Tin Plating

TIN ALLOY ELECTROPLATING

RANDHIR KUMAR SINGH ASST PROFESSOR OPJIT

CONTENT

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.

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.

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 "Whisker" shown above growing between pure tin-plated hook terminals of an

electromagnetic relay similar to MIL-R-6106 (LDC 8913)

"Dendrites" shown above are NOT the same phenomenon as "whiskers"


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)2

Lead fluoborate Pb(BF4)2

Copper fluoborate Cu(BF4)2

Fluoboric acid HBF4

Boric acid H3BO3

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 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)


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

Bath formulations

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


Dark deposit 1. Organic contaminations 2. Low additive 3. Low temperature

1. Carbon treat 2. Ad additive 3. Increase temperature

Problems and troubleshooting

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

Tin alloy electroplating in methane sulfonic solutions

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)

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

Bath formulations


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



Tin electroplating in sulfate solutions Bath ingredients:

Stannous sulfate SnSO4 Sulfuric acid H2SO4 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 : 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)


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



Brittle deposit 1. Organic contaminations 2. Metallic contaminations


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)


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


3. Adjust current density or

increase anode area

Low conductivity

1. Low temperature

2. Low free potassium

hydroxide

3. Low tin concentration


2. Ad potassium hydroxide

3. Ad potassium stannate

Spongy dark deposit Stannous tin formation Add hydrogen peroxide


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Tin Plating