Electrodeposition
Electrodeposition is the process of coating a
thin layer of one metal on top of a different metal
to modify its surface properties.
Objectives:
• To achieve the desired electrical & corrosion
resistance.
• To reduce wear & friction.
• To improve heat tolerance.
Applications:
• Automotive parts.
• Printed circuitry and electrical contacts.
• General engineering components.
• Gold-silver wares and jewelry.
• Decorative wares.
• Production of micro parts for MEMS.
Corrosion & Energy Storage Materials Lab.
Electroplating Setup
Corrosion & Energy Storage Materials Lab.
Equilibrium Potential
For Cu2+ + 2e Cu
ECu2+/Cu = EoCu2+/Cu + (RT/zF) ln [Cu2+]
= 0.340 + 0.059/2 ln [Cu2+] at 25 ℃
Noble
Corrosion
Cu2+
ECu2+/Cu 0.34 V
Cu
Electroplating
Active
Corrosion & Energy Storage Materials Lab.
Atomistic Aspects of Electrodeposition
For charge transfer reaction in metal/solution
interface:
M(in lattice) Mz+ (hydrated in sol.) + ze-
M,lattice
M,ad
Actually, Mz+, sol.
M, adsorbed Mz+,sol
M, lattice
Corrosion & Energy Storage Materials Lab.
Ion Transfer Mechanism
Step-edge ion transfer mechanism:
1) A direct transfer to kink site
2) A direct transfer to the step-edge site other than a
kink, the transferred metal ion diffuses along the
step edge until it finds a kink site.
Step edge
Terrace ion transfer mechanism:
A metal ion is transferred from the solution to the flat
face of the terrace region. At this position the metal
ion is in the adion state having most of the water of
hydration. It is weakly bound to the crystal lattice.
From this position it diffuses on the surface, seeking
a position of lower energy. The final position is a
kink site.
Corrosion & Energy Storage Materials Lab.
Electrode Kinetics
At equilibrium, I.H. ic O.H.
Free ia
ia
M Mz+ + ze, at E = Eeq , energy
ic
ia = i c = i o
G‡
(io : exchange current density)
M Mz+
Metal ia ic io Solution
Reaction coordinate
-
-
+
+
- +
-
-
+
+
-
-
+
+
Capacitor
Corrosion & Energy Storage Materials Lab.
Electrode Kinetics
At anodic polarization, I.H. ic O.H.
at E = Ep > Eeq , Free ia
energy
ia
M Mz+ + ze-,
ic
a = Ep - Eeq =overvoltage M
ie.a = ia - ic f(a)
Metal Solution
i
η βlog a
a a for anodic polar. Mz+
io ia ic
ic
η βlog
c c for cathodic polar. Reaction co-ordinate
io
Corrosion & Energy Storage Materials Lab.
Polarization Diagram
• Polarization : An electrode is no longer at equil. when a net current
flows from or to the surface. The extent of potential change caused by
net current to or from an electrode, measured in volts, is overpotential
(η). η = f ( ie ), in which ie is current flowing through external wire.
Anodic current
ia ηa
η βlog
a a for anodic polar. Ba=2.3RT / zF
io =0.059 / z
ic E Er
η βlog
c c for cathodic polar.
io
ηc
where, io = exchange current density
Cathodic current
Er = equil. potential, or rest potential
Ba, Bc : Tafel constant, -0.05 V < B < 0.15 V
log io log |io|
Corrosion & Energy Storage Materials Lab.
Influence of Mass Transport on Electrode Kinetics
(Conc. Polarization)
For M+z + ze- M Conc.
M M+Z
CB
iL = DzFCB/
CS
c = RT/zF ln (1 - i/iL) : Nernst Layer
OHP
IHP
= 0.059/z log (1 - i/iL) at 298 K
iL is increased by
• higher solution concentration, CB;
• higher temperature which increases
diffusivity, D;
• higher solution agitation, which decreases
.
