Effect of morphology and hydrogen evolution on porosity of electroplated cobalt hard gold

Article abstract of DOI:10.1149/1.3430076

The impact of plating parameters on the porosity of cobalt hard gold electrodeposited from a cyanide bath is presented. Both constant current and pulsed current deposition were studied by electrochemical and microscopy methods. As observed previously, pores tend to decorate grain boundaries, defects, and other morphology features. Focused ion beam scanning electron microscopy indicates that perhaps only a small fraction of pores seen on the surface of the ～350 nm thick gold film extends to the electroplated Ni underlayer. The density of pores depends on morphology and current efficiency, both of which were found to vary significantly with plating conditions. Faceted surface features correlate with a higher pore density. For a similar morphology, the pore density increases with the hydrogen generation rate, estimated from current efficiency measurements. Pulse plating conditions, with relatively high peak current and frequency, result in deposits with lower porosity due to the capability of generating finer grains while influencing hydrogen evolution rates.

Full text of DOI:10.1149/1.3430076

Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
D411
0013-4651/2010/157͑7͒/D411/6/$28.00 © The Electrochemical Society
Effect of Morphology and Hydrogen Evolution on Porosity
of Electroplated Cobalt Hard Gold
,z
b
a
Zhengwei Liu,a, Min Zheng,
Robert D. Hilty, and Alan C. West
*
b,**
aDepartment of Chemical Engineering, Columbia University, New York, New York 10027, USA
bTyco Electronics, Technology, Middletown, Pennsylvania 17057, USA
The impact of plating parameters on the porosity of cobalt hard gold electrodeposited from a cyanide bath is presented. Both
constant current and pulsed current deposition were studied by electrochemical and microscopy methods. As observed previously,
pores tend to decorate grain boundaries, defects, and other morphology features. Focused ion beam scanning electron microscopy
indicates that perhaps only a small fraction of pores seen on the surface of the ϳ350 nm thick gold film extends to the
electroplated Ni underlayer. The density of pores depends on morphology and current efficiency, both of which were found to vary
significantly with plating conditions. Faceted surface features correlate with a higher pore density. For a similar morphology, the
pore density increases with the hydrogen generation rate, estimated from current efficiency measurements. Pulse plating condi-
tions, with relatively high peak current and frequency, result in deposits with lower porosity due to the capability of generating
finer grains while influencing hydrogen evolution rates.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3430076͔ All rights reserved.
Manuscript submitted November 6, 2009; revised manuscript received April 15, 2010. Published May 21, 2010.
Electrochemically deposited cobalt hard gold is important in con-
nector applications because of its favorable properties, including
low contact resistance, high corrosion resistance, and high wear
resistance.1 The high price of gold and its considerable usage make
it an important task to improve gold deposit properties, allowing
potential reduction in gold usage. A major impediment to reducing
gold film thickness is the corrosion of the underlying substrate, the
rate of which is directly impacted by the porosity of the gold layer.2
The porosity of various electrodeposited metal coatings was re-
viewed by Notter and Gabe.3 For hard gold, the porosity has been
investigated on several occasions including studies of the methodol-
ogy for porosity evaluation,4 the performance evaluation of different
plating bath compositions and plating techniques,2,5 the correlation
between porosity and film thickness,6 and the structural study of the
electrodeposits.5,7-9 However, to the best of our knowledge, the fun-
damental factors that affect porosity have not yet been identified and
investigated systematically. Some related studies can be divided into
two categories: One is from the structural viewpoint and the other is
about hydrogen evolution.
Among various methods for studying porosity, structural studies
by electron microscopy3 have been helpful for the characterization
of pore shape and size. Researchers7,8 found that the pores predomi-
nately appear at the boundaries of features ͑or grains͒ of the micro-
structure and are uniformly round, suggesting that they were formed
by hydrogen bubbles produced during the plating process. In an
attempt to explain the effect of deposit morphology on porosity,
Popov et al.10,11 explained the relationship between porosity and
roughness for nickel electrodeposits. For gold electroplating, such
analysis has not yet been performed.
hydrogen evolution rate on porosity. Based on our findings, we pro-
posed the plating conditions to achieve lower porosity.
