Lecture_03 Electroplating and Electroless

Material & Metalurgi

Electroplating

Electroplating Electroplating process •

The process can produce thin film on metal with electrolysi electrolysiss method.

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Cathode  Substrate Anode  Coating material •

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Non-consumable (only for conductor)

Medium (electrolyte)  such as alkali, acid, salt  •

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Consumable (as conductor and coating material) and

Current in electrolyte  ions move from anode to cathode Electrolyte molecules can be dissolved in water become (+) and (-) charge

Rectifier

Deposition mechanism •

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Solvent molecules is polarized take place in surrounding of Metal ions. Somehow, electrical Double Layer (EDL) near cathode surface as dielectric layer (as barrier of surface from ions) Driving force (from voltage) + chemical reactions  Metals ions move to cathode surface to capture electron from cathode. In the same time they are deposited on surface In equilibrium condition, after the ions become atoms afterwards they will be taken place on the surface and adapt with atomic structure of cathode

The principle of Electroplating •

Metal volume of anode to cathode  = . . . 

where: V = volume (cm3) , E = cathode efficiency, C = constant (cm3/A.s), I = current (A), t = time (s) •

The thickness film (d)  

=

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;  A = surface area (cm2)

Electrolyte •

The electrolyte must contain the dissolved salt of the metal to be deposited.

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The salts dissolved in water and form ions

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For example: CuSO4 dissolved in water form Cu++ and SO4- -

A simple estimation of ion in electrolyte •

Copper in copper cyanide Molecular weight of Cu(CN)4 = 167.5 Atomic weight of copper = 63.5 % Copper =(63.5/167.5).100 = 38% So, if there is 15 gms/liter  the copper content would be 5.7 gms

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Copper in copper sulphate Molecular weight of CuSO4 = 159.5 Atomic weight of copper = 63.5 % Copper =(63.5/159.5).100 = 40% So, if there is 200 gms/liter  the copper content would be 80 gms

Electroplating characteristics and challenge •

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Deposition temperature < 100 oC (no distortion) Can be used to increase the hardness of substrate Adhesion up to 1000 Mpa Thickness depend on time and current Maximum of deposition rate 75 m/hour Non homogen thickness can be caused from non homogen current Uncoated area can be designed by covering the area using masker

Electroplating types •

Zinc plating (galvanizing) • •

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Nickel plating •

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for corrosion protection, automotive part, decorative coating for ferrous, brass etc. First layer for Cr coating

Tin plating • •

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For ferrous product to reduce corrosion rate Example: bold and nut, contact

For reduce corrosion rate. Often used for food cans coating

Cooper plating •

For decorative coating and print circuit board.

Electroplating types •

Chromium plating • • • •

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decorative coating for automotive part, kitchen equipment etc. The hardest coating using electroplating For wear resistance coating on piston, cylinder hydrolic, air craft component etc HARD CHROMIUM PLATING is produced by electrodeposition from a solution containing chromic acid (CrO 3) and a catalytic anion in proper proportion.

The metal so produced is extremely hard and corrosion resistant. The process is used for applications where excellent wear and/or corrosion resistance is required. This includes products such as piston rings, shock absorbers, struts, brake pistons, engine valve stems, cylinder liners, and hydraulic rods. Other applications are for aircraft landing gears, textile and gravure rolls, plastic rolls, and dies and molds.

Zinc Electroplating •

Zinc is less noble than steel  sacrificial anode

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Zinc electrolyte •

Zinc chloride bath •

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Zinc chloride 3 oz per gallon, 20 grams per liter Ammonium chloride 20 oz per gallon, 120 grams per liter

Zinc oxide bath •

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Zinc oxide 1.0 oz per gallon, 6 grams per liter Sodium hydroxide 9.0oz per gallon, 55 grams per liter Dextrin 1% addition by weight

Faults in Zinc electroplating* Fault

Reason

Remedy

Rough deposit

1. Current density too high

Reduce the current density

2. Caused by suspended matter in the electrolyte

Filter the electrolyte through a filter paper or cloth

Low conductivity of electrolyte

1. Zinc electrolyte add ammonium chloride to the solution (2  – 4 oz/gallon)

Deposits rough and electroplating sluggish (i.e. lack of thickness)

2. Zinc electrolyte 0.25oz of zinc oxide and 1-5oz of sodium hydroxide per gallon of solution Electrolyte appears to be a rusty colour

Iron form the component being electroplated is dissolved into the solution

Zinc electrolyte add 50 ml of hydrogen peroxide, stir well and leave to settle. When settled, carefully decant off the clear solution

The deposit is patchy

The pre-treatment clean is faulty

Strip the deposit off to the metal by immersing the component in 30% sulphuric acid or 15% hydrochloric acid until all the zinc is removed. Go back through the pretreatment and replate

Nickel electroplating •

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Ni offers good corrosion resistance Nickel deposit are more noble than steel and steel become anodic and therefore dissolved  important to have good pre cleaning and avoid pores and discontinuities Nickel electrolyte* •

Watt’s Nickel 

nickel suphate 38.5 oz per gallon, 240 grms per liter; nickel chloride 7.2 oz per gallon 45 grms per liter; boric acid 4.8 oz per gallon, 30 grms per liter