Concentration polarization only becomes
important when the current density
approaches iL
Corrosion & Energy Storage Materials Lab.
Combined Cathodic Polarization
E – log i plot E – i plot
E Linear region Butler-Volmer
E
1 exponential
relationship (the
Er Er
ct Exponential region
Purely activation
controlled current)
io Ba 2
C 3
Mixed control
(activation and
mass transport) 4
log ic iL iL
T = ct + c The limiting current density is;
= Bc log i/io + 2.3RT/zF log(1 - i/iL) iL = nFD/ cb
(D : diffusion coefficient of Mz+
: diffusion layer thickness
n : number of electrons involved in the reaction
F : Faraday constant)
Corrosion & Energy Storage Materials Lab.
Influencing Factors in Electrodeposition
The morphology and composition of electrodeposits vary significantly, and
depend on:
• Current density
• The nature of the anions or cations in the solution
• Bath composition and temperature
• Solution concentration
• Power supply current waveform
• The presence of impurities
• Physical & chemical nature of substrate surface
Corrosion & Energy Storage Materials Lab.
Faraday’s Law
• Faraday’s Law: The amount of electrochemical reaction that occurs at an electrode is
proportional to the quantity of electric charge (Q) through an electrochemical cell.
• The weight of a product of electrolysis is w, then
w = ZQ (Z : the electrochemical equivalent)
Q = It, it follows that w = ZIt
• Production of one gram equivalent of a product at the electrode (weq) in a cell needs 96487
Coulombs.
• The electrochemical equivalent of a metal Z(M) is the weight in grams produced or
consumed by one coulombs of charge.
weq = Awt/n
Awt : the atomic weight of metal deposited on the cathode
n : number of electrons involved in the deposition reaction
• When Q = 1,w(Q=1) = Z
weq = 96487Z
Z = weq/96487 = weq/F = Awt/nF w = ZQ = AwtQ/nF
Corrosion & Energy Storage Materials Lab.
Current Efficiency
• Current efficiency is the ratio between the
actual amount of metal depositing (or
dissolved) Ma to that calculated
theoretically from Faraday’s law Mtotal
in %.
C.E. = (Qa / Qtotal) x 100 = (Ma / Mtotal) x 100
• C.E. indicates the faction of total current
that generates desired products.
• C.E in plating bath depends on:
• Electrolyte or bath
• pH and agitation
• Current density
• Concentration of chemical component
Corrosion & Energy Storage Materials Lab.
Nucleation and Growth of Electrodeposit
Layer growth
: a crystal enlarges by a spreading of discrete layers.
3D crystallites growth mechanism
: a coherent deposit is built up as a result of
coalescence of these crystallites.
Corrosion & Energy Storage Materials Lab.
Effects of Overpotential on the Nucleation Rate
2r adsorbed atom
When a nuclei with radius r and height h is formed from
adsorbed atoms,
h
G = -r2h/V + 2rh substrate
( V : atomic volume, : interface energy, k : overpotential
: chemical potential difference between adsorbed atom and atom in neclei. (=Zek))
Critical radius (Rc) and critical free energy (G*) for stable nucleation;
Rc = h3/6Zek
G* = h42/6Zek
Nucleation rate J
J = K1 exp(-G*/RT) = K1exp(-K2/k)
Higher overpotential smaller or finer nuclei
Corrosion & Energy Storage Materials Lab.
Effects of Overpotential on Surface Morphology of Sn
Smooth Sn
compact
Potential (V)
dendrite
Powdery Sn
Powdery structure is obtained powdery
at high cathodic overpotential.
Cathodic current density (A/cm2)
Corrosion & Energy Storage Materials Lab.
Influence of Overpotential on Deposits of Cu
30 seconds at 0.2 A/cm2
0.2 A/cm2
0.3 A/cm2
0.4 A/cm2
30 seconds at 0.3 A/cm2 30 seconds at 0.4 A/cm2
Corrosion & Energy Storage Materials Lab.