Experimental
The substrate used in this study was a 1 mm diameter copper
wire sealed in a 5 mm epoxy cylinder. Before plating, the cross
section of the copper wire was polished by polishing paper and 300
nm alumina polishing beads until it had a mirrorlike finish. Then it
was ultrasonically washed in deionized ͑DI͒ water for 15 min and
immersed in 10% sulfuric acid for 10 min for activation. A nickel
sulfamate bath was used for plating a nickel undercoating layer with
a thickness of 1 ␮m at a constant current density of 50 mA cm−2
.
The nickel sulfamate bath contained 90 g/L ͑1.53 M͒ nickel sulfa-
mate ͑measured as nickel metal͒, 38 g/L ͑0.61 M͒ boric acid, and 15
g/L ͑0.094 M͒ nickel bromide ͑measured as bromide͒; pH was about
3.4. All depositions were conducted between 40 and 50°C.
After nickel plating, the substrate was rinsed with DI water to
remove any nickel electrolyte residue. A commercially available co-
balt hard gold bath ͑Metalor Gold, Metalor Technologies͒ was used
for gold plating. This Metalor Gold bath contained gold salt ͑0.04 M
potassium gold cyanide salt͒, conducting salt ͑a mixture of organic
acid salt and potassium oxalate monohydrate͒, acid salt ͑citrate-acid-
based͒, brightener ͑mixture of potassium hydroxide and aromatic
organic compounds͒, and cobalt additive. The pH of this bath was
around 5. The temperature of the plating bath was set to be 45°C
using a water bath ͑Isotemp 3016 Fisher Scientific͒. The gold de-
posit has a purity of about 98.9% as gold containing primarily Co
with trace amounts of C, H, O, N, and K.
DC plating and pulse plating were employed to deposit the gold
layer onto the nickel underlayer. Detailed plating parameters are
indicated in the Results section. Both nickel and gold deposition
processes were controlled by a Princeton Applied Research model
273A potentiostat. The hydrodynamic conditions were controlled by
using a rotation rate of 2500 rpm, controlled by a Pine Instruments
rotator. This high rotation rate leads to a small diffusion layer thick-
ness in a range achieved in reel-to-reel plating, which is employed in
industry.
The characterization of the microstructure of the deposits in-
cluded the study of morphology and roughness. The scanning elec-
tron microscope ͑SEM͒ images for morphology characterization and
the side-view images of the pores were obtained by a focused ion
beam ͑FIB͒ SEM ͑FEI-Nova NanoSEM͒ at a working potential of 5
kV and a working distance of around 5 mm. Top-view SEM images
used for porosity characterization were obtained using a Hitachi
4700 SEM. Roughness data were measured using a surface profiler
͑Alpha Step IQ͒. Each roughness measurement was repeated five
times. The roughness number is the average roughness obtained
from a scan range of 200 ␮m.
Regarding hydrogen evolution, which is a common side reaction
in metal plating processes,12 there are other facts to support its im-
portance in porosity formation. The current efficiency, a measure-
ment of hydrogen evolution, is reported to be lower than 40%13,14 in
cobalt hard gold plating, with a precise value depending on the
particular bath. A high atomic percentage of hydrogen, about
6.9%,15 has been found in the deposit. The presence of hydrogen in
gold deposit is in the form of hydrogen gas trapped in the voids due
to the low solubility of hydrogen in the gold lattice.16 From these
facts, the hypothesis that the pores are a result of hydrogen bubbles8
seems reasonable.
To elucidate key factors of porosity formation, we conducted a
systematic study on deposits from constant current ͑dc͒ and pulse
plating16-20 conditions. We emphasize the impact of morphology and
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: zl2173@columbia.edu
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Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
Figure 1. SEM images of samples ob-
tained by dc plating at 20 mA cm−2 for
͑a͒ 25, ͑b͒ 50, and ͑c͒ 100 s.
(a)
(b)
(c)
The current efficiency was evaluated by a method developed by
Cheh.17 A 0.1 M potassium cyanide electrolyte was used to electro-
chemically strip, at a current density of 6 mA cm−2, the gold until
the nickel undercoating layer is exposed. The number of coulombs
was counted based on the applied current and stripping time. In this
method, the end of the stripping process is indicated by a substantial
change in potential when the nickel is exposed. Current efficiency
obtained from measuring the amount of gold plated by dissolving
the gold in 0.1 M HCl solution and measuring the gold concentra-
tion by an atomic absorption spectroscope ͑Buck Scientific, model
200A͒ was used to confirm the accuracy of the coulometric stripping
method.
