Silver plating

• Characteristics of silver
• Silver strike
• Cyanide silver plating
• Non-cyanide silver plating
• Properties of some silver alloys

Characteristics of silver

Symbol: Ag

Atomic number: 47

Atomic weight: 107.87

Classification: Metal

Crystal structure: Face Centered Cubic (FCC)

Characteristic properties of silver:

• Very high Thermal properties#thermal conductivity (highest of all metals);
• Very high electrical conductivity (highest of all metals) and low contact electrical resistance;
• High ductility;
• Good chemical resistance (silver is attacked only by nitric acid , hot sulfuric acid and ozone);
• Bright lustrous appearance.

The metals substrates which are plated by silver:

• Gold
• Copper
• Nickel
• Steels. Prior to silver plating the steel surface is coated by copper/copper-silver from copper/copper-silver strike solution.
• Aluminum. Prior to silver plating aluminum surface is zincated.

The most reliable and widely used process of silver deposition is cyanide silver plating.

Cyanide silver is the Electroplating process utilizing an electrolyte containing silver cyanide solution and some free cyanide and operating at PH value not less than 8.
Free cyanide prevents precipitation of silver cyanide salt from the solution, provides electrical conductivity of the electrolyte and helps dissolution of silver anodes.
Cyanide silver plating is used for decorative and engineering applications.
Common cyanide silver process includes two stages: silver strike coating followed by general silver plating.
Silver strike

Silver strike is a very thin deposition of silver from an electrolyte containing low concentration of silver ions and high concentration of free cyanide as complexing agent.
Silver is a noble metal having high value of standard electrode potential. When a metal with lower electrode potential (less noble) is dipped into a silver cyanide solution the metal atoms are displaced by silver ions, which are deposited on the surface. This electroplating process is called immersion deposition. Immersion deposition is characterized by weak adhesion to the substrate.
Silver strike prevents immersion deposition of silver providing good adhesion of the coating to the workpiece surface. Silver strike coating has dull powdery appearance and low mechanical strength therefore it is used only as an adherent layer over-coated by bright silver.

Silver strike process
Bath compositions for silver strike

• Silver(Ag): 0.3 oz/gal (2.2 g/l)
• Potassium cyanide (KCN): 13 oz/gal (100 g/l)
• Water: reminder

• Silver cyanide (AgCN): 0.5 oz/gal (4 g/l)
• Potassium cyanide (KCN): 13 oz/gal (100 g/l)
• Water: reminder

• Silver cyanide (AgCN): 0.6 oz/gal (4.5 g/l)
• Sodium cyanide (NaCN): 10 oz/gal (75 g/l)
• Water: reminder

• Silver potassium cyanide (KAg(CN)₂): 1 oz/gal (7.5 g/l)
• Potassium cyanide (KCN): 11 oz/gal (80 g/l)
• Water: reminder

Copper-silver strike for steel substrates

• Silver potassium cyanide (KAg(CN)₂): 0.4 oz/gal (3 g/l)
• Copper cyanide: 1.25 oz/gal (10 g/l)
• Potassium cyanide (KCN): 11 oz/gal (80 g/l)
• Water: reminder

Silver strike for nickel/nickel coated substrates

• Silver cyanide (AgCN): 0.12 oz/gal (1 g/l)
• Sodium cyanide (NaCN): 6.7 oz/gal (50 g/l)
• Water: reminder

Operating conditions:

• Anodes: silver or stainless steel
• Cathode current density: 20-25 A/ft² (2.2-2.7 A/dm²)
• Voltage: 6-8 V
• Temperature: room
• Striking time: 0.5-1 min, 5 min for barrel plating
• The plated parts should be electrically connected before entering the bath )prevent immersion deposition)

Cyanide silver plating

Prior to silver plating the parts are treated in a silver strike solution.
The Grain structure and the level of brightness (bright, semi-bright, dull) of the cyanide platings is controlled by additives (brightening agents). Sulfur containing organic materials (products of reaction organic compounds with carbon disulfide, ammonium thiosulfate, sodium thiosulfate).