Pulse and Pulse-Reverse Plating
• In dc-plating, constant current is used, and the
rate of arrival of metal ions depends on their
diffusion coefficient (electrode-to-part spacing and
agitation).
• In PC and PRC, a modulated current waveforms
are used to get a better leveling of the deposit,
and to minimize the porosity, contamination, etc.
• The morphology of some metal and alloy deposits
were found to be superior to the dc-plated
deposits.
• Complex current wave forms can be generated by
using pulse rectifier: unipolar and bipolar pulses.
Corrosion & Energy Storage Materials Lab.
Pulse deposition technique
The main effects of pulse deposition ;
1) Under pulse deposition conditions
the Nernst diffusion-layer is split into
two diffusion-layers. Because of the
thin Nernst layer, high current
density can be applied.
2) High current density & high
overpotential increase the nucleation
rate, then fine deposit can be formed.
Corrosion & Energy Storage Materials Lab.
Three major wave form in pulse deposition
Rectangular-pulse deposition
The waveform consists of pulses of a current or
potential of a rectangular shape separated by
intervals of zero current or potential.
Periodic reverse deposition
The applied current or potential is periodically
switched from cathodic to anodic polarization.
Superimposed sinusoidal deposition
The waveform is the sum of a sinusoidal
alternating (ac) wave current and a direct cathodic
current (dc). If the amplitude of the sine wave is
greater than the dc offset, then the wave form
consists of both a cathodic and an anodic portion.
Corrosion & Energy Storage Materials Lab.
Characteristics of Unipolar Currents
• To characterize a train of current pulses, three
applied pulse period
parameters need to be known:
current ip
peak pulse current density (ip)
pulse length (ton)
interval between pulses (toff) ton toff
• The average current density iav is measured, and it is
expressed for PC as: iav = (ip x ton)/(ton + toff) ip : peak current density
• The duty cycle (γ) which represents the portion of the ton : current on-time
time in each cycle when the current is ON.
• It is defined by γ = ton/(ton + toff) ; iav = ip x γ toff : current off-time
Duty cycle = ton/(ton+toff)
Corrosion & Energy Storage Materials Lab.
Leveling and Throwing Power
• Throwing power:
- The ability of electroplating solution to deposit almost
equal thickness on both recessed and prominent areas. deposit
- Ni plating (also other acidic Cu and Zn baths) show
poor throwing power. If the C.E. values are close to substrate
100 % at low and high current density values, then the
macroscopic irregularities on the cathode will lead to
non-uniform deposits.
- Alkaline baths have better throwing power, since the
metal ions are present as complex ions.
deposit
• Leveling:
- The ability of electroplating solution to fill in defects and substrate
scratches on the surface preferentially is called leveling.
Corrosion & Energy Storage Materials Lab.
Effects of Additives on Electrodposits
additive
• Levelers : Levelers are adsorbed on
the peaks, and more metal is deposited deposit
on the recessed areas than peaks.
substrate
• Grain refiner : Grain refiner is
adsorbed on the surface, and suppress
substrate
the lateral growth, resulting grain
refinement.
deposit
• Stress reducer : Stress reducer is
adsorbed in deposited layer, and substrate
reduce residual stress that occurs from
cracks formed during deposition.
H2
• Wetting agent : Wetting agent reduces
the surface tension of H2 bubble substrate
adsorbed on the surface of cathode,
and the metal is deposited more easily.
Corrosion & Energy Storage Materials Lab.
Influences of Additive and Pulse Current on Electrodeposits
additiv
e
20Pb-80Sn deposited by DC current
No additives
↑overpotential
with additives
Additives
Corrosion & Energy Storage Materials Lab.
