gold immersion deposition on electroless nickel substrates

Journal of The Electrochemical Society, 154 �12� D662-D668 �2007�D662

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Gold Immersion Deposition on Electroless Nickel SubstratesDeposition Process and Influence Factor AnalysisHaiping Liu, Ning Li,z Sifu Bi, and Deyu Li

Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China

An immersion gold-plating process on electroless nickel substrate was investigated. The effects of gold salt, trisodium citrate, bathtemperature, and pH on the gold-immersion-deposition process are also discussed. The study was performed by measuring themixed potential-time curves in situ and electrochemical impedance spectroscopy in combination with X-ray fluorescence spec-trometry �XRF� and atomic force microscopy �AFM� surface analysis. Electrochemical measurements and XRF results show thatboth the deposition rate and the mixed potential changed during the gold-deposition process. These variations reflect the changeof electrode surface state. AFM analysis shows that the morphology of nickel surface changed greatly at the initial stage of golddeposition. A model for understanding the gold-immersion-deposition process was proposed based on the experimental data. Theeffects of KAu�CN�2, trisodium citrate, pH, and temperature on gold-immersion deposition are significant, via affecting the goldfilm formation and the deposition rate. The optimal conditions of aqueous solution for the gold-immersion plating were deter-mined. The experimental results support the explanation of the proposed model.© 2007 The Electrochemical Society. �DOI: 10.1149/1.2790281� All rights reserved.

Manuscript submitted May 4, 2007; revised manuscript received August 13, 2007. Available electronically October 17, 2007.

0013-4651/2007/154�12�/D662/7/$20.00 © The Electrochemical Society

An electroless nickel �EN� layer is frequently used in printedcircuit boards and semiconductor assemblies for its excellent com-position uniformity, corrosion resistance, good solderability, andhardness.1-3 However, EN deposit is active and susceptible to oxi-dation and passivation in air, resulting in decreased solderability.Therefore, a gold layer is deposited directly after EN plating.2,4,5

Electroplated gold has long been used for surface metallizationsuch as connectors, printed circuit boards, wire bonding pads, andsolder ball pads. However, the conventional gold electroplatingneeds electricity supply wiring for plating, which limits the high-density circuit pattern and may cause signal delay and noise.5-7 Im-mersion gold process is now used for its stable bath and processsimplicity. Thus, electroless nickel/immersion gold is widely used asa final finish for microelectronic packaging technology.8-11

The immersion gold process on the EN substrate is a displace-ment reaction that involves Ni atom dissolution into the gold solu-tion, Au+ reduction, and deposition on EN substrate.8,9 The potentialdifference between the Ni and Au element is the driving force ofimmersion plating.12,13 This process can be shown as follows

Anodic Ni → Ni2+ + 2e− ENi2+/Ni = − 0.257 V �1�

Cathodic Au+ + e− → Au0 EAu+/Au = 1.692 V �2�

Overall Ni + 2Au+ → Ni2+ + 2Au �3�Therefore, immersion plating is a combination of two different

simultaneous electrochemical reactions, namely, anodic and ca-thodic reactions; the overall electrode potential can be called mixedpotential. According to mixed-potential theory, the rates of the twodifferent simultaneous reactions at one electrode are independentfrom each other and depend only on the electrode potential.14 Thepolarization curves for independent anodic and cathodic processeshave been used to predict the mixed potential. The polarizationcurves can be obtained by the potential scan technique and by thesteady-state galvanostatic method.14-16 However, the mixed potentialobtained by the former method is dependent on the scan rate, andthe mixed potential obtained by the latter method is not valid if thecatalytic activity of the electrode surface changes with potential overthe range of interest. Therefore, an in situ method for measuring themixed potential �also called open-circuit potential� was extensivelyused to evaluate different immersion metal plating processes suchas immersion copper on silicon,17 magnesium alloy,18 and alu-minum19,20 or gold deposition on nickel.20

z E-mail: [email protected]

address. Redistribution subject to ECS terms128.97.90.221aded on 2014-05-07 to IP