Faults in Nickel electroplating* Fault

Reason

Remedy

Pitting of deposit

Accidity of solution too high, nickel content low, boric acid too low

Adjust pH to between 3 and 5 with aquaeos solution of sodium hydroxide. Add 3 oz per gallon of Nickel sulphate. Add 0.5 oz per gallon of boric acid

No enough coverage of component

Electrolyte temperature too low, or low current density

Increase electrolyte temperature to 50  – 55oC. Increase current density

Poor adhesion and may be of burnt appearance

Poor cleaning, too high pH (alkanity), too high current density

Strip off the nickel plate, pre clean and re plate. Add diluted hydrochloric or sulfuric acid until pH 3  – 5

Material & Metalurgi

Electroless coating

Electroless nickel coating •

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Definition  ELECTROLESS NICKEL PLATING is used to deposit material without the use of an electric current. Electroless nickel plating methods: •

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The coating is deposited by of nickel ions by hypophosphite, aminoborane, or borohydride compounds. from solutions of nickel chloride and boric acid at 70 °C (160 °F) and vapor at 180 °C (360 °F).

Electroless nickel solutions contain •

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of nickel, usually nickel sulfate to supply electrons for the reduction of nickel (heat) (chelators) to control the free nickel available to the reaction to resist the pH changes caused by the hydrogen generated during deposition (exultant) to help increase the speed of the reaction (stabilizers) to help control reduction

Reduction agent •

Sodium Hypophosphite Baths

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Aminoborane Baths •

N-dimethylamine borane (DMAB)-(CH3)2 NHBH3, and Hdiethylamine borane (DEAB)--(C2H5)2 NHBH3

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Sodium Borohydride Baths

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Hydrazine Baths

Sodium Hypophosphite Baths •

The majority of electroless nickel used commercially is deposited from solutions reduced with sodium hypophosphite

hypophosphite

orthophosphite Absorbed Hydrogen Nickel on the surface reduced by absorbed active Hydrogen Low efficiency : Usually 5 kg (10 lb) of sodium hypophosphite is required to reduce 1 kg (2 lb) of nickel, for an average efficiency of 37%

Energy •

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Very low deposition rate at temperatures below 65 °C (150 °F) At temperatures above 100 °C (212 °F), electroless nickel solutions may decompose The preferred operating range for most solutions is 85 to 95 °C (185 to 205 °F).

Complexing agent •

To

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and to control the reaction so that it occurs only on the catalytic surface, Complexing agents are organic acids or their salts, also

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as hydrogen ions are produced by the reduction reaction. (-) reduce the speed of deposition

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Accelerator •

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To overcome reduce the speed of deposition, organic additives, called accelerators or exultants, are often added to the plating solution in small amounts. In hypophosphite-reduced solutions, succinic acid is the accelerator most frequently used.

Inhibitor •

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The reduction reaction in an electroless nickel plating bath must be controlled so that deposition occurs at a predictable rate and only on the substrate to be plated. To accomplish this, inhibitors, also known as stabilizers, are added. Electroless nickel plating solutions can operate for hours or days without inhibitors, only to decompose unexpectedly. Decomposition is usually initiated by the presence of colloidal, solid particles in the solution. The addition of inhibitors can have harmful as well as beneficial effects on the plating bath and its deposit.

Advantages •

Good resistance to corrosion and wear

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Excellent uniformity

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Solderability and brazeability

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Low labor costs

Limitations •

Higher chemical cost than electroplating

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Brittleness

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Poor welding characteristics due to contamination of nickel plate with nickel phosphorus deposits Need to copper strike plate alloys containing significant amounts of lead, tin, cadmium, and zinc before electroless nickel can be applied Slower plating rate, as compared to electrolytic methods

Electroless Copper Plating •

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The first commercial applicability of electroless copper was reported in the mid-1950s with the development of plating solutions for plated-through-hole (PTH) printed wiring boards. Copper baths of the 1950s were difficult to control and very susceptible to spontaneous decomposition. Electroless copper solutions resembling today's technology were first reported in 1957 by Cahill with the report of alkaline copper tartrate baths using formaldehyde as reducing agent Electroless copper plates much more slowly, and is a much more expensive process, than electrolytic copper plating.

Components of Electroless Copper Plating •

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The minimum necessary components are and a . The metal salt: Copper salt, as Cu(II), A number of common reducing agents have been suggested for use in electroless copper baths: formaldehyde, dimethylamine borane, borohydride, hypophosphite, hydrazine, sugars (sucrose, glucose, etc.), and dithionite. In practice, however, virtually all commercial electroless copper solutions have used .

Formaldehyde

Complexing agents for electroless copper baths •

Because simple copper salts are insoluble at pH above about 4, the use of alkaline plating media necessitates use of a complexing, or chelating, component. Historically, complexing agents for electroless copper baths have almost always fallen into one of the following groups of compounds: •

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Tartrate salts Alkanol amines, such as quadrol (N,N,N',N' tetrakis(2hydroxypropyl)ethylenediamine) or related compounds EDTA (ethylenediamine tetraacetic acid) or related compounds