Effects of bath type on depostion of Cu
1. Acid bath : Low polarization; difficult to
deposit Cu initially. Once Cu is present on
cathode surface, subsequent high speed
deposition is undertaken. Without adding
agents in electrolyte, the deposit is dull and
uneven. High current efficiency (90 %)
2. Complex bath : High polarization ;yield
deposits of finer grain size, brighter, and
more lustrous appearance. Low current
efficiency (70 %)
Corrosion & Energy Storage Materials Lab.
Uniform Electrodeposit through the Use of Complex Baths
• In most complex baths, the deposition potentials are amenable to hydrogen
evolution which competes with metal deposition such that C.E. falls as
current density is increased. This results in a more uniform deposit on
cathodic macro irregularities.
• When the ions are complexed, they encounter high concentration
polarization (CP). If the CP is high, the micro throwing power is rather poor.
• The ability to produce a deposit over a surface including recesses is called
covering power.
Corrosion & Energy Storage Materials Lab.
Alloy Electrodeposition
Generally, metals whose E° values differ by more than 0.2 V can be codeposited from
simple salt solutions.
In the case of large difference in E° values between two metals, complex ion bath for
noble metal should be used.
Ex) Cu-Zn alloy deposition in 25-50 g/L Cu(CN)2, 30
g/L Na2CO3, 10-30 g/L Zn(CN)2, 10-30 g/L NaOH, 50-
75 g/L NaCN bath.
Potential E
Cu(CN)43- Cu
Cu and Zn form cyanide complexes
2NaCn + CuCN Na2Cu(CN)3
Cu(CN)32- + CN- Cn(CN)43-
ZnO22- Zn
2NaCN + Zn(CN)2 Na2Zn(CN)4
Zn(CN)42- + 4OH- ZnO22- + 4CN- + 2H2O Current density, i
Cn & Zn can be codeposited from Cu(CN)43- & ZnO22-.
Corrosion & Energy Storage Materials Lab.
Electroforming
Electroforming: a process used for making metallic articles with tight
dimensional tolerance.
e- • By depositing a metal into or on to a
mold or mandrel, a free-standing
metal object is made.
• Useful for the production of originals
and making exact copies of originals.
mold
Corrosion & Energy Storage Materials Lab.
Advantages & Applications of Electroforming
Advantages Applications
• Micro components and prostheses
• Accuracy of reproduction
• Complex wave guides
• Production of foils and
• Metal bellows
mesh-products
• Reflectors, Nose cones
• Manufacture of complex • Heat exchangers, micro filters
shaped objects
• Decorative ware
When combined with lithography, electroforming is extremely useful for
making micro parts, and overcomes the difficulty of traditional machining.
Corrosion & Energy Storage Materials Lab.
Lithography Process
X-rays
Mask
Resist Mold
Substrate
Develop Embossing
Resist
Plating μ-structure
Corrosion & Energy Storage Materials Lab.
Electroforming process
Introduction
■ Copper electroplating is a wide-spread and important process in the electronics
industry.
■ Copper foil (12~70 ㎛) used in the printed circuit boards industries is mostly
manufactured by electrodeposition process (electroforming) of copper.
Roller
Copper foil
Cathode drum
Electrolytic cell
Anode (Pb)
Electrolytyte
Cu2+ SO4-
Rotating cathode drum
Corrosion & Energy Storage Materials Lab.
Manufacturing Process of Cu foil
Manufacturing Process of Copper Foil
Base foil
Cathode drum
roll sheet
Surface treatment slitter sheeter
(Nodule formation, Stain proof)
Corrosion & Energy Storage Materials Lab.
Electroless Plating
No power supply is necessary to drive
the deposition reaction. The overall
reaction is ;
Mz+sol + Redsol Mlattice + Oxsol
catalytic surface
catalytic surface
Mz+sol + ze Mlattice
catalytic surface
Redsol Oxsol + me
Current-potential curves for reduction of Cu2+
ions and for oxidation of reducing agent Red,
formaldehyde, combined into one graph.