In addition, electrochemical impedance spectroscopy �EIS� waswidely used to investigate qualitative information on the substratesurface/solution interaction at various electroless plating process21,22

and the corrosion performance of EN layers.23,24

According to the immersion plating mechanism, the immersiongold process is considered as a controlled corrosion process of ENsubstrate.9 The corrosion of EN substrates during electroless goldplating is investigated by Shaigan and Ashrafizadeh.4 However, theexact mechanism of immersion gold and the corrosion of EN sub-strate at the initiatory process are not explained by them. In thispaper, an immersion gold-plating process on EN substrate was thor-oughly investigated. The purpose of this research was to clarify themechanism of the immersion gold process and to confirm the influ-ences of the gold salt, trisodium citrate concentration, bath tempera-ture, and pH on the process. The study was performed by measuringthe mixed potential-time curves in situ and EIS in combination withX-ray fluorescence spectrometry �XRF� and atomic force micros-copy �AFM� surface analysis.

Experimental

Bath composition and electrode making.— The chemicals usedin this research were all of analytical grade. The immersion gold-plating solution composition and conditions used in this work arelisted in Table I.

For the effect of immersion gold bath pH and temperature, thepH and temperature vary from 5.0 to 8.0 �adjusted by dilute NaOHor HCl� and from 70 to 90°C, respectively. The effect of KAu�CN�2�from 0 to 3 g L−1� and trisodium citrate concentration �from 10 to35 g L−1� was investigated.

The substrates were 2–3 �m EN layers plated on copper sheetswith dimensions of 10 � 10 � 0.25 mm; these samples were pre-pared by treating the substrate with a series of pretreatment stepsbefore displacement gold deposition. They were first degreased in anacid cleaner �GT-210� for 5 min and then etched in a mixed solutionof sodium hyposulfite �100 g L−1� and 10% sulfuric acid for 1 min.After that they were dipped in 5% sulfuric acid for 2 min. In order

Table I. Basic bath composition and operating conditions for im-mersion gold plating.

KAu�CN�2 1 g L−1

Trisodium citrate 25 g L−1

NH4Cl 40 g L−1

pH 7.0 ± 0.2Bath temperature 85 ± 1°CAgitation Without

) unless CC License in place (see abstract). ecsdl.org/site/terms_use of use (see

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D663Journal of The Electrochemical Society, 154 �12� D662-D668 �2007� D663

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to activate the samples for subsequent EN plating, they were im-mersed in 30 mg L−1 of palladium sulfuric and 30 mL L−1 of sulfu-ric acid for 3 min. Then EN plating was performed for 10 min �bathcomposition and conditions are listed in Table II�, and immersiongold plating was performed for 10 min.

Mixed potential and EIS.— For both mixed potential and EISmeasurements, the Cu substrates were first sealed using insulatedadhesive tape, except for a 1 � 1 cm effective working area, thencoated by an EN layer. After thoroughly rinsing in deionized water,the specimens were immediately dipped into gold solutions as work-ing electrodes for the mixed potential-time curves measurements,while EIS measurement was carried out at steady-state potential.

Mixed potential was recorded as a function of time in a cell withtwo electrodes, a working electrode, and a reference electrode, usinga CHI electrochemical station �model 630A�. For EIS a three-compartment Pyrex cell was used. A Luggin capillary was placednear the working electrode to minimize the solution resistance. Thecounter electrode was a platinum plate, and the reference electrodewas a saturated calomel electrode �SCE�. All potentials were deter-mined with respect to this reference electrode and were controlledby a conventional potentiostat with a programmer.

The EIS measurement was performed in a Gamry electrochemi-cal station �USA� with a frequency range from 100 kHz to 0.1 Hz, abias potential of 0.0 V �vs open-circuit potential of steady state�, anda 5 mV ac perturbation. The software of ZsimWin version 3.00 writ-ten by Dr. Bruno Yeum was used for EIS data analysis.

XRF and AFM analysis.— XRF �Brukeraxs S4 Explorer, Ger-many� was performed to confirm gold deposit weight and the depo-sition rate at different deposition times. The XRF spectra wereevaluated by the automatic analysis program Spectraplus.

