this PPT covers plating techniques like electro plating and elctroless plating.

1. DEPARTMENT OF CHEMISTRY
Industrial Metal Finishing

2. UNIT-II
Industrial Metal Finishing
Introduction – technological importance of metal finishing- methods of
metal finishing - manufacturing of electronic component-PCB fabrication-
essential of metal finishing: polarization, decomposition potential and
overpotential - surface preparation - Electroplating – process - effect of
plating variables on the nature of electrodeposit - electroplating of
chromium and silver. Electroless plating - electroless copper plating on
printed circuit board - electroless nickel plating process -Distinction
between electroplating and electroless plating- advantages of electroless
plating.

3. Metal Finishing
Metal finishing is a process of modifying surface properties of metals by
deposition of a layer of another metal or polymer on its surface or by the
formation of an oxide film.
Metal finishing was introduced as a decorative finish, but the increasing
demand for parts with prescribed specifications has led to vast
technological developments in the field.
Technological importance of metal finishing:
• To increase the decorativeness of metal surface
•Impart higher corrosion resistance
•Improved wear resistance
•Providing electrical and thermal conducting surface
•Impart thermal resistance and hardness
•Providing optical and thermal reflectivity

4. Methods of Metal Finishing:
• Electroplating of metals and alloys
•Electroless plating of metals and alloys
•Thermal spray coating
•Vapour deposition technology
•Chemical vapour deposition coating
•Chemical conversion coating
Manufacturing of Electronic Components
Printed Circuit Board Fabrication
The following steps have been adopted to manufacture PCB
i. Components Layout Designing
It is nothing but planning the positions of different components constituting
the circuit and then showing their interconnections as per the circuit
diagram

6. ii. PCB Layout printing
Once the customer approves the design, the layout is printed on any photo
basic gloss transparent paper.
iii. Transferring PCB Layout onto PCB laminate (Substrate preparation)
PCB layout is attached onto the copper laminate by applying heat and
pressure on the assembly
and this step is known as substrate preparation. Now the assembly contains
copper laminate with
PCB layout attached to it.
iii.Etching
The purpose of etching is to remove unnecessary copper traces from the
substrate.
The most commonly used etching solutions are ferric chloride or hydrochloric
acid.
iv. Drilling
The next step in the PCB fabrication process is drilling holes to attach PCB
components utilizing

7. vi. Solder mask applications
To protect copper circuitry oxidation, damage and corrosion the entire panel
is coated
with a liquid solder mask.
vii. Assembling
All the electronic components are assembled onto the respective holes in the
board
 Terminals and Connectors
 Resistors
 Switches
 Capacitors
 Network components
 Diodes
 Transistors
 Integrated circuits
viii. PCB Testing
The finished boards are then sent for functional as well as electrical testing

8. x. Cutting individual PCBs from the production panel
The final manufacturing stage is cutting individual PCBs from the
production panel.
Generally, PCB manufacturers utilize computer-controlled milling
machines or routers to cut
individual PCBs without damaging other boards in the panel.
vii. Final inspection and packaging
A team performs a final check on the finished assembly to find out any
obvious
defects like scratches.
Thus, PCB fabrication involves several steps that must be done with
utmost care.
Any flaw in these manufacturing processes will affect the performance,
functionality and
durability of the final assembly. PCBs that are fabricated by following
the right fabrication

9. Essentials of Metal Finishing
The three important factors that governs the process of metal finishing
are:
Polarization, decomposition potential and over potential
1) POLARIZATION:
Definition: “ It is a process where there is a variation of electrode potential
due to slow supply of metal ions from bulk of the solution to the vicinity of
the electrode”.
Polarization depends on several factors:
1) Size, shape and composition of electrode.
2) Electrolyte concentration and its conductivity.
3) Temperature.
4) Products formed at electrodes.
5) Rate of stirring of electrolyte.

10. 2) DECOMPOSITION POTENTIAL (ED):
Definition: “ The minimum external voltage that must be applied in order to
bring about continuous electrolysis of an electrolyte ”
Determination :
The decomposition voltage can be determined using an electrolytic cell.
In the electrolysis of ZnI2, at low voltage no reaction occurs and there is
slight increase in current on increasing voltage. On increase the voltage
above 1.30V, there is an abrupt increase in the current and Zn and iodine
are liberated at the electrodes.
The reaction products exerts a back emf , and offers resistance to the flow
of current till the applied voltage overcomes the back emf.
Eback = Ecathode - Eanode = 0.54- (-0.76) = 1.30V
The cell emf is, therefore equal to the decomposition voltage ED which is
experimentally found to be 1.3V for Zn and I2 cell.
In general, ED of an electrolyte may be equal to the emf of the cell
developed due to the products of electrolysis.
ED = Eback = Ecathode - Eanode

