What would we do without it?
Where and how is it used?
1 Tin - brief history
• Known about for over 3000 years • Alloyed with Cu to make bronze (3300 - 1200BC) • Alloyed with Cu, and/or Sb, Bi, Pb, Ag - Pewter, Britannia metal & other alloys. Used for tin toys, whistles, etc. (before plastics/zinc diecasting). • Applied to copper pans to obviate the metallic taste of Cu in food
• Alloyed with Pb for soft soldering (As, Sb, Bi, Ag also used in small amounts)
• Used in pottery glazing (as stannic oxide) • Electroplated onto steel sheet, wire and electronic components • Organo tin compounds used for reaction catalysts, intermediates plus pesticides, biocides and wood preservatives
2 Metal contents in earth’s crust
Approximate content in ppb
•Sn 2 •Pb 14 •Cu 50 •Zn 75 • Al 82,000 Typical cost of tin is $22,300/tonne (Ca £13.60/Kg) For comparison copper is $7,200 (Ca £4.39/kg) (Exchange rate of 1.64 $/£)
3 Estimated global tin mine reserves Tonnes - 2011
Current consumption is said to be ca 25,000 tonnes p.a. 4 Global tin usage 2011
5 Tin mining and ore treatment
Mining • Veins of tin extracted by digging • Dredging of alluvial deposits • Gravel pumping of tin bearing gravels and shales • Open cast mining - not common
Ore separation • Screening, wet and dry gravity methods • Magnetic and electrostatic separation.
(The amount of treatment needed depends on the grade of ore and type of deposit).
6 Smelting from ore
Initial refining
• Cassiterite (SnO2) crushed • Roasted with carbon, limestone & silica @ 1200 - 1300oC
SnO2 + C Sn + CO2 • Molten tin tapped off • Slag containing tin is reprocessed Further refining • Ingots fused at just above m.p. and molten tin tapped off • Molten tin “tossed” in air to oxidise impurities that are skimmed off (dross) • Dross that contains significant tin is refined again to recover the tin • Non alloyed impurities stay as solid slag. Useful impurities • Sb, Ta, Ni, W are all useful metals recovered from dross..
7 Ancient smelting
8 More modern smelting
9 Tin refining process
10 Atomic structure
11 Isotopes • 10 stable isotopes and a further 28 unstable ones Below is a list of the more common ones
12 Basic properties of tin • Atomic number 50 • Atomic weight 118.71 • Density (White Sn - beta) 7.37 g/cc • Density (Grey Sn - alpha) 6.99 g/cc • Melting point 231.93oC • Boiling point 2,602oC • Crystal structure (Beta) Tetragonal • Crystal structure (Alpha) Diamond cubic • Creaks when bent (Tin cry) Crystal twinning (intergrowth) • Isotopes 10 stable, 28 unstable • Toxicity (metal) V. low (0.02% in food – nausea?) • (Organo tin compounds) Can be very toxic 13 Allotropes
Gold & Tungsten Alpha tin Beta tin (for comparison) W Au
Gamma tin also exists but only at high temperatures (160 - 200oC)
14 Tin pest • Pure beta tin transforms to Alpha tin spontaneously below 13.2oC.
• Process is autocatalytic and promoted by trace Ge.
• Impurities (Sb, As, Pb, In, Ag) reduce the transformation to below 0oC
• Alpha tin has a volume that is ca 27% greater than beta tin
• Scott in the Antarctic said to have lost kerosene from drums with pure tin soldered seals.
• Napolean’s troops said to have tin tunic buttons disintegrate in Russia
• Risk of electronic equipment failure (Especially with Pb use restricted)
• Pure tin solders now contain Sb, Cu &/or Ag (improve strength/reduce tin pest risk) • Alpha tin reverts to Beta tin when heated to Ca 230oC
15 Tin/gold intermetallic
• Tin & gold in contact undergo solid state diffusion • Process takes several months at R.T.