The surface morphology of the coatings was examined by AFM�Molecule Imaging, Inc., USA�. AFM measurement is in contactmode using ultralever tips with nominal spring constant at25 N m−1. The AFM images were taken from four samples thatwere immersed in the deposition solution for periods of 0–20 s anddried in air.

Results and Discussion

Deposition process analysis by XRF.— In order to confirm thecharacteristics of the gold deposition process, we studied immersiongold deposition in a solution shown in Table I by XRF analysis.Table III shows the gold weight at different deposition times. Ac-cording to Table III, after 2 s of EN electrode dipping into immer-sion gold bath, the gold is deposited on the nickel surface. After 5 sof dipping, the gold weight shows a prompt increase. Because theweight quantity of gold deposit is so small, it is hardly recognizablefrom the color change of the electrode surface. After 10 s of depo-sition, the gold-deposit weight is 0.708 mg dm−2 and the surfacepresents a slight yellow glistening. After 20 s of deposition, the

Table II. Bath composition and operating conditions for electro-less nickel plating.

NiSO4·6H2O 27 g L−1

NaH2PO2·H2O 29 g L−1

Lactic acid 25 mL L−1

Malic acid 12 mL L−1

pH 5.0 ± 0.2Bath temperature 88 ± 1°CAgitation Without

Table III. Au weight on the surface of Ni–P substrate at different de

Deposition time �s� 0 2 5 10Au weight �mg dm−2� 0.000 0.069 0.223 0


surface of the deposit exhibits a yellow-white color; as the dippingtime prolongs ��50 s�, the color of surface appears visibly golden.

The variations of gold-deposition rate with time are shown inFig. 1. It indicates that the gold-immersion-deposition process couldbe divided into three phases: �i� t � t1 �t1 = 20 s�, deposition be-gins and simultaneously the deposition rate increases rapidly. At20 s of deposition, the deposition rate is 0.0368 �m min−1, which isthe largest value in the immersion gold process. �ii� t1 � t � t2,deposition rate begins to decrease; �iii� t � t2, the decrease trend ofdeposition rate reduces. At 150 s �t2�, the deposition rate is only0.0142 �m min−1; after that time, the rate decreases slowly. The fastincrease of deposition rate at the first stage indicates that the corro-sion rate of nickel substrate is also accelerated.

Surface image at initial immersion gold growth stages.— TheAFM morphology during the initial gold deposition process isshown in Fig. 2. The surface of EN coating exhibits a visible nodu-lar structure �Fig. 2a�. After dipping into the immersion gold bathfor 5 s, the surface of the nodular structure becomes slightly coarse�Fig. 2b�. As the deposition time prolongs, the nodular structuredisappears and evolves into a smaller convex columnlike structure�Fig. 2c and d�. AFM analysis shows that the surface morphology ofthe EN coating changes greatly at the initial stage of gold deposi-tion. These results are in good agreement with the fastest depositionrate, which implies that the corrosion rate of nickel substrate at theinitial stages of immersion gold deposition is also very fast.

Characteristics of mixed potential-time curves.— Figure 3shows the mixed potential-time curve during the immersion goldprocess in a solution shown in Table I. Based on the characteristicsof the curve, the immersion gold process can be divided into threedistinct phases: �i� t � t1, electrode potential drop. At t = 20 s, thepotential reaches the lowest value, −0.621 V. �ii� t1 � t � t2, thepotential positive shifts and at t = 150 s, the potential is −0.554 V.�iii� t � t2, the potential attains the plateau potential and then main-tains at −0.550–0.560 V.

From the experimental results, it is clear that the changes inmixed potential and deposition rate during the immersion gold pro-cess could reflect the variation of the electrode surface state. Notethat the vertical drops of the potential at the beginning of the curve

ion times.

20 50 100 150 2002.330 4.353 5.137 7.690 7.930

Figure 1. Characteristics of gold immersion deposition rate-time curve insolution shown in Table I.

posit

.708



D664 Journal of The Electrochemical Society, 154 �12� D662-D668 �2007�D664

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may be attributed to the slow response of the measurement appara-tus. After that the negative shifting of the potential could be relatedto the abundant adsorption of electronegative particles.