Pulse plating with a rectangular wave form has been used in gold
electroplating,17,22-25 and multiple parameters result in the same time
average current density, iavg. We compared deposits obtained from
the same iavg. Specifically, we hold iavg constant and vary frequency
f and peak current density ipeak
ton
iavg = ipeak
ϫ
͓1͔
ton + toff
1
f =
͓2͔
ton + toff
The porosity was characterized by microscopic observation2,3 of
the electrodeposits’ surface, combined with image processing and
analysis. The backscattered electron mode of the SEM provided the
best contrast. In this mode, pores appeared to be darker compared to
gold. Based on this difference in brightness and contrast, the pores
are analyzed by an NIH-developed software, ImageJ, to give the
number density of pores and size statistics from processed images.
The detailed image process and analysis can be found in Ref. 18.
Through Eq. 1, the duty cycle, ton/͑ton + toff͒, is changed in the
range of 0.1–0.2 based on iavg and ipeak
.
Results and Discussion
Morphology of electrodeposits from various plating con-
ditions.— The morphology of electrodeposits can be affected by
many parameters including applied current density, plating time,
bath composition, and substrate properties. In this study, we keep
the bath composition and substrate constant to focus on the effect of
plating condition on morphology and current efficiency, which are
important factors in estimating porosity.
(a)
(b)
In Fig. 1, the morphology of samples evolves with plating time
over time scales of interest in this work. After 25 s, hemispherical
surface features are observed. The sizes of these features grow until
they reach a transition time, after which the feature becomes faceted.
Similar trends were observed in nickel hard gold plating onto copper
substrate19 and in other electroplated metal systems.20 In a connector
application, a surface with faceted features results in a substantial
increase in the contact resistance, making it an unacceptable candi-
date for good connectors. Thus, the optimal plating condition must
be able to avoid that.
(c)
(d)
Figure 2 shows the morphology of cobalt hard gold electrode-
posits from dc plating at different current densities. The time of
deposition is adjusted to achieve roughly the same film thickness
͑estimated from current efficiency͒. From the images, it is found that
the morphology changes dramatically with current density. At lower
current densities, the grains are faceted with a large size, up to
several hundred nanometers. At higher current densities, the grain
structure started to become round with a small size from 10 to 30
500 nm
nm. For intermediate current densities, for instance, 10 mA cm−2
,
large faceted grains and small round grains coexist, indicating a
gradually changing effect of current density on grain structure. This
change in grain size has been explained well in a previous paper.21
At lower current densities, crystal growth is favored compared to
crystal nucleation. This leads to large grains. At higher current den-
sities, more energy is provided to overcome the energy barrier.
Nucleation is promoted at a greater rate than growth.
(e)
Figure 2. SEM images of samples obtained by dc plating. All samples have
an approximate thickness of 350 nm at different current densities: ͑a͒ 2.5, ͑b͒
5, ͑c͒ 10, ͑d͒ 25, and ͑e͒ 50 mA cm−2. The thickness estimates are obtained
from Faraday’s law with a current efficiency correction.
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Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
D413
the potential response changes substantially with frequency at a
given ipeak and iavg. At 0.1 Hz, the potential at the end of the on-time
is close to the potential response at ipeak for dc plating. At the end of
the off-time, the potential is close to the open-circuit potential. As
the frequency increases, the potential responses during the on-time
and off-time both move toward the potential response of dc plating
at iavg. At a frequency of 100 Hz, the potential responses during the
whole pulse duration are close to dc plating at iavg. A detailed dis-
cussion of the impact of frequency on potential response can be
found in Ref. 23.
As shown in Fig. 2 and 3, at the lower current densities, pores are
located between adjacent grains, with many at the triple junctions of
the grains. At higher current densities, pores appear to be randomly
distributed throughout the coating. In some samples, there is a pat-
tern for pore formation, favoring areas near defects on substrates
and undercoating layers, such as a polishing line. However, for most
samples that have been carefully polished or used a freshly plated
nickel undercoating, pores from the defects can be eliminated. In all
conditions, most of the pores are round and larger than 10 nm,
suggesting that they were not formed by polymeric material
inclusions,5 which are smaller than a few nanometers. However,
from the present study, we cannot definitely conclude that the pores
are indeed the result of hydrogen bubbles.