11. 3) OVER POTENTIAL (VOLTAGE):
DEFINITION: “The excess voltage that has to be applied above the
theoretical decomposition potential for continuous electrolysis.”
Overvoltage is represented by η.
In general, for continuous electrolysis to take place , the applied voltage
should be equal to or slightly more than the decomposition potential. In
few cases even when voltage decomposition potential is reached,
electrolysis will not occur and sometimes the applied voltage has to
exceed the theoretical voltage by 1V for continuous electrolysis. This is
known as over potential or over voltage.
Over voltage = Experimental decomposition voltage – Theoretical
decomposition voltage
The Over potential for a given electrolyte depends on:
1) Nature of electrodes.
2) Nature of substance deposited.
3) Current density
4) Temperature
5) Rate of stirring of electrolyte

12. The hydrogen overvoltage of a few metals at current density of 10 A/m2 are

13. Surface preparation
Need for cleaning the surface
If the surface of the substrate contains any dirt, grease, oxides or other materials
metal at these points is prevented. Hence, surface pretreatment through chemical
important.
Surface preparation involves the following steps:
1. Degreasing
Removal of oil, grease and other organic impurities from the surface of the substra
with organic solvents like trichloroethylene and methylene chloride.
For cleaning PCBs and other electronic components , 1,1,1-trichloroethane is use
Degreasing may also be done by alkali cleaning keeping the object cathodic.
The higher pH of the alkali helps in hydrolysis of the fat and the hydrogen liberate
fatty acids.

14. 2. Descaling
This step involves the removal of scale and oxide films.
Pickling
The object is immersed in 10 percent H2SO4 in order to remove excess alkali
from alkali treatment, scales
and rust present on the surface. This is called pickling.
Oxide scales may be better removed by pickling in H2SO4 keeping the object
anodic.
Polishing
Polishing of the object may be carried out mechanically or electrochemically.
In mechanical polishing silicon carbide grinding wheel is used.
Electropolishing involves the anodic dissolution of the metal.
Sandblasting
In this method, very finely divided material is propelled at high speed to clean or
etch a surface.
For this sand used, hence the name sandblasting.

15. Electroplating
 It is a process in which the coating metal is deposited on base metal when electricity is
passed through an electrolytic solution having the soluble salt of coat metal.
 Coat Metal - Metal to be coated
 Base Metal - Metal to be plated
 Anode - Inert material of good conductivity (graphite) or coat metal
 Cathode - Base metal
 Electrolyte - soluble salt of a metal ( to be plated)
Process
The surface treated article is made as cathode of an electrolytic cell. Anode &
cathode are immersed in an electrolyte solution kept in an electroplating tank. The tank is
made of glass, enameled iron and stoneware. When electricity is applied the metal ions from
the electrolyte are migrated and deposited as metal over the cathode. Plating is an redox
reaction. At Anode: M → Mn+ + ne- (oxidation occurs)
At cathode: Mn+ + ne- → M (reduction occurs)

16. Effect of Plating Variables on Nature of Electrodeposit
The important factors which affects electroplating process are as follows
1. Current density of deposition
2. Metal ion concentration and electrolytes
3. Complexing agents
4. Organic additives
5. pH
6. Temperature
7. Throwing power of plating bath
1.Current density of deposition
At low current density – surface diffusion is faster than electron transfer, results in smooth
deposition
At high current density – surface diffusion may not reach the most favorable positions and mass
transport predominates in solution, this results in bad deposit with rough and powdery deposit.

17. 2.Metal ion concentration and Electrolytes
 High metal concentration decreases mass transfer and also affects the quality of plating.
 High concentration of electrolyte increases the conductivity of plating bath and
sometimes acts as buffer solution
3.Complexing agents - addition of suitable complexing agents
 Converts metal ion into complex ion to get finely grained adherent deposit
 To prevent the reaction of plating ion with cathode metal (Ex. Plating Cu on iron or steel)
 To prevent passivation of anodes so that anodes dissolve easily and to increase current
efficiency
 To improve the throwing power of the plating bath
 Frequently used complexing agents are cyanides, hydroxides and sulphamates

18.  4. Organic additives
 Additives modify the structure, morphology and properties of electrodeposit.
 Additives includes brighteners, levelers, structure modifiers and wetting agents.
a. Brighteners - Brighteners are added to produce bright and microscopically fine
deposit.
 Ex. Aromatic sulphones/sulphonates and molecules containing -CN, N=C=S or C=O
groups
 b. Levelers
 Levelers are added to get uniform thickness and reduces rapid deposition at
particular region
 Brighteners also acts as levelers. Ex. Sodium allyl sulphonate is used as leveler for
nickel deposition
 c. Structure modifiers or stress relievers
 To alter the deposit properties and to modify the structure of deposit stress relievers
are added. Ex. Saccharin

19. d. Wetting agents
Wetting agents are added to release the H2 gas bubbles formed during electroplating
process.
They also improve the leveling, uniformity of deposit and to reduce brittleness of
deposit. Ex. Sodium lauryl sulphate
5. pH
Low pH – releases H2 gas and results in burnt deposit, High pH- surface gets coated with
insoluble hydroxides
Optimum pH range for plating is 4 to 8 and to get desired pH buffer is used.
Ex. Borate buffer for Ni plating & citrate buffer for Au plating
6. Temperature
Plating is carried out between 35° C to 60°C to avoid corrosion and decomposition of
additives