6Sn + Au AuSn6 (Amorphous crystals)
Example of solder joint failure caused by a gold plated component lead
16 Tin whiskers Tin plated component leads with whiskers
Electron-photomicrographs of whiskers
Several major failures of electronic equipment have been caused by tin whiskers 17 Main modern uses of tin
Soft soldering of copper and other metals •Plumbing • Electric/electronic equipment Electrodeposition • Steel sheet for making cans (food etc). (“Tin” cans patented in 1810 using steel sheet tinned by hot dip process) • Wire (mainly Cu) for electrical/electronic applications • Printed circuit board etch resists & solderable finishes • Electronic component leads to be subsequently soldered Engine bearing shells (Babbitt metal) • Alloyed with Cu and either Sb & Pb or both • Other bearing alloys with elements such as In
18 Modern uses of tin Continued Ceramics • Stannic oxide in ceramics glazes - opaque white finish Mixed with
other metal oxides to create colours examples V2O5 - yellow, Cr2O3 -pink, Sb2O5 - blue/grey Electronic components • Stannic oxide used in some electronic resistors Textiles • Stannous chloride used as a mordant (dye fix) for printing Organic tin compounds • Organo-tin compounds in plastics, wood preservatives, pesticides and fire retardants Glass making • Molten tin used as a base for float glass manufacture
19 Soft soldering
Typical tin alloys used
• Pure tin with traces of Ag, Cu or Sb (for improved strength) • 63% Sn, 37% Pb (Tinmans solder - eutectic composition) • 30% Sn, 70% Pb (Plumbers solder) • 90% Sn, 10% Pb (High temperature solder - ca 220oC) • 10% tin, 90% Pb (High temperature solder - ca 310oC) • 50% Sn, 49% Zn, 1% Cu (For galvanised steel) • 60% Sn, 38% Pb, 2% Cu (Reduces wear of Cu irons) • 62% Sn, 36% Pb, 2% Ag (High strength - electronics) • 43%Sn, 43% Pb, 14% Bi (Low temperature solder 144 - 163oC)
20 Tin lead phase diagram
21 Soldering • For tin and tin alloys to solder items together the tin must form an intermetallic bond with the components. Surfaces to be clean & free from: • Surface oxides, sulphides etc. • Embedded abrasives • Deformation resulting from abrasion.
Substrate with surface furrows and cavities exposed after surface dissolution
Etching a small amount of metal from the surface obviates the problem
22 Surface preparation
• Remove any visible contamination, corrosion products etc. • If surfaces have been abraded apply a light etch • Apply a flux to remove any trace oxides during soldering • Many mildly acidic compounds will act as fluxes. Typical flux types • Electronic applications - Rosin with less than 0.5% halide • General electrical - Rosin with less than 1% halide (Rosin is the resin from pine trees and is essentially abietic and pimaric acids) • Steels - Acidic zinc chloride, ammonium chloride (Ni & Al both have tenacious oxides and require special treatment) • Residues need to be removed to avoid subsequent corrosion 23 Wave soldering of printed circuits
24 Other metals with MP below 400oC for Soldering?
Metal MP Issue • Cadmium 321oC - Toxic • Indium 157oC - Viable alternative • Lead 327oC - Will not readily wet copper steel etc. • Lithium 180oC - Reactive!! • Bismuth 271o - Bi/Pb fuses at 124oC More a substitute for lead •Polonium 254oC - Toxic!!! • Sodium 98oC - Reactive!! • Thallium 304oC - Toxic
(Tin - 232oC)
25 Indium alternative to tin in solder
Useful for soldering gold components - no inter-metallic issue Indium costs Ca $650/Kg (£406/Kg) Tin is Ca £13.6/Kg Estimated content of the earth’s crust is 100 ppb Tetragonal structure (like tin) and creaks when bent 26 Alternatives to soldering
Both plumbing and electronics
• Welding (spot welding if the heat is not a problem)
•Brazing (expensive because of Ag content and heat can be a problem)
• Adhesives (susceptible to surface preparation)
• Compression joints (metal and plastic)
27 Bronzes
• Wide range of Cu/