Cyanide silver bath compositions:

• Silver cyanide (AgCN): 4 oz/gal (30 g/l)
• Potassium cyanide (KCN): 6 oz/gal (45 g/l)
• Potassium carbonate (K₂CO₃): 4 oz/gal (30 g/l)
• Water: reminder

• Silver(Ag): 5 oz/gal (37.5 g/l)
• Free potassium cyanide (KCN): 15 oz/gal (110 g/l)
• Potassium carbonate (K₂CO₃): 13 oz/gal (100 g/l) maximum
• Water: reminder

• Silver(Ag): 4 oz/gal (30 g/l)
• Free potassium cyanide (KCN): 16 oz/gal (120 g/l)
• Potassium hydroxide (KOH): 0.8 oz/gal (6 g/l) maximum
• Water: reminder

Barrel plating:

• Silver(Ag): 3 oz/gal (22.5 g/l)
• Free potassium cyanide (KCN): 15 oz/gal (110 g/l)
• Potassium carbonate (K₂CO₃): 13 oz/gal (100 g/l) maximum
• Water: reminder

Operating conditions:

• Anodes: silver or stainless steel
• Cathode current density: 25 A/ft² (2.5 A/dm²)
• Temperature: room
• Cathode agitation: cathode rod movements 6-15 ft/min (2-6 m/min)
• Filtration: at least 1 turnover/hour
• Silver consumption: 0.13 oz/A*hr (4 g/A*HR)
• Time for deposition 1 µm (0.04 µinch) at 1.5 A/dm²: 1 min.

Non-cyanide silver plating

Environmental and safety considerations have stimulated development of cyanide-free silver plating formulations.

Some of these new non-cyanide processes have been already implemented in the electroplating industry.

Besides of non-poisonous composition cyanide-free silver plating provides good adhesion without silver strike treatment.
Non-cyanide bath compositions:

• Silver(Ag): 0.5-1.0 M
• Potassium carbonate (K₂CO₃): 0.2 M
• Succinimide (C₄H₅NO₂): 0.5-0.7 M
• Water: reminder

• Silver iodide: 8 oz/gal (60 g/l)
• Sodium iodide: 67 oz/gal (500 g/l)
• Polyvinyl alcohol 0.13 oz/gal (1 g/l)
• Sodium thiosulfate 0.16 oz/gal (1.2 g/l)
• Water: reminder

• Silver as silver chloride: 5.3 oz/gal (40 g/l)
• Sodium thiosulfate 67 oz/gal (500 g/l)
• Potassium metabisulfite 4 oz/gal (30 g/l)
• Water: reminder

Properties of some silver alloys

(Materials Data)

• Commercially pure silver
• Sterling silver
• Silver-ABA

Electrolytic co-deposition

Electrolytic co-deposition is an Electroplating method of incorporation of non-metallic particles into metallic coatings obtained from electrolytes containing the particles in a suspended state.

The second phase particles dispersed within the metal base form a metal matrix composite, properties of which may be significantly different from those of the pure metallic deposit.

The metals used as matrices for electrolytic co-deposition:

• Nickel (Ni)
• Copper (Cu)
• Silver (Ag)
• Chromium (Cr)
• Cobalt (Co)
• Iron (Fe)
• Lead (Pb) alloys
• Aluminum (Al)
• Gold (Au)
• Zinc (Zn)

The following substances are used as the second phase particles:

• Aluminum oxide (Al₂O₃)
• Carbide ceramics|Silicon carbide (SiC)
• Tungsten carbide (WC)
• Diamond (C)
• Silicon oxide (SiO₂)
• Graphite (C)
• Thermoplastic Polytetrafluoroethylene (PTFE)
• Molybdenum disulphide
• Boron nitride (BN)

Content of the dispersed phase is commonly 2-27 oz/gal (appr. 15-200 g/l).

• Mechanism of electrolytic co-deposition
• Effect of electroplating process parameters on co-deposition
• Wear resistant coatings with incorporated hard particles
• Co-deposition of solid lubricant particles
• Co-deposition of corrosion protection coatings

Mechanism of electrolytic co-deposition

• The second phase particles suspended in the electrolyte adsorb the positively charged metal ions. The particles gain positive electric charge.
• The particles surrounded by the positive metal ions reach the cathode surface driven by the electrostatic attraction and the electrolyte convection.
• The particles stick to the cathode surface and discharge. The particles may either stay on the cathode surface or disconnect from it. Bonding force retaining the particles on the cathode surface is determined by the relationship between the interfacial energies particle-electrolyte, particle-cathode and cathode electrolyte.
• The metal ions are deposited on the cathode surface around the particles. The particles are incorporated into the metallic deposit.

