetermined pressure. The ternary sys-tem was then constantly agitated using a cross-mag-netic stirrer bar at a speed of 400 rpm under a desir-able constant temperature. The bulk electroplating solution commenced after 30 min of agitation and the entire electroplating reaction for each run was carried out for same amount of electric charge at various operation conditions. Based on Faraday’s law, the electroplating time was executed from 0.5 to 1.5 h depending on the conditions of current density and the thickness deposited gold film was obtained around 5.0±0.3 µm at the same quantity of electricity.

Analysis

The microscopic images of gold deposited film were obtained using a Hitachi S-4700I High-Resol-ution Scanning Electron Microscope & Energy Dis-persive spectrometer. The surface features and aver-age surface roughness values (Ra) were measured using atomic force microscopy (Veeco CP-II, Thermo-Microscopes Co. Ltd, USA). Micro-hardness values were measured using an AKASHI Vickers AVK-C2

hardness machine, with a weight of 50 g. For each sample, considering that the gold films from both the front and back of the brass substrate were also given, each hardness value is actually based on 15 mea-surements [9].

RESULTS AND DISCUSSION

Surface observation

Gold electroplating was operated at a current density of 0.3 A/dm2 in a constantly agitated ternary system of Sc-CO2 (100 ml), and the electroplating solution (100 ml) and the surfactant (0.01 mol/L to the electroplating solution) under conditions 55 °C and 101 bar. The SEM analysis in Figure 2 showed that the size of grains in deposited gold film taken from supercritical plating is in the range of 100 nm. There-fore, a nanometer order electroplating is possible by a hybrid of traditional direct current (DC) electroplating and Sc-CO2 techniques with an emulsion. It involves the electrochemical reaction in emulsions formed by Sc-CO2 and aqueous electrolyte with surfactant PEOPPO. The emulsion particle size ranges typically from several nanometers to several millimeters and can be controlled with surfactants to within a relatively narrow size distribution [10]. When the electroplating solution comes in contact with the cathode, the nuc-leation and the crystal growth can occur. However, when the Sc-CO2 comes in contact with the cathode, the nucleation and the crystal growth cannot occur [11]. This result indicates that Sc-CO2 plays an impor-tant role in increasing the quality of plated film higher because Sc-CO2 electroplating solution emulsion has performed characteristics with a higher diffusion coef-ficient, lower viscosity and the surface tension. This makes the deposited atoms on the cathode move

Figure 2. SEM analysis of deposited gold film obtained at 55 °C, 0.3 A/dm2 and 101 bar.

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more easily into the depressions of place than when using only electroplating solution; thus, films of excel-lent characteristics were obtained [12].

In addition, the average hardness of deposited gold films was 125.2 HV by supercritical electroplat-ing, which is much higher than the hardness 85.6 HV of the gold film produced from the electroplating solution only. Since the hardness value of this plated film in the emulsion has been increased over 50%, there is a possibility that an impurity could have been included in the gold matrix and influence the metal structure. In order to confirm that the plated film is composed of pure gold, we performed measurements using an energy dispersive spectrometer of as-dep-osited 5 µm thick coating made from the emulsion of supercritical CO2 at 101 bar, 55 °C and 0.3 A/dm2 (Figure 3). It can be observed from Figure 3 that the purity of deposited gold film is over 99.9%. The studies on the morphology of deposited gold film per-formed by other investigators [13-14] show that the grain sizes caused by the inhibition of crystal growth as well as inclusions with incorporated impurities play the most important role in determining critical physical properties of hard gold. Thus, we could indicate that the higher hardness of the plated film produced by our method originated from the small grains of the gold and the small grain strengthening that occurs in our system.

Screening the significant variables

Since various parameters potentially affect the supercritical plating process [15], the optimization of experimental conditions represents a critical step in the development of a supercritical gold plating method for hard and soft gold in industrial application. These parameters make it difficult to select the required conditions for subsequent reliable quantif-ications. Several studies have been conducted rec-ently regarding electroplating technology using dense

CO2, such as a supercritical fluid transported chem-ical deposition within a nanometer-scale casting. Electrochemical studies on Sc-CO2 have also been reviewed by other investigators in the past few years [16]. However, there have been no reports within the literature of Sc-CO2 being applied to their practical applications, because of complex of theory and mech-anism. In order to have a better understanding on the role of each process parameter, interactions among process parameters and optimization of process para-meters as well as responses, a statistical analysis is essential. In order to clarify the hardness values of deposited films, an experimental statistical method was applied to designs and analysis of experimental results to deal the preparatory conditions of nano-meter-sized gold film from supercritical electroplating process.

In this study, Design Expert software (version 7.1, Stat-Ease Inc., Minneapolis, MN, USA, 2008) was used for experimental designs and analysis of experimental results. Response surface methodology (RSM) is an empirical modeling technique used to evaluate the relationship between a set of the con-trolled experimental variables and the measured res-ponses. A prior knowledge and understanding of the process and process variables under investigation are necessary for achieving a more realistic model. The effect of five parameters including current density (X1), pressure (X2), temperature (X3), the concentra-tion of gold (X4) and surfactant (X5) were investigated using a 25−1 resolution V fractional factorial design to determine the significant variables that affected hard-ness of deposited gold film. Each variable had two levels to be examined for a high (+1) and low level (−1). The high and low levels selected represented the extremes of normal operating ranges. Table 1 shows the variables and levels in detail.

In variable screening, a 25-1 fractional factorial design (FFD) with V resolution was applied to test the

Figure 3. EDS analysis of deposited gold film obtained at 55 ºC, 0.3 A/dm2 and 101 bar.

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significant variables. The screen experimental design and results are shown in Table 2. Depending on the operating conditions, the hardness of gold film varied from 81.8 to 158.8 HV. Results from analysis of variance are shown in Table 3. Based on experi-mental statistical analysis for F-test method, it can be seen from Table 3 that the model F-value of 43.64 implies the model is significant for quality of deposited gold films navigating the design space. In this case the values of “Prob > F” less than 0.100 indicate the model terms are significant, it is proven that current density, pressure and temperature are significant model terms, whereas the concentration of gold and surfactant did not exert significantly effect on hard-ness of deposited gold film because the values of “Prob > F” greater than 0.100. The three significant variables – current density, temperature and pressure – will be further investigated for the following opti-mization process.

Table 2. 25-1 fractional factorial design and experimental results

Trial no.

Variables Hardness, HV

X1 X2 X3 X4 X5

1 -1 -1 -1 -1 1 103.3

2 1 -1 -1 -1 -1 94.4

3 -1 1 -1 -1 -1 157.5

4 1 1 -1 -1 1 130.9

5 -1 -1 1 -1 -1 88.5

6 1 -1 1 -1 1 83.4

7 -1 1 1 -1 1 138.1

8 1 1 1 -1 -1 120.9

9 -1 -1 -1 1 -1 90.7

10 1 -1 -1 1 1 81.8

11 -1 1 -1 1 1 158.8

12 1 1 -1 1 -1 133.3

13 -1 -1 1 1 1 93.3

14 1 -1 1 1 -1 82.4

15 -1 1 1 1 -1 143.2

16 1 1 1 1 1 122.6

Table 3. Analysis of variance for screening experiments

Source Sum of squares

Degree of freedom

Mean square

F-Value Prob.>F

Model 10597.89 5 2119.58 43.64 <0.0001

X1 948.64 1 948.64 19.53 0.0013

X2 9273.69 1 9273.69 190.93 <0.0001

X3 368.64 1 368.64 7.59 0.0203

X4 6.76 1 6.76 0.14 0.7169

X5 0.16 1 0.16 0.0033 0.9554

Residual 485.70 10 48.57

Cor Total 11083.59 15

Optimization

Based on Box-Behnken design for 3 variables, a set of 17 experiments was carried out [18] and expe-rimental results are shown in Table 4. The experi-mental results were analyzed using statistical methods appropriate to the experimental design used. Design Expert 7.1 was used to analyze the experimental results. According to the RSM methodology, a poly-nomial model was used to fit the independent vari-ables using the following equation:

= + + + ++ + +

0 1 1 2 2 3 3

12 1 2 13 1 2 23 2 3

Y B B x B x B x

B x x B x x B x x (1)

where Y is the response (hardness of deposited gold film), xi are the variables, B0 is the constant coef-ficient, Bi and Bii refer to the coefficients of linear and interaction terms.

Table 4. Box-Behnken design and experimental results

Trial no. Variable Hardness, HV

X1 X2 X3 Measured Predicted

1 -1 -1 0 93.3 88.7

2 1 -1 0 83.8 87.1

3 -1 1 0 157.7 155.7

4 1 1 0 123.5 129.9

5 -1 0 -1 120.2 123.1

6 1 0 -1 125.2 120.0

7 -1 0 1 119.7 121.2

8 1 0 1 103.2 99.1

9 0 -1 -1 90.8 89.6

10 0 1 -1 157.3 153.4

11 0 -1 1 85.6 86.1

12 0 1 1 135.5 132.3

13 0 0 0 115 115.4

14 0 0 0 115 115.4

15 0 0 0 115 115.4

16 0 0 0 114 115.4

17 0 0 0 113 115.4

Table 1. Variables and their levels for 25-1 fractional factorial design

Variable Code level

-1 0 +1

X1: Current density, A/dm2 0.1 0.3 0.5

X2: Pressure, bar 1.0 101 201

X3: Temperature, °C 45 55 65

X4: Gold ion conc., mol/L 0.06 0.08 0.10

X5: Surfactant conc., mol/L 0.01 0.02 0.03

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In the study, the hardness of deposited gold film data was processed for Eq. (1) including ANOVA to obtain the interaction between the process variables and the response. The quality of the fit of polynomial model was expressed by the coefficient of deter-mination, R2, and statistical significance was checked by the F-test in the program [19-20]. The regression analysis gave the following regression model:

= − + −− − + −

1 2

3 1 2 1 3 2 3

119.92 10.85 19.23

7.38 8.90 0.75 3.30

Y x x

x x x x x x x (2)

where x1 is the coded variable of current density, x2 is the coded variable of pressure and x3 is the coded variable of temperature.

The R2 (determination coefficient) of the regres-sion equation obtained from analysis of variance is 0.9260 (a value > 0.75 indicates aptness of the model), which means that the model can explain 92.60% variation in the response. Compared the measured and predicted hardness of deposited gold film in Table 4, it can be observed that most of the standard residuals should lie in the interval of 2.55 with respect to its observed response. Therefore, the predicting response surface equation confirms that the equation gives a reasonable fitting to the experi-mentally observed data.

Based on analyzing the Eq. (2) with statistical experimental method, the optimal condition for deposited gold film should be obtained for industrial application of hard and soft gold navigating the design space. For example, the operation condition should be controlled at 128 bar, 54.3 °C and 0.102 A/dm2 if the required hardness of hard gold application needs to be 125 HV. If the designed hardness of gold film was 90 HV for soft gold application, the optimal con-dition could be executed at 75.2 bar, 52.3 °C and 0.44 A/dm2. Confirmation experiments were performed and the results showed that the difference between the predicted values and the measured values is within 15%. Statistical optimization method overcomes the limitations of classic empirical methods and is proven to be a powerful tool for the optimization of gold film deposition.

CONCLUSIONS

High quality gold films with fine grains have been developed using a new electroplating method involving the emulsion of a supercritical carbon dioxide, an electroplating solution and a surfactant, PEOPPO. The films plated using this method have a uniformity of the surface and Vickers hardness better than the results obtained by using conventional elec-troplating methods. Analysis of the composite mater-

ials by SEM allowed the measurement of grain size in neighborhood of 100 nm. It is also highlighted that the developed medium showed a nanometer-sized gold film with hardness ranging from 83.8 to 157.7 HV has been obtained for industrial application of soft and hard gold by combining the optimal settings of those variables. Data from the present investigation have shown that gold film deposition is dependent mainly on current density, pressure and temperature. On the basis of Box-Behnken design, using response surface methodology, a theoretical approach calculated from numerical calculation is in agreement with the expe-rimental data.

Acknowledgment

The authors sincerely appreciate the financial support of the Nation Science Council of the Republic of China (99-2221-E-242 -007) for this work.

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JAU-KAI WANG

JIR-MING CHAR

Department of Applied Chemistry & Material Science, Fooyin University,

Ta-Liao Hsiang, Kaohsiung City, Taiwan, R.O.C.

NAUČNI RAD

OPTIMIZACIJA TVRDOĆE FILMA OD ZLATA DOBIJENOG SUPERKTIČNIM GALVANOTEHNIČKIM PROCESOM METODOM ODZIVNE POVRŠINE

U radu je korišćeno necijanidno zlatno kupatilo za galvanotehničko dobijanje zlatnog filma

na supstratu od mesinga koristeći emulziju superkritičnog ugljen dioksida. Tvrdoća

nanešenog zlatnog filma, kao zavisne promenljive, iskorišćena je u optimizaciji procesnih

galvanotehničkih parametara statističkim eksperimentalnim metodama. U cilju nalaženja

optimalnih uslova analiziran je efekat gustine struje, pritiska, temperature i sastava

rastvora. Za određivanje karakteristike metalnih filmova korišćeni su elektronski mikroskop

i mikro ispitivač tvrdoće. Ovo ocenjivanje značajnih promenljivih je izvršeno u skaldu sa

parcijalnim faktorijelnim planom 25-1 i V rezolucionom metodom. Eksperimentalni rezultat

pokazuje da su značajne promenljive koje imaju uticaj na nanošenje zlatnog filma: gustina

struje, pritisak i temperatura. Korišćenjem Box–Behnken dizajna i metode odzivne povr-

šine, razvijen je regresioni model fitovanjem eksperimentalnih rezultata sa polinomnom

jednačinom. Na osnovu njega moguće je određivanje optimalnih operativnih uslova za

definisanu tvrdoću u opsegu od 83,8 do 157,7 HV za meke zlatne filmove i tvrde

industrijske filmove.

Ključne reči: galvanotehnički proces, zlato, optimizacija, statistička eksperi-mentalna metoda.