As the EN electrode begins dipping into the gold plating solu-tion, Au�CN�2

− and other electronegative particles �such as citrateanion� begin to adsorb on the EN surface, resulting in a negativeshifting of the potential. In the EN surface, the Au+−Ni galvanic cellis formed. Ni atoms oxidize; Au�CN�2

− reduces and deposits on the

Figure 2. �Color online� Typical AFM morphologies of the coatings at diffe

Figure 3. Mixed potential-time curves of immersion gold on nickel substratein solution shown in Table I.


EN surface. With the adsorption of electronegative particles andAu�CN�2

− increase on the electrode, the potential continues its nega-tive shift and the gold deposition accelerates. When the adsorptionof these electronegative particles at the surface attains the saturationstate �t = t1�, the potential moves to the lowest, and the depositionrate arrives the fastest. As the gold deposition proceeds �t1 � t� t2�, the nickel surface of the electrode is gradually displaced bythe gold deposit, accompanied with a positive shifting of the poten-tial and a decrease of the deposition rate. As the deposition time isprolonged �t � t2�, the potential reaches the plateau value, attrib-uted to the buildup of gold film on nickel substrate.

Characteristics of immersion gold deposition at plateau poten-tial.— According to the mechanism of immersion plating, the im-mersion gold reaction would be terminated once gold deposit coversthe nickel substrate.8,9 However, the above tests show that the golddeposition reaction continues for a long time after the gold deposithas already built up on the nickel substrate. A reasonable interpre-tation of such information requires input from other experiments.Therefore, the EIS measurements were conducted during the immer-sion gold process. The measurement was performed after EN sub-strate was immersed in the gold solution for 180 s, which coincidedwith the plateau stage on the mixed-potential plot �Fig. 3�. Duringthis period, the gold deposit had already built up. The impedancespectra in a solution shown in Table I is presented as Fig. 4, and anequivalent electrical circuit model �insert Fig. 4� was applied tosimulate the Au coating/solution interface. The values of electro-chemical parameters obtained by curve-fitting using the equivalentcircuits are given in Table IV. In this equivalent circuit model, RL isthe solution resistance between the Au coating and the tip of the

mmersion gold times. �a� 0, �b� 5, �c� 10, and �d� 20 s.
rent i



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Luggin capillary, Cdl is the double-layer capacitance of Au coating,and Rr is the charge-transfer resistance of the electrochemical reac-tion. Ra and Ca provide information about the adsorption species atthe Au coating/plating solution interface.

From Table IV, it can be seen that a low value of RL�1.185 � cm2� is mainly due to the contribution of the gold-platingsolution. Rr is 86.05 � cm2 and Ra is 73.91 � cm2. It was reportedthat the double-layer capacitance �Cdl� values of metal coatings, i.e.,the EN layer, are in the range of 8–32 �F cm−2.24,25 The Cdl value isrelated to the porosity of the coating. In this study, the Cdl value�1540 �F cm−2� is much higher than that of the general metal coat-ing, which confirms the porosity of immersion gold film on the ENsubstrate. Moreover, the value of double-layer capacitance �Ca� ofthe adsorption layer �4970 �F cm−2� is higher compared to that ofporous Au coating, which indicates that the adsorption layer has amore porous nature than the Au coating. These results demonstratethat the immersion gold deposit is porous. The tiny holes on the golddeposits expose the nickel surface to the gold bath and result incontinuous deposition at the plateau potential.

Gold-immersion-deposition model.— According to the aboveanalysis, following the basic mechanism of immersion gold plating,a model �Fig. 5� is proposed for the understanding of the gold-immersion-deposition process. In this model, the gold plating pro-cess could be divided into three sections:

1. �AuLm�− diffuse from the bulk to the surface of nickel elec-trode and adsorb on the electrode surface.

2. In the EN surface, an Au+−Ni galvanic cell is formed. Niatoms oxidize, and the adsorbed �AuLm�− reduces and deposits onthe EN surface.

3. After the nickel surface is basically covered by gold deposit,the Ni surface continues to oxidize and dissolve through the tinyholes on the gold deposit; gold atoms deposit on the outer layer ofthe deposit.