Effect of electroplating process parameters on co-deposition

• Metal ion concentration. Higher ion concentration leads to denser adsorption of the ions on the particles surface resulting in increase of the driving force of the co-deposition. Increased concentration of the positively charged metal ions enhances the deposition of the particles suspended in the electrolyte.
• Additives promote deposition of the second phase particles from the electrolytic suspensions. Additives promote adsorption of the metal ions on the particle surface and stabilize suspension preventing particles agglomeration.
• Current density. Increased current density enhances incorporation of the particles into the deposit.
• Electrolyte agitation increases convection and therefore enhances the flux of the particles reaching the cathode surface however too intensive agitation may cause adverse effect caused by disconnection and removal of the particles by turbulent streams of the electrolyte.
• Electrolyte temperature. Elevated temperature increases the electrolyte flow and the ions mobility due to lower viscosity and density. Higher temperature also causes stronger bonding between the particles and the cathode surface. Thus increased temperature enhances incorporation of the particles into the deposit.

Wear resistant coatings with incorporated hard particles

Metal Matrix Composites with dispersed hard particles as the second phase possess enhanced wear resistance and abrasion resistance.
The composite coating reinforced by hard particles have also improved mechanical properties (hardness, strength) than plain metallic coatings.

The most popular wear resistant composite coatings are nickel based. Nickel matrix composites with various dispersed phases (Al₂O₃, SiC, WC, diamond, SiO₂) are fabricated by electrolytic co-deposition from Nickel Sulfamate and Watts electrolytes. Nickel based wear resistant coatings are used in abrasive tools, measuring tools and gauges, moving details of machines and engines.

Anti-friction coating of Engine bearings consisting of lead-tin-copper alloy and reinforced by alumina (Al₂O₃) is fabricated by electrolytic co-deposition from electrolyte solution of lead, tin and copper with alumina particles.
Aluminum matrix material reinforced by silica (SiO₂) is prepared from AlCl₃-dimethylsulfone electrolyte containing fine silica particles.

Incorporation of dispersed hard particles by electrolytic co-deposition is also used for obtaining wear resistant coatings of copper, cobalt, silver, gold and zinc.
Co-deposition of solid lubricant particles

Incorporation of solid lubricant particles into a metallic coating improves its antifriction properties: decreases the coefficient of friction and increases Seizure resistance (compatibility) and wear resistance.

Particles of Thermoplastic Polytetrafluoroethylene (PTFE), molybdenum disulfide, graphite and boron nitride (BN) are used as the second phase of solid lubricant in the composite antifriction coatings.

The most popular composite antifriction coatings are nickel based with co-deposited PTFE.Copper-graphite and cobalt-BN are other examples of depositions with incorporated particles of solid lubricants.

Co-deposition of corrosion protection coatings

Composite coating containing incorporated dispersed non-metallic particles demonstrate enhanced corrosion and oxidation resistance.

Strong compounds (oxides, nitrides, carbides, borides) are used as the second phase in corrosion resistant coatings: Al₂O₃, Cr₂O₃, TiO₂, SiO₂, SiC, TiC, Cr₃C₂, Si₃N₄, ZrB.Nickel, cobalt and chromium are commonly used as the matrix materials in corrosion resistant composite coatings.

Examples of corrosion and oxidation resistant composite coatings: Ni-Al₂O₃, Ni-TiO₂, Ni-Cr₂O₃, Ni-TiC, Co-Cr₃C₂, Cr-ZrB.

Sputter bearing overlays

 Sputter overlays are deposited by Physical Vapor Deposition (PVD) method.
Sputtering utilizes argon ions for bombarding a cathodically connected target, made of the coating material (normally Al20Sn or Al40Sn).

