tin plating
DESCRIPTION
corrosion prevention techniqueTRANSCRIPT
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.
Tin alloys used for electroplating
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 alloys used for electroplating
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 alloys used for electroplating
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 alloys used for electroplating
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
Tin alloy electroplating in fluoborate 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 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 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
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
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
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
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
Problems and troubleshooting
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)
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
Problems and troubleshooting
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)
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
Problems and troubleshooting
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