(b)
(a)
(c)
(d)
500 nm
Cross-sectional view of pores from FIB-SEM.— Pores can be
divided into “through-pores” and “masked pores” depending on
whether they extend from the underneath layer to the surface of the
coating.3 Through-pores are most susceptible to attacks from a cor-
rosive environment. To verify whether the pores we observed on the
coating surface are through-pores, FIB-SEM was used. In Fig. 5a-c,
the images are obtained by exposing a pore. The lighter layer is
gold; and the darker layer is the nickel undercoating. In Fig. 5a, the
pores are hidden in the coating layer, indicating that the pores are
not straight. As the ion beam continuously removes the Au coating
in the direction of the target pore as shown in Fig. 5b, it is also
found that the size of the pore is not constant. When finally the
sidewall of the pore is removed, we can clearly see that the pore
fully extended through the entire Au coating layer. However, most of
the pores are not like this particular one, which is approximately
vertical to the substrate. Figure 5d shows a cross section of the gold
layer, between platinum, which was later deposited to protect the
gold surface in FIB-SEM characterization, and the nickel undercoat-
ing layer. Pores are randomly distributed throughout the whole cross
section. The round shape and size distribution are similar to the
observation from the top view in Fig. 2 and 3.
Figure 3. SEM images of samples from pulse plating at the same time
average current density iavg of 10 mA cm−2. All samples have an approxi-
mated thickness of 350 nm. The ipeak and f are ͑a͒ −100 mA cm−2, 1 Hz; ͑b͒
−100 mA cm−2
,
100 Hz; ͑c͒ −50 mA cm−2
,
1
Hz; and ͑d͒
−50 mA cm−2,100 Hz, respectively.
Figure 3 reveals the morphology of pulse plating samples. All the
samples were obtained at the same iavg = 10 mA cm−2 and differ-
ent ipeak and f. It is found that a higher ipeak results in smaller grains.
It is also found that the lower frequency generates a sample with
morphology close to the dc plating at ipeak. At higher frequencies,
small round grains started to merge to form larger faceted crystals,
getting close to the appearance of the surface topography seen for dc
plating at iavg. The impact of pulse plating parameters on deposit
morphology is not unique to the cobalt hard gold case. Similar ob-
servations have been made in other metal electroplating studies.11,23
The impact of frequency on morphology may be related to the
overpotential at which deposition takes place. As indicated in Fig. 4,
Roughness of electrodeposits from different plating con-
ditions.— Roughness can also affect the porosity.3 For nickel
electroplating,10,11 surfaces with higher roughness result in a larger
porosity. We studied the roughness of electroplated cobalt hard gold.
The data are shown in Fig. 6. It is found that the roughness changes
with the grain size. At lower current densities ͑5 and 10 mA/cm2͒,
roughness is substantially larger than the roughness obtained at
higher current densities for dc plating and for pulse plating samples.
At higher dc plating rates and for films obtained by pulse plating,
small, round grains lead to a smoother surface. This change in
roughness with grain structure was also revealed in nickel hard gold
plating.19
-0.4
-0.6
End of on time
End of off time
-0.8
Open Circuit Potential
DC Potential at ipeak
DC Potneital at iavg
-1.0
-1.2
-1.4
Evaluation of current efficiency by coulometric stripping
method.— During the gold plating process, with the gold reduced,
hydrogen evolution reaction happens as a side reaction. Current ef-
ficiency indicates the ratio of the current due to gold reduction with
total applied current. In Fig. 7, current efficiency is plotted as a
function of current density for dc plating. The current efficiency
increases to a maximum value near 20 mA cm−2, and then de-
creases with increasing current density.
0.1
1
10
100
Frequency (Hz)
Figure 4. The variation in the electrode potential at the end of the on
͑squares͒ and off ͑triangles͒ times as a function of frequency. In all cases,
ipeak = 100 mA cm−2 and iavg = 20 mA cm−2. Results are taken at the end
of the cycle after 10 s of plating. The solid line is the open-circuit potential.
The long dashed line is the potential at ipeak for dc plating. The short dashed
line is the potential at iavg for dc plating.
For the pulse plating study, the time average current density iavg
was maintained at 20 mA cm−2, which had the maximum current
efficiency in dc plating. The current efficiency is plotted in Fig. 8 as
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D414
Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
Figure 5. Side view of pores in the CoHG
layer. ͓͑a͒-͑c͔͒ The images of the same
sample after different etch times by using
the FIB. Starting from the original,
etched-through rectangular feature, the
FIB progressively removes material in the
direction of the indicated pore. ͑d͒ A
cross-section view of a sample obtained
from dc plating.
