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