DEPARTMENT OF CHEMISTRY

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Industrial Metal Finishing-PCB

Dr.A.Geetha

Associate Professor

Department of Chemistry

Kongu Engineering College

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.

Metal Finishing

INTRODUCTION:

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• The term metal finishing involves many processes carried out to modify the surface

properties of a metal by depositing a layer of another metal or a polymer.

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• No single metal or alloy can have all ideal properties like corrosion resistance, wear

resistance, mechanical, electrical and thermal properties.

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• Therefore metal finishing is one of the method employed to improve the properties.

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.

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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.

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• Technological importance of metal finishing:

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• 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
• To enhance chemical resistance.
• To improve mechanical strength (hardness).
• Manufacturing electrical and electronic components like printed circuit boards, capacitors etc..

Methods of Metal Finishing

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• Electroplating of metals and alloys

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• Electroless plating of metals and alloys

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• Thermal spray coating

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• Vapour deposition technology

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• Chemical vapour deposition coating

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• Chemical conversion coating

Essentials of Metal Finishing

The three important factors that governs the process of metal finishing are:

Polarization, decomposition potential and over potential.

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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”.

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Thus polarisation is,

• Development of over potential in an electrolytic cell is called polarization.
• Magnitude of over potential is directly proportional to the extent of polarization.

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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.

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Types of Polarization:

The observed polarization can be of two types.

*Concentration polarization.

*Kinetic or Activation polarization.

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Polarization is an electrode phenomenon, The electrode potential is given by the Nernst’s equation, Where E⁰ = standard electrode potential and [ Mⁿ⁺ ] is the metal ion concentration surrounding the electrode surface at equilibrium.

Explanation: Consider an electrolytic cell under operation. When current is being passed, positive ions are produced at the anode and are consumed at the cathode. If the diffusion of ions in the electrolyte is slow, there will be an accumulation of positive ions in the vicinity of anode. Similarly, there will be a depletion of ions in the vicinity of cathode. Under these conditions, the anode and cathode are said to be polarized. This type of polarization is known as concentration polarization.

1. Variation of concentration polarization

During electrolysis, metal ions in the vicinity of cathode get reduced. If metal ions are not supplied sufficiently, the concentration of metal ions at cathode decreases, this leads to decrease in electrode potential as evident from Nernst Equation.

E = E⁰+ 2.303RTlog Mⁿ⁺

Concentration polarization is generally overcome by stirring the electrolyte.

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Factors affecting the electrode polarization:

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1. Nature of the electrode [size, shape & composition]

2. Electrolyte concentration

3. Temperature

4. Rate of stirring of the electrolyte

5. Products formed at the electrode

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Thus, Large electrode surface, low [M_n+ ] concentration, continuous stirring decreases polarization

In electrochemistry, concentration polarization denotes the part of the polarization of an electrolytic cell resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.

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Video link for the Topic: Factors governing the process of METAL FINISHING

https://youtu.be/sZU1xX86tqc

https://www.youtube.com/watch?v=EkuS2KjlNIk

https://www.youtube.com/watch?v=atXU8RcydME

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2. Activation Polarization:

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The deposition of metal on cathode involve several steps and if any one of the steps involved is slow , it will result in polarization. Activation polarization can be overcome by applying excess voltage or Overvoltage

2) DECOMPOSITION POTENTIAL (ED):

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Definition:

“The minimum external voltage that must be applied in order bring about continuous electrolysis of an electrolyte ”

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Determination :

In the electrolysis of water, at low voltage no reaction occurs and there is slight increase in current on increasing voltage. On increase the voltage above 1.68V, there is an abrupt increase in the current and sudden evolution of H₂ at cathode and O₂ at anode takes place. The reaction product H₂ and O₂ set in galvanic cell leading to a back emf , Eback which opposes applied voltage.

Eback = Ecathode -Eanode= 1.23V

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Decomposition Potential( ED) , for electrolysis splitting of water is calculated using equation,

ED = Eback+ ᾐ

ED = 1.23V + 0.45V

= 1.68V

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The applied minimum voltage 1.68V is the decomposition voltage for the electrolysis of water.

Determination : 2^nd example

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.

E_back = E_cathode - E_anode = 0.54- (-0.76) = 1.30V

The cell emf is, therefore equal to the decomposition voltage E_D which is experimentally found to be 1.3V for Zn and I₂ cell.

In general, E_D of an electrolyte may be equal to the emf of the cell developed due to the products of electrolysis.

E_D = E_back = E_cathode - E_anode

The applied minimum voltage 1.30V is the decomposition voltage for the electrolysis of ZnI₂.

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Decomposition potential [Ed] Definition: Decomposition potential is defined as the minimum voltage that must be applied in order to carry out continuous electrolysis of an electrolyte.

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The decomposition potential is determined using an electrolytic cell as shown in figure:

Example: In the electrolysis of water, a pair of platinum electrodes immersed in a solution of an acid. It is found experimentally that a potential of about 1.7V must be applied to the cell before there sets in a continuous evolution of H2 and O2 .The voltage at which the current increases suddenly is called Ed of the electrolyte.

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iii. Over voltage (over potential) (η) Definition: Over voltage is defined as the excess voltage that has to be applied above the theoretical decomposition potential to start the electrolysis. η = [Ed] experimental -[Ed] theoretical Example: For electrolysis of water using smooth platinum electrodes, The theoretical decomposition potential using Pt electrode is 1.23 V. The experimental decomposition potential using smooth platinum electrode is 1.7 V. η = 1.7-1.23 = 0.47V A graph of variation of current w. r. to applied potential

3) OVER POTENTIAL (OVER VOLTAGE):

DEFINITION:

“The excess voltage that has to be applied above the theoretical decomposition potential for continuous electrolysis.” Overvoltage is represented by η.