(a)
(b)
(c)
Pt
Au
500 nm
Ni
(d)
efficiency. Both morphology and current efficiencies vary with plat-
ing conditions, complicating the analysis of any systematic investi-
gation. However, by varying f and holding ipeak and iavg constant for
pulse plating, we can have deposits with different current efficien-
cies and with a relatively small difference in morphology ͑shown in
Fig. 3͒ and roughness ͑indicated in Fig. 6͒. These have been used to
study the influence of current efficiency on porosity. Results are
shown in Fig. 9. We found an approximately linear relation between
porosity and current efficiency, indicated by the dashed line for
pulse plating conditions with ipeak of 100 mA cm−2. Higher peak
current promotes nucleation over growth. Thus, a smooth surface
with fine grains can be achieved. However, a very high peak current
leads to a low current efficiency. 100 mA cm−2 is chosen as a mod-
erate peak current density to realize two requirements. Samples with
similar current efficiency, however, may be useful for analyzing the
impact of microstructure because the hydrogen evolution rate is the
same. Focusing on the points in the circle in Fig. 9, pulse plating
generates fewer pores than dc plating. This may be explained by the
difference in morphology and roughness of the samples. Surface
roughness and grain size are larger for deposits obtained from
a function of frequency f for different ipeak. The duty cycles are
changed according to ipeak. The results indicated that the current
efficiency falls between the dc current efficiencies obtained at ipeak
and iavg. At low frequencies, the current efficiency is close to the
value of ipeak. With the increase in f, current efficiency moves to-
ward the value of iavg. This gradual change in current efficiency with
f can be related to the chance of potential response with f, shown in
Fig. 4.
Correlation of hydrogen evolution with porosity.— In our study,
as we doubled the thickness from about 350 to 700 nm, porosity
decreased for about 45% in dc plating. This fairly agrees with the
porosity study carried out by other methods.2,6 To investigate critical
factors to control porosity, we chose to study the samples from dif-
ferent plating conditions with the same film thickness. Figure 9
shows the dependence of porosity on current efficiency for dc and
pulse plating. Because we hypothesize that the porosity depends on
both microstructure and current efficiency, deposits with similar
morphology are good candidates to investigate the impact of current
0.024
0.022
0.020
0.018
0.016
0.014
0.012
0.010
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
Plating Condition
0
10
20
30
40
50
60
70
80
90
100
Current Density (mA cm-2)
Figure 6. Sample roughness obtained by plating under a variety of dc plat-
ing conditions. The reported values indicate current density and pulse plating
conditions at ipeak = 100 mA cm−2 and iavg = 20 mA cm−2 at different fre-
quencies.
Figure 7. The measured current efficiency as a function of applied dc cur-
rent density.
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Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
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30
0.8
0.7
0.6
0.5
0.4
0.3
0.2
25
20
15
-2
ipeak 50 mA cm
-2
ipeak 100 mA cm
-2
ipeak 200 mA cm
-2
10
DC plating
DC 20 mA cm
DC 50 mA cm
PP at ipeak100 mA cm-2
-2
-2
PP at ipeak 200 mA cm-2
DC 100 mA cm
5
0.1
1
10
100
0
5
10
15
20
60
Hydrogen Evolution Current (mA cm-2)
Frequency (Hz)
Figure 8. The measured current efficiency as a function of the frequency for
three peak current densities. In all cases, the time average current density
iavg = 20 mA cm−2. Horizontal lines provide a reference of current effi-
Figure 10. The variation in pore density with the rate of hydrogen evolution.
For the pulsed plating data taken with ipeak = 100 mA cm−2, a linear varia-
tion is observed.
ciency results obtained for dc plating conditions at the indicated iavg
.
gen is produced from the reduction of hydrogen ions, creating
bubbles in some cases. Once a bubble is big enough, it detaches
from the surface. New bubbles are formed continuously and follow
the same life cycle as their ancestors. During the off-time of a pulse,
hydrogen bubbles may coalesce and detach from the deposit surface.
In this complex process, the rate of hydrogen evolution is important.