20. Applications
Extensively used in
engineering &
industrial applications like rods,
gun bores, rollers,
Mold surfaces, etc.,
7. Throwing Power of the plating bath
 Ability of a plating bath to give a uniform and even deposit on the entire surface of the object is
measured by its throwing power.
 Throwing power is good if the distribution of the deposit is uniform, irrespective of the shapes
of the object.
 Throwing power of bath is determined by Haring-Blum Cell.
Electroplating of Chromium
 Chromium plating is applied for wear resistance, lubrication and oil retention
 Decorative chrome plating involves plating of nickel onto article before plating chrome
 Anode – Pb with 7% Sn or Sb
 Cathode – Article to be plated
 Electrolyte – H2CrO4 & H2SO4
 Current Density – 17-20( mA/cm2)
 Temperature – 45°C – 60°C

21. Electroplating of Silver
 Mainly used in photovoltaic ( solar) market & Decorative
purpose
Anode – Ag or inert material
Cathode – Article to be plated
Electrolyte – Silver cyanide dissolved in sodium cyanide
Current Density – phosphate up to 20( A/dm2)
Temperature – 65°C
Plating rate - 15 µm/min

22. It is chemical process used for depositing certain metals on a verity of
materials including metal and plastics.
It is also used to deposit a conductive surface on a non-conductive object to improve
its electroplating.
 It is widely used for machine frames, base plates, fixtures.
 It is used in machine parts where metal -to-metal wear applications are required which
cannot be achieved by using conventional oils and greases.

23. Two kinds of copper plating in the manufacture of printed circuit board.
 It happens only on a conductive surface via electrochemistry method whereby
electrons are obtained from a DC power sources to reduce metal ions to the metalic state.
 Electroplating baths cannot be used for non-conductive surfaces because the electrons
cannot flow.
 The hole walls represent the dielectirc portion of a circuit board, which are non-conductive
 In that, the mechanism is similar but the source of electrons is chemical reducing
agent.
 Hence non conductive surfaces may be metalized by employing electroless baths, since
baths contains their own source of electrons.

24.  Therefore, electroless copper plating is needed to render the board conductive for subsequent
through-hole electroplating
 Copper sulphate - source of copper
 Formaldehide - reducing agent (to reduce the cupric ions to metalic copper)
 Caustic - basic medium (palladium-catalyst)
 Chelating agents - like amines, gluconates, tartrates etc., It govern the plating rate, influence
the properties of deposit and the bath stability.
Chemical reaction is,
At anode : 2HCHO + 4OH- → 2HCOO- + H2 + 2e-
At Cathode : Cu2+ + 2e- → Cu
Net redox reaction : 2HCHO + 4OH- + Cu2+ → 2HCOO- + H2 + Cu
 Reduction proceeds through a cuprous state.
 Excess of cuprous oxide formation will cause the reduction reaction to proceed out of control.

25.  The formation of cuprous oxide, air is bubbled slowly through the electrodes copper
solution and small complexing agents are added.
In the manufacturing of printed circuit boards:
 Electroless copper plating is utilized to metalize the entire board with a thin deposit copper
to render the board conductive for subsequent through hole electroplating.
 Chelating agent- creates difficulties and interferes with wastewater treatment.
 reducing agent, formaldehyde- human health hazard.
 instability of the electroless copper bath creates difficulties in process control.

26.  The metal ion is reduced to metal only on a specific surface, which must have a
catalyst present before the reaction can begin.
 Bath solution- Nickel sulphate.
∆
NiSO4 + NaH2PO2 + H2O → Ni plating + NaHPO3 + H2SO4
catalyst
Electroless Nickel solution consists
 NiCI2 (20 g/l) - Source for Nickel
 NaH2PO2 (20 g/I) -Reducing agent
 NaCH3OO buffer (10 g/I) - complexing agent
 sodium succinate - Exhaultant
 pH - 4.5
 Temperature- 93⁰C

27. At anode : NaH2PO2 + H2O → NaH2PO3 + 2H+ + 2e-
At cathode : Ni+2 + 2e- → Ni
Net redox reaction: NaH2PO2 + H2O + Ni+2 → NaH2PO3 + 2H+ + Ni
The H+ ions are released in above reaction, and decrease the pH of the medium.
Ni2+ ions and sodium hypophosphite are consumed during the redox reaction.
 The bath possesses excellent throwing power.
 this method is suitable for plating the objects having intricate shapes.
 The deposits are free from pores, hence there is better corrosion resistance.
 Plating gives harder surface, it gives wear resistance.
 Ni plating on AI enhance the solderability, also provides a non magnetic underlay in
magnetic components.

30.  Uniform plate thickness on complex part geometrics.
 Process can be used on both metallic and non metallic subtracts.
 No need for a DC power supply to drive the process.
 The deposition may not be as thick as with electrolytic plating, but the coating deposits on
the surface more evenly.
 An object with electroless plating provides superior strength for the item it covers.
 Electroless plating does not require a conductive surface.
 Limited Bath Life
During electrolytic processing, the metal in the solution constantly replenishes the metal
ions in the liquid.