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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.

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Over voltage = Experimental decomposition voltage – Theoretical decomposition voltage

= 1.63V –1.23V= 0.45V

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Overvoltage for electrolytic splitting of water is formed .

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The Over potential for a given electrolyte depends on:

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1) Nature of electrodes.

2) Nature of substance deposited.

3) Current density

4) Temperature

5) Rate of stirring of electrolyte

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The hydrogen over voltage is the over voltage required for the liberation of hydrogen at cathode during electrolysis. This is important in electroplating process.

The hydrogen overvoltage of a few metals at current density of 10 A/m² are listed below:

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• A printed circuit board (PCB) mechanically supports and electrically connects electrical) mechanically supports and electrically connects electrical or electronic components) mechanically supports and electrically connects electrical or electronic components using conductive) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.
• Printed circuit boards are used in all but the simplest electronic products. They are also used in some electrical products, such as passive switch boxes.
• Printed circuit boards (PCBs) are usually a flat laminated composite made from non-conductive substrate materials with layers of copper circuitry buried internally or on the external surfaces. They can be as simple as one or two layers of copper, or in high density applications they can have fifty layers or more.
• The Printed Circuit Board (PCB) is very important in all electronic gadgets, which are used either for domestic use, or for industrial purpose. PCB design services are used to design the electronic circuits. Apart from electrically connecting, it also gives mechanical support to the electrical components.
• Products that may contain PCBs include: Transformers and capacitors. Electrical equipment including voltage regulators, switches, re-closers, bushings, and electromagnets.

• Manufacturing of Electronic Components
• Printed Circuit Board Fabrication
• The following steps have been adopted to manufacture PCB
• 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

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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 advanced laser drilling techniques.

v. Electroless copper deposition

The next step in the PCB fabrication process is the chemical deposition of very thin layer of copper on the walls of the holes.

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vi. Solder mask applications

To protect copper circuitry oxidation, damage and corrosion the entire panel is coated

with a liquid solder mask.

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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

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viii. PCB Testing

The finished boards are then sent for functional as well as electrical testing to ensure optimum performance and quality Capacitors, Network components, Diode Transistors, Integrated circuits

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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.

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vii. Final inspection and packaging

A team performs a final check on the finished assembly to find out any obvious defects like scratches.

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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 process will last long and offer superior performance for many years.

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Video link for PCB Manufacturing��https://youtu.be/_GVk_hEMjzs

Surface preparation

Need for cleaning the surface

If the surface of the substrate contains any dirt, grease, oxides or other materials , electrodeposition of the metal at these points is prevented. Hence, surface pretreatment through chemical or mechanical processes is important.

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Surface preparation involves the following steps:

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1. Degreasing

Removal of oil, grease and other organic impurities from the surface of the substrate may be done by washing with organic solvents like trichloroethylene and methylene chloride.

For cleaning PCBs and other electronic components,1,1,1-trichloroethane is used.

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 liberated reduces unsaturated fatty acids.

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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

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3. Rinsing and drying

Substrate is then washed with distilled water, and dried before it is sent for electroplating.

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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.

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• 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)

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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.

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At Anode: M → Mⁿ⁺ + ne^- (oxidation occurs)

At cathode: Mⁿ⁺ + ne^- → M (reduction occurs)

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.

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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

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• 4. Organic additives
• Additives modify the structure, morphology and properties of electrodeposit.
• Additives includes brighteners, levelers, structure modifiers and wetting agents.
• 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

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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

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

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Applications

Extensively used in engineering &

industrial applications like rods, gun bores, rollers,

Mold surfaces, etc.,

Electroplating of Chromium

Electroplating of Silver

• Mainly used in photovoltaic ( solar) market & Decorative purpose

Anode – Ag 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

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Low cyanide bath

High cyanide bath – Barrel plating applications

Electroless Plating (Autocatalytic plating)

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It is chemical process used for depositing certain metals on a verity of materials including metal and plastics.

Advantage of electroless plating

It is also used to deposit a conductive surface on a non-conductive object to improve its electroplating.

Uses

• 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.

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Electroless Copper plating on Printed Circuit Board:

Two kinds of copper plating in the manufacture of printed circuit board.

1. Acid copper electroplating:
1. 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.
2. Electroplating baths cannot be used for non-conductive surfaces because the electrons cannot flow.
3. The hole walls represent the dielectirc portion of a circuit board, which are non-conductive

2. Copper Electroless plaitng:

• 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.

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• Therefore, electroless copper plating is needed to render the board conductive for subsequent through-hole electroplating

Electroless plating bath copper consists

• 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.

• To inhibit the formation of cuprous oxide, air is bubbled slowly through the electroless copper solution and small complexing agents are added to the solution.

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.

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Disadvantages of this electroless plating:

• 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.

Electroless (autocatalytic plating) Nickel plating process:

• 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.

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NiSO₄ + NaH₂PO₂ + H₂O → Ni plating + NaHPO₃ + H₂SO₄

^catalyst

Electroless Nickel solution consists

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• NiCI₂ (20 g/l) - Source for Nickel
• NaH₂PO₂ (sodium hypophosphite)(20 g/I) -Reducing agent
• NaCH₃OO (10 g/I) - buffer
• sodium succinate - complexing agent cum Exhaultant
• pH - 4.5
• Temperature- 93⁰C

The electrode reaction

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.

Advantages:

• The bath possesses excellent throwing power.
• this method is suitable for plating the objects having intricate(complex) 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.

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Advantage of Electroless plating over Electroplating

• 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.

Disadvantages of Electroless Plating

• Limited Bath Life

During electrolytic processing, the metal in the solution constantly replenishes the metal ions in the liquid.