In Fig. 10, hydrogen evolution current densities, calculated using
Eq. 3, and current efficiency, ␩, from Fig. 8, are plotted against
porosity
smaller current densities in dc plating than samples from pulse plat-
ing, for conditions with similar current efficiency. For samples with
large faceted grains, hydrogen bubbles prefer to form at the junc-
tions of these grains. At the junctions, hydrogen nuclei have a
smaller critical volume than at the grain faces. Because the growth
rate of faceted grains is generally slow, this provides more time for
hydrogen bubbles at the junctions to grow into a relatively “stable”
phase, which exceeds the critical volume. The attached hydrogen
bubbles are then trapped by deposited gold to form pores. Another
explanation for this preferential growth of pores at the junctions of
large faceted grains is the accumulation of nonmetallic impurities26
at the junctions. A smoother surface with small and uniform round
grains, which is produced by pulse plating, cannot provide such
preferential growth locations as adjacent large faceted grains do.
Thus, a less porous coating is achieved. The advantage of pulse
plating for generating less porous coating has also been reported by
other researchers.22,23
ihydrogen = itotal͑1 − ␩͒
͓3͔
where ihydrogen is the current due to hydrogen evolution, itotal is the
applied current, and ␩ is the current efficiency. In pulse plating
conditions, itotal is iavg. Two zones are seen in this figure. The one on
the left with the solid line is from dc plating at lower current den-
sities. In this zone, both the hydrogen evolution rate and microstruc-
ture change significantly. Because both the grain structure and
roughness change and because the hydrogen evolution rate is small,
we hypothesize that the microstructure is the dominant factor in this
region. However, as the hydrogen evolution rate increases and the
microstructure size decreases, the rate of hydrogen evolution domi-
nates in the region denoted with the dashed line.
Hydrogen evolution in the acidic hard gold plating process is a
complex process involving many stages. In the dc plating process
and during the cathodic portion of the pulse plating process, hydro-
30
25
20
15
Impact of peak current and frequency on pore size in pulse plat-
ing.— Pore size is also an important factor for corrosion resistance
because the corrosion product of the underlayers migrates to the
surface of the coating. The size of pores also depends on hydrogen
evolution and microstructure. In Fig. 11, we demonstrate the impact
of ipeak and frequency on pore size in pulse plating conditions. For
the same iavg, ipeak and frequency are both significant factors in
determining the pore size. A larger ipeak leads to larger pores at the
same frequency, and a higher frequency results in smaller pores at
the same peak current. At a higher ipeak, hydrogen is produced at a
higher rate. Hydrogen bubbles grow to a large size in a very short
time before detaching or dissolving from the coating surface. The
impact of frequency on pore sizes may be that fewer coulombs are
passed in a given on-time.
10
DC plating
PP at ipeak 100 mA cm-2
PP at ipeak 200 mA cm-2
5
Conclusions
0.3
0.4
0.5
0.6
0.7
0.8
Both the microstructure and hydrogen evolution affect the poros-
ity of cobalt hard gold electroplating. At low current densities, the
microstructure is the dominant factor. At higher current densities in
dc plating and for pulse plating conditions, the rate of hydrogen
evolution plays a dominant role. To achieve a lower porosity, both of
these factors need to be considered.
Current Efficiency
Figure 9. The variation of pore density with current efficiency. The pore
densities are obtained from samples with a film thickness of ϳ750 nm. As
indicated by the circled data, there is no one-to-one correspondence because
other factors, such as microstructure, are important.
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Journal of The Electrochemical Society, 157 ͑7͒ D411-D416 ͑2010͒
300
List of Symbols
ipeak 200 mA cm-2
ipeak 100 mA cm-2
f
iavg
ihydrogen
ipeak
frequency, Hz
250
200
150
100
50
average current density, mA cm−2
current density due to hydrogen evolution, mA cm−2
peak current density, mA cm−2
itotal applied current containing current due to Au reduction and hy-
drogen evolution, mA cm−2
toff length of off-time in between the pulses, s
ton length of each pulse period, s
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Frequency (Hz)
Figure 11. The average pore size obtained under different pulsed plating
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Through the correlation of porosity with microstructure and cur-
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process to minimize porosity. The key requirements to achieve this
goal are fine grains, a smooth surface, and a relatively high current
efficiency. Pulse plating with a relatively high frequency and low
peak current appears promising.
Acknowledgments
The authors are grateful to Tyco Electronics for the financial
support during the course of this study.
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