Applications;
Cu electroless Ni-P electroless deposition
deposition for via for corrosion resistance
hole
Corrosion & Energy Storage Materials Lab.
Advantages of Electroless Plating
• No electrical contact is needed.
• It is possible to plate both conductive and insulating surfaces, provided the
surfaces are first sensitized.
• It is readily adaptable for three-dimensional coverage.
• No field lines are present, and this enhances deposit uniformity.
Metallizing Nonconductors
• Certain parts or components whose functions are fully utilized
only when the properties of both a metal and non-metal are
combined.
• Generally a part is made of plastic or ceramic, and the metal
added to its significant surfaces to impart specific metallic
properties.
1. For electrical conductivity, as in PCB.
2. For metallic appearance, as in the buttons, door knobs, wheels
in toys, etc.
Corrosion & Energy Storage Materials Lab.
Developing Good Adhesion Metal Deposition on Polymers
• For metallizing nonconductors, their surfaces need to be mechanically
roughened, or etched, or made hydrophilic. Nonconductors need pre-
conditioning of their surfaces.
• Polymers: Polyimides, Polysulfones, etc.
surface etching : Cr2O3/H2SO4 (0.5:1.0)
• Fluorocarbons: Na metal in anhydrous NH3 or THF
• Need to be treated further in sensitizing, nucleating (catalytic), accelerator
solutions prior to plating in electroless solution.
Corrosion & Energy Storage Materials Lab.
Electroless Plating Process
Surface etching (e.g. Rinse water
sulfuric + chromic acids)
“Catalyze” Sensitize (e.g. Sn(II))
mixed colloid
Activate
(e.g. KF, HF)
Catalyze Pd(II))
Electroless bath
Sensitization and catalysis mean the absorbing of agents from a solution of Sn2+and/or
Sn4+. A simplified model of sensitization and catalysis process is that the sensitizing ion
reduce the active metal from catalyst solution, which most often is PdCl2.
Sn2+ + Pd2+ → Sn4+ + Pd
Corrosion & Energy Storage Materials Lab.
Anodizing
• Anodizing is an electrochemical process in which the part is made the anodic
electrode in a suitable electrolyte. Sufficiently high voltage is deliberately
applied to establish the desired polarization to deposit oxygen at the surface
(O2 overvoltage). The metal surfaces or ions react with the oxygen to produce
adherent , oxide coatings, distinguishing the process from electrobrightening
or electropolishing processes.
• Industrial anodizing processes are confirmed mainly to Al, Mg and Ti alloys.
Anodic coating applications include:
1. Protection : corrosion, wear and abrasion resistance.
2. Decorative : clear coatings on polished or brightened surfaces, dyed (color)
coatings.
3. Base for subsequent paint or organic coating.
Corrosion & Energy Storage Materials Lab.
Anodic oxide film of Al
4Al + 6H2SO4 = 2Al2O3 + 6SO3 + 3H2 + 6H+ + 6e- + 1250 kJ
Heat generation
↑ Al oxide film breakdown
Formation of porous oxide layer
Formation of anodic oxide film of Al
a : thin barrier layer
b : rough surface
localized dissolution
c : formation of pores
d : growth of anodic oxide film
Corrosion & Energy Storage Materials Lab.
Structure of Anodic oxide film of Al
Cell
Porous
wall
oxide
Aluminum
Barrier layer
Corrosion & Energy Storage Materials Lab.
Template-Assited Nanowire Fabrication
Corrosion & Energy Storage Materials Lab.
Alumina Templates on Silicon Wafers (I)
Corrosion & Energy Storage Materials Lab.
Alumina Templates on Silicon Wafers (II)
SEM image of porous alumina anodized 40 nm Bi nanowires deposited in the
in 4 wt.% H2C2O4 at 45 V. porous alumina template on a silicon
The pore diameter is ~ 44 nm. wafer with a conducting adhesion layer.
Aluminum
Corrosion & Energy Storage Materials Lab.