Sputtering process:

• Evacuation of the vacuum chamber and introduction of argon gas.
• Heating of argon, which converts to plasma state: positively charged of argon ions form).
• Etching and activation of the substrate (bearing) surface. At this stage the bearings are cathodes and the positive argon ions bombard the substrate surface and clean it. Etching and activation stage provides good adhesion of the deposit to the bearing surface.
• Ni or NiCr diffusion layer is deposited. The cathode now is a target made of Ni/NiCr. The argon ions bombard the target knocking the atoms of nickel (nickel and chromium) out from the target. The atoms moving off the target meet the substrate surface and stick to it. Thickness of the diffusion layer is 0.00004”-0.00008” (1-2 µm).
• Deposition of the overlay (Al20Sn or Al40Sn). At this stage the cathode is a target made of aluminum-tin alloy (Al20Sn or Al40Sn). Atoms of the target are knocked out by the high energy ions and deposit on the substrate surface forming AlSn overlay.

Sputtering method provide extremely homogeneous distribution of tin within aluminum matrix. Hardness of aluminum-tin sputter material is about 90 HV, which is three times higher than hardness of aluminum-tin alloy prepared by conventional methods (casting).Cast copper based bearings or high strength aluminum based bearings are commonly plated by sputter overlays.
Load carrying capacity of sputter bearings is highest of all bearing materials: 14500-17400 psi (100-120 MPa).

The disadvantages of sputter bearings: high production cost (slow deposition process) and poor soft anti-friction properties (compatibility, conformability, embedability).
Improvement of anti-friction properties and cost reduction are achieved by a combination of a sputter bearing shell in the high load position (conrod) with a common tri-metal bearing shell in the less loaded position (cap).
Sputter bearings are mainly used as connecting rod bearings in highly loaded diesel engines with direct fuel injection system.

Vapor deposition

There are two basic vapor deposition processes:

• Physical Vapor Deposition (PVD)
• Chemical Vapor Deposition (CVD)

Physical Vapor Deposition (PVD)

Physical Vapor Deposition (PVD) is the process involving vaporization of the coating material in vacuum, transportation of the vapor to the substrate and condensation of the vapor on the substrate (part) surface.
Vaporization of the coating material stock may be made by one of the following methods:

• Evaporation
• Sputtering
• Arc Vaporization

Sputtering is a Physical Vapor Deposition method, utilizing argon ions for bombarding a cathodically connected target, made of the coating material.
Atoms of the target are knocked out by the high energy ions and deposit on the substrate surface.
Sputtering process scheme is shown in the picture:
 
Metals, alloys, ceramics and some polymers may be deposited onto metals, ceramics and polymers by Physical Vapor Deposition method.
Applications of PVD:

• TiN, TiAlN, TiCN and CrN coating for cutting tools;
• AlSn coating on engine bearings, diamond like coating for valve trains;
• Coating for forming tools;
• Anti-stick wear resistant coating for injection molds;
• Decorative coatings of sanitary and door hardware.

Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) – the process, in which the coating is formed on the hot substrate surface placed in an atmosphere of a mixture of gases, as a result of chemical reaction or decomposition of the gases on the substrate material.
Applications of CVD:

• Integrated circuits;
• Optoelectrical devices;
• Micromachines;
• Fine powders;
• Protective coatings;
• Solar cells;
• Refractory coating for jet engine turbine blades.

Phosphate coating

Phosphate coating (phosphating) is a conversion coating consisting of an insoluble crystalline metal-phosphate salt formed in a chemical reaction between the substrate metal and a phosphoric acid solution containing ions of metals (zinc, iron or magnesium).

Conversion coating is a film of a chemical compound formed in the reaction of the substrate substance with another substance. This reaction distinguishes conversion coating from a conventional coating applied on the substrate surface without changing its chemical state. Examples of conversion coating are Anodizing (electrochemical process of growing oxide film on the surface of anodically connected metal in an acidic electrolyte solution) and Black oxide (coating formed on the metal surface as a result of a chemical reaction of the metal atoms with an oxidizing agent).

Phosphating coatings are applied to steels, cast irons and aluminum alloys in order to increase their corrosion resistance, improve the anti-friction properties (break-in, wear resistance, ant-galling, coefficient of friction) and provide strong adhesion bonding for subsequent painting or other organic coating.

• Chemistry of phosphating process
• Zinc phosphate
• Iron phosphate
• Manganese phosphate
• Stages of phosphating process

Chemistry of phosphating process

The main components of a phosphating solution are:

• Phosphoric acid (H₃PO₄);
• Ions (cations) of bivalent metals: Zn²⁺, Fe²⁺, Mn²⁺;
• Accelerator - an oxidizing reagent (nitrate, nitrite, peroxide) increasing the coating process rate and reducing the grain size of the deposit.

When a metal part is immersed into a phosphating solution (for example zinc phosphate) the following chemical reactions start:

Iron dissolves in the phosphoric acid solution:
3Fe + 6H⁺ + 2PO₄^3- = 3Fe²⁺ + 2PO₄^3- + 3H₂

Consumption of phosphoric acid for the reaction causes reduction of the acidity of the solution in the layer adjacent to the metal surface. Solubility of zinc phosphate in the neutralized solution is lowering resulting in prcipitation of the salt and its deposition on the substrate surface:
3Zn²⁺ + 2PO₄^3- = Zn₃(PO₄)₂
Zinc phosphate

Zinc phosphate coating is applied when increased corrosion resistance is required. Zinc phosphate withstands 240 hours of neutral salt test.
A wide range of coating weights may be obtained: from very thin fine crystal films to heavy deposits with weight up to 4 g/ft² (40 g/m²).
The coating color is gray of different tins: from light to dark. Finer zinc phosphate crystals produce darker color. Dark gray color is also characteristic for the high carbon steel substrates.
Zinc phosphate coatings may be applied by using immersion or spray technique.
Light and medium weight zinc coatings do not require substrate surface activation. The substrate surface should be acid activated prior to heavy coating deposition.
Zinc phosphate is used not only for non-coated Steels and cast irons but also for galvanized (zinc plated) steel parts.

Iron phosphate

Iron phosphate coating is applied when strong adhesion of a subsequent painting is required.
In contrast to the solutions for Zinc phosphate and Manganese phosphate coatings, in which the metal ions are a constituent of the composition, to the iron phosphate solution iron ions are provided by the dissolving substrate.
Iron phosphate coatings have very fine Grain structure.
Iron phosphate is translucent therefore its color depends on the steel surface quality. The common color is blue or bluish-brown.
Iron phosphate is applied mostly by spray (three-stage or five-stage) method but immersion technique is also used.
The coating weight is typically in the range 20-100 mg/ft² (0.2-1.0 g/m²).
Manganese phosphate

Manganese phosphate coatings is applied when wear resistance and anti-galling properties are required. Manganese phosphate also possesses the ability to retain oil, which further improves anti-friction properties and imparts corrosion resistance to the coated parts.
Magnesium phosphate coatings typically have black color with a slight brown tint, intensity of which depends on the content of manganese oxide in the coating.
Manganese phosphate is applied by immersion method. The substrate surface should be acid activated prior to coating.
The coating weight is typically in the range 500-4000 mg/ft² (5-40 g/m²). Because of their good anti-friction properties and corrosion resistance iron phosphate coatings are widely used for combustion engine parts (camshafts, piston rings, cylinder liners, gear parts), weapon mechanisms and other parts working with friction.

Stages of phosphating process

• Cleaning. The part is mechanically cleaned and degreased in an alkaline solution.
• Hot water rinsing at about 170ºF (77ºC).
• Pickling (acid cleaning). Oxide films and rust stains are dissolved in acid.
• Acid activation (if necessary).
• Water rinsing.
• Phosphating by immersion or spraying method. Typical operating temperature is about 150ºF (66ºC). Manganese phosphate coating is applied at 170-200ºF (77-93ºC). The treatment time is varying in the range 2-40 min.
• Water rinsing.
• Drying.

Tin alloy electroplating

         Characteristics of tin

• Applications of electroplated tin alloys
• Tin alloys used for electroplating
• Tin alloy electroplating in fluoborate solutions
• Tin alloy electroplating in methane sulfonic solutions
• Tin electroplating in sulfate solutions
• Tin electroplating in stannate solutions

Characteristics of tin

Symbol: Sn

Atomic number: 50

Atomic weight: 118.71

Classification: Metal

Crystal structure: Tetragonal

Tin is soft ductile silver white metal.

Characteristic properties of tin and tin alloys:

• Excellent corrosion and tarnish resistance;
• Excellent cosmetic appearance;
• Excellent solderability;
• Very good ductility (malleability);
• Non-Toxicity;
• Good anti-friction properties (low friction, high galling resistance).

Applications of electroplated tin alloys

• Electronics and semiconductors industry

Tin Electroplating is widely used in manufacturing printed circuit boards (PCBs), printed wiring boards (PWBs), electronic components. Most electric circuit connections are made by Soldering therefore the surfaces of the conductors being connected are coated by tin or a tin alloy having excellent solderability. Additionally tin coating protects the components and connections from corrosion in aggressive atmosphere. Thickness of tin coatings used in electronics is usually up to 0.0005” (0.012 mm).

• Food containers and packages

Many food and beverage cans, food storage containers, food handling equipment are tin plated.

• Engine bearings

Tin-copper and lead-tin-copper alloys are used in tri-metal sliding bearings as anti-friction coating of 0.0005”-0.001” (0.012-0.025 mm) thick. In addition to this very thin (0.04 μinch / 1μm) pure tin coating over the bearing surface is used for better cosmetic appearance and corrosion protection.

Tin alloys used for electroplating

Most tin base alloys have been developed as non-toxic lead-free alternatives of the traditional tin-lead solder 63Sn-37Pb.

Electroplating process of tin base lead-free alloys requires strict control of the electrolyte composition and other process parameters. Small deviations in the deposited alloy composition may result in large changes in the melting point.

Another disadvantage of most tin base lead-free alloys is their proneness to form tin whiskers - mono-crystal tin filaments growing on the surface of tin base alloy. Whiskers growth is driven by the internal compressive stresses in the deposit caused by either parameters of the electroplating process or external factors (mechanical, thermal, environmental).
Long whiskers formed on a lead extend to other leads and may bridge across them causing catastrophic shorts of the circuit.
The following measures reduce the risk of whiskers formation: low brighteners plating solutions, annealing immediately after plating at 300-340°F (150-170°C) for 3-1 hours, reflow after the plating, nickel barrier preventing diffusion of copper from the substrate to the tin coating.

• Pure tin

There are two types of electroplated pure tin: bright tin and matte tin.

Bright tin is coated in electroplating solutions containing brighteners - organic additives causing formation of fine Grain structure deposit. Bright tin coating have excellent cosmetic appearance, however they are characterized by high internal stresses and contain increased amount of organics.
Matte tin coatings are made in electrolytes without additions of brighteners. Matte tin has dull appearance but the level of internal stresses in matte tin depositions is much less than in that of bright tin.

Pure tin has been used in food package applications and as cosmetic overlay.
Recently pure tin has been introduced as non-toxic replacement of lead containing solders. Maximum service temperature of pure tin solders is higher due to higher melting temperature of tin (450°F / 232°C).
Matte tin (in contrast to bright tin) is characterized by low whiskers growing therefore it is used in electronics.

• Tin-lead

Tin-lead alloys (eg.63Sn-37Pb) were very popular for electroplating of electronic components. The composition 63Sn-37Pb is eutectic point of the binary Sn-Pb system therefore the melting point of the alloy is lowest of all Sn-Pb alloys: 361°F (183°C).
Now toxic lead containing alloys have been replaced by lead-free alternatives.

• Lead-tin-copper

Alloys 87Pb-10Sn-3Cu, 83Pb-14Sn-3Cu, 82Pb-10Sn-8Cu are used for deposition of anti-friction layer on sliding engine bearings. Lead provides good anti-friction properties of the coating, tin imparts corrosion resistance, copper increases hardness and fatigue strength.

• Tin-copper

Eutectic composition Sn-0.7Cu with the melting point 441°F (227°C) is the most popular non-toxic Sn-Cu alloy. The presence of copper increases the alloy strength but makes it brittle. Other disadvantages of the alloy are its poor wetting and proneness to form whiskers.

• Tin-silver

Sn-3.5Ag, Sn-3Ag are typical tin-silver lead-free alloys possessing good solderability, high maximum service temperature and mechanical strength. The alloy disadvantages are relatively high cost and proneness to form whiskers.

• Tin-silver-copper

Eutectic composition Sn-3.5Ag-0.7Cu has relatively low melting point 423°F (217°C), moderate wettability, good strength and fatigue strength. Sometimes up to 3% of bismuth is added to the alloy to improve wettability and decrease the melting point.

• Tin-bismuth

Eutectic composition 42Sn-58Bi having very low melting point 280°F (138°C) is used in some low temperature applications. The alloy has good wettability and low proneness to whiskers however it is brittle. Sn-Bi alloys are incompatible with lead containing materials because of formation of ternary eutectic with extremely low melting point 204°F (96°C). The eutectic locating along the grain boundaries causes drop of mechanical properties.

• Tin-zinc

The alloy Sn-9Zn has a melting point 388°F (198°C). The alloy strength and fatigue strength are higher than those of tin-lead alloy. The disadvantages of the alloy are poor wettability and low corrosion resistance.

Tin alloy electroplating in fluoborate solutions

Bath ingredients:

Tin fluoborate Sn(BF₄)₂Lead fluoborate Pb(BF₄)₂Copper fluoborate Cu(BF₄)₂Fluoboric acid HBF₄Boric acid H₃BO₃Organic brighteners (additives)
Deionized (DI) water

Operating conditions:

Temperature: 70-100°F (21-38°C)
Agitation: Solution and/or cathode rod, no air agitation
Anodes composition: similar to the coating composition
Anode/Cathode surface areas ratio: ≥1
Filtration: continuous with minimum 2 bath turnovers per hour, no carbon
Cathode current density: 20-70 A/ft² (2.2-7.6 A/dm²)

Bath formulations

Tin alloy electroplating in fluoborate solutions
	Tin		Lead		Copper		Fluoboric acid		Boric acid
Coating	oz/gal	g/l	oz/gal	g/l	oz/gal	g/l	oz/gal	g/l	oz/gal	g/l
Pure tin (100Sn)	5	37					26	200	4	30
90Sn-10Pb	10	75	1.3	10			23	175	4	30
60Sn-40Pb	7	52	4	30			17	128	4	30
10Sn-87Pb-3Cu	1.3	10	9	68	0.33	2.5	17	128	4	30

Problems and troubleshooting

Problem	Cause	Corrective action
Burning at high current densities	1. Low metals concentration 2. Too high current density	1. Adjust metals concentrations 2. Adjust current density
Treeing at high current densities	1. Low additive concentration 2. Low acid concentration	1. Ad additive 2. Ad acid
Roughness	1. Foreign particles in bath 2. Stannic tin 3. Sulfate/chloride contaminations	1. Filter 2. Filter 3. Increase rinsing and filter the bath
Poor throwing power	1. Low acid concentration 2. Metallic contaminations	1. Ad acid 2. Dummy bath at 1-2 A/ft² (0.1-0.2 A/dm²)
Poor solderability	1. Organic contaminations 2. Metallic contaminations	1. Carbon treat 2. Dummy bath at 1-2 A/ft² (0.1-0.2 A/dm²)
Poor adhesion	Poor substrate cleaning	Improve cleaning
Brittle deposit	1. Organic contaminations 2. Metallic contaminations	1. Carbon treat 2. Dummy bath at 1-2 A/ft² (0.1-0.2 A/dm²)
Dark deposit	1. Organic contaminations 2. Low additive 3. Low temperature	1. Carbon treat 2. Ad additive 3. Increase temperature

 

Tin alloy electroplating in methane sulfonic solutions

Electroplating in methane sulfonic acid solutions is more controllable process than deposition in fluoborate solutions. It allows to obtain high quality tin base coatings of consistent chemical composition.

Bath ingredients:

Stannous methane sulfonate
Lead methane sulfonate
Copper methane sulfonate
Methane sulfonic acid (MSA)
Organic brighteners (additives)
Deionized (DI) water

Operating conditions:

Temperature: 70-100°F (21-38°C)
Agitation: Solution and/or cathode rod, no air agitation
Anodes composition: similar to the coating composition
Filtration: continuous with minimum 2 bath turnovers per hour, no carbon
Cathode current density: 10-40 A/ft² (1.1-4.3 A/dm²)

Bath formulations

Tin alloy electroplating in Methane sulfonic acid solutions
	Tin		Lead		Copper		MSA
Coating	oz/gal	g/l	oz/gal	g/l	oz/gal	g/l	oz/gal	g/l
Pure tin (100Sn)	6	45					26	200
90Sn-10Cu	6.7	50			0.67	5	26	200
90Sn-10Pb	3	22	0.4	3			26	200
60Sn-40Pb	2	15	1	7.5			26	200

Problems and troubleshooting

Problem	Cause	Corrective action
Burning at high current densities	1. Low metals concentration 2. Too high current density	1. Adjust metals concentrations 2. Adjust current density
Treeing at high current densities	1. Low additive concentration 2. Low acid concentration	1. Ad additive 2. Ad acid
Roughness	1. Foreign particles in bath 2. Stannic tin	1. Filter 2. Filter
Poor adhesion	Poor substrate cleaning	Improve cleaning

 

Tin electroplating in sulfate solutions

Bath ingredients:

Stannous sulfate SnSO₄Sulfuric acid H₂SO₄Organic brighteners (additives)
Deionized (DI) water

Operating conditions:

Temperature: 70-100°F (21-38°C)
Agitation: Solution and/or cathode rod, no air agitation
Anodes composition: pure tin
Filtration: continuous with minimum 2 bath turnovers per hour, no carbon
Cathode current density: 10-40 A/ft² (1.1-4.3 A/dm²)

Bath formulations

Tin 6 oz/gal (45 g/l)
Sulfuric acid 16 oz/gal (120 g/l)

Problems and troubleshooting

Problem	Cause	Corrective action
Burning at high current densities	1. Low metals concentration 2. Too high current density	1. Adjust metals concentrations 2. Adjust current density
Treeing at high current densities	1. Low additive concentration 2. Low acid concentration	1. Ad additive 2. Ad acid
Roughness	1. Foreign solid particles in bath 2. Stannic tin	1. Filter 2. Filter
Poor throwing power	1. Low acid concentration 2. Low tin concentration	1. Ad acid 2. Add stannous sulfate
Poor solderability	1. Organic contaminations 2. Metallic contaminations	1. Carbon treat 2. Dummy bath at 1-2 A/ft² (0.1-0.2 A/dm²)
Poor adhesion	Poor substrate cleaning	Improve cleaning
Brittle deposit	1. Organic contaminations 2. Metallic contaminations	1. Carbon treat 2. Dummy bath at 1-2 A/ft² (0.1-0.2 A/dm²)
Dark deposit	1. Organic contaminations 2. Low additive 3. Low temperature	1. Carbon treat 2. Ad additive 3. Increase temperature

 

Tin electroplating in stannate solutions

Bath ingredients:

Potassium stannate K₂SnO₃•3H₂O
Free potassium hydroxide KOH
No additives are required
Deionized (DI) water

Operating conditions:

Temperature: 150-180°F (66-82°C)
Agitation: Solution and/or cathode rod
Anodes composition: pure tin, steel, stainless steel
Filtration: continuous with minimum 2 bath turnovers per hour
Cathode current density: 30-100 A/ft² (3.2-11 A/dm²)

Bath formulations

Potassium stannate 13.5 oz/gal (100 g/l)
Free potassium hydroxide 2 oz/gal (15 g/l)

Problems and troubleshooting

Problem	Cause	Corrective action
Low cathode efficiency	1. Low tin concentration 2. Low temperature 3. High current density	1. Ad potassium stannate 2. Increase temperature 3. Adjust current density
Low anode efficiency	1. Low free potassium hydroxide 2. Low temperature 3. High current density	1. Ad potassium hydroxide 2. Increase temperature 3. Adjust current density or increase anode area
Low conductivity	1. Low temperature 2. Low free potassium hydroxide 3. Low tin concentration	1. Increase temperature 2. Ad potassium hydroxide 3. Ad potassium stannate
Spongy dark deposit	Stannous tin formation	Add hydrogen peroxide