The influence factors analysis.— It is known that immersiongold deposition would be influenced by bath pH, temperature,

Figure 4. Complex-plane impedance plot during the immersion gold processat the plateau potential. The calculation data is the simulation result accord-ing to the equivalent-circuit insert in this figure.

Table IV. Electrochemical parameters of Au coating during immersiosimulation.

Parameters R1 �� cm2� Cd ��F cm−2�Values 1.185 1540


KAu�CN�2 concentration, and so on. Therefore, the effects of theabove factors on deposition rate and deposition process were inves-tigated.

Effect of KAu�CN�2 concentration.— The dependence of mixedpotential-time curves and deposition rate on KAu�CN�2 concentra-tions are presented in Fig. 6. Figure 6a indicates that the nickel-electrode potential maintains steady at −0.64 V without KAu�CN�2in the bath, while with the addition of KAu�CN�2, the plateau po-tential shifts to the positive direction. The time for the potential toreach the plateau value decreases with increasing KAu�CN�2 con-centration.

According to the basic mechanism of immersion gold plating, thecathodic reaction of gold deposition could be expressed as the fol-lowing

Au�CN�2− + e− → Au0 + 2CN− �4�

Therefore, the potential of gold deposit �EAu�CN�2−/Au� could be de-

scribed as follows by the Nernst equation

process in a solution shown in Table I obtained by equivalent circuit

Rr �� cm2� Ca ��F cm−2� Ra �� cm2�86.05 4970 73.91

Figure 5. �Color online� Schematic representation of different stages of im-mersion gold plating based on Ni–P substrate: �a� section 1, �b� section 2,and �c� section 3.

n gold




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EAu�CN�2−/Au = EAu�CN�2

−/Au0 + �2.3RT/F�ln �m + �2.3RT/F�ln CAu+

�5�

Here �m represents the stability constant of Au�CN�2−.

Because the gold film has already built up on the nickel substratewhen the electrode potential attains the plateau value, it could beconcluded that the variation of the plateau potential agrees with thevariation of the potential of gold deposit �EAu�CN�2

−/Au�. According toEq. 5, a higher Au+ concentration could increase the EAu�CN�2

−/Au,which indicates that the plateau potential of immersion gold in-creases accordingly.

Figure 6b shows that the deposition rate increases with an in-crease of KAu�CN�2 concentration. The deposition rate increasesfrom 0.0587 to 0.0676 �m �10 min�−1 in the range of 0.5–2.0 g L−1

KAu�CN�2. However, as the KAu�CN�2 concentration exceeds2.0 g L−1, the increase becomes negligible. Therefore, the concen-tration of KAu�CN�2 may vary in the range from 1.0 to 2.0 g L−1

depending on the desired deposition rate.

Effect of trisodium citrate.— Figure 7 shows the effect of trisodiumcitrate on the mixed potential-time curves and the deposition rate.Both the minimum potential and the plateau potential shifts morenegative with increased trisodium citrate concentration �Fig. 7a�.However, the time for the potential to reach the plateau value is notsignificantly affected by the trisodium citrate concentration. Fromthe proposed deposition model, increasing citrate concentrations in-dicate increasing adsorption of the electronegative ion on the sur-face; hence, the electrode potential decreases.

Figure 6. Dependence of the �a� mixed potential-time and �b� deposition rateon KAu�CN�2 concentration for immersion gold.


Figure 7b shows that at first the deposition rate increases slightlywith trisodium citrate. However, as trisodium citrate concentrationexceeds 25 g L−1, the deposition rate decreases gradually. There-fore, in this immersion gold bath, the optimal sodium citrate con-centration range is 20–30 g L−1.Effect of pH.— The influence of pH on the mixed potential-timecurves and deposition rate for immersion gold deposition is shownin Fig. 8. With increasing the solution pH from 5.0 to 8.0, the timefor the potential to reach the plateau value becomes short and theplateau potential shifts to the negative direction for about 76 mV�Fig. 8a�. Figure 8b presents the deposition rate increases with anincrease of pH monotonically. The deposition rate increases from0.0497 to 0.0678 �m �10 min�−1 as the pH increases from 5.0 to8.0. Considering the excessive attacking of the alkaline bath to theNi substrate, the appropriate pH range is 6.0–7.0.

These influences of pH on the potential and the deposition ratemay be related to the adsorption changes of metal complexes on theelectrode surface. Au�CN�2

−, citrate and ammonia chloride are con-tained in the gold-plating solutions. In addition, as the immersiongold proceeds, a small quantity of Ni2+ is also produced. In a lowerpH value, the Au+ usually exists as Au�CN�2

−, and Ni2+ is in Ni–citrate �or Ni–cyanide� complexes. However, by gradually increas-ing the pH value, free ammonia is present in the solution.26 Inthe presence of ammonia, these binary complexes for Au and Nicould be evolved as Au�CN�2�NH3�n and Ni�citrate��NH3�n �orNi�cyanide��NH3�n�, respectively. These ternary complexes havestronger adsorption than both binary complexes,26,27 which indicatesthat the absorption of the electronegative ion on the electrode sur-

Figure 7. Dependence of the �a� mixed potential-time and �b� deposition rateon trisodium citrate concentration for immersion gold.




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face also increases. Therefore, increasing of the bath pH value couldresult in the negative shifting of the electrode potential. Also, in-creasing the electronegative ions �reaction active ions� on the elec-trode surface could accelerate the deposition rate.

Effect of bath temperature.— Figure 9 presents the dependence ofthe mixed potential-time curves and deposition rate on the tempera-ture. The positive shifting of the plateau potential is very slight inthe working temperature range 70–90°C �Fig. 9a�. This result isconsistent with Eq. 5. Moreover, the deposition rate linearly in-creases with temperature �Fig. 9b�, indicating that the corrosion rateof Ni substrate also increases. Therefore, the optimal temperaturerange is 75–85°C.

For an understanding of the effective mechanism of temperatureon the deposition rate, the enthalpy change of the reaction ��Hf

��involved in gold immersion deposition �Reaction 6� is listed below�in order to calculate the reaction enthalpy, here, the Au+, Ni2+ ionsare proposed to complex with cyanide�

2Au�CN�2− + Ni0 → 2Au0 + Ni�CN�4

2− �6�

The �Hf� for Au�CN�2

− and Ni�CN�42− are 242.3 and 367.8

�kJ mol−1�, respectively. The enthalpy change of Reaction 6 is−116.8 �kJ mol−1�, indicating that Reaction 6 is endothermic.Therefore, increasing temperature is favorable to Ni oxidation andAu reduction �Reaction 6�. As a result, the deposition rate of immer-sion gold increases with increasing bath temperature.

Overall, for both the mixed potential and the deposition-ratemeasurements during the immersion process, the results show sig-nificant changes in plateau potential and the deposition rate under

Figure 8. Dependence of the �a� mixed potential-time and �b� deposition rateon the pH for immersion gold.


various conditions. These changes reflect information originatingfrom the gold-film formation and deposition process.

Conclusion

1. Both the deposition rate and the mixed potential have signifi-cant changes during the gold deposition process, which reflects thevariation of the electrode surface state. The morphology of the elec-troless nickel surface changed greatly at the initial gold-depositionstage. These results are in good agreement with the highest deposi-tion rate.

2. A model for understanding the gold-immersion-deposition pro-cess is proposed which can explain the influences of KAu�CN�2,trisodium citrate concentration, pH, and temperature on the deposi-tion process.

3. Increasing KAu�CN�2 concentration or the bath pH and tem-perature can increase the gold-deposition rate. Based on experimen-tal data, the optimal working conditions for gold-immersion platingwere determined, which is important for practical applications ofdeveloped solutions.

Acknowledgments

This work was financially supported by the Specialized ResearchFund for the Doctoral Program of Higher Education �20030213007�,the Heilongjiang Province Natural Science Foundation �E0325�, andSuzhou Macderlun Chemical Company, Limited.

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Figure 9. Dependence of the �a� mixed potential-time and �b� deposition rateon the bath temperature for immersion gold.




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gold immersion deposition on electroless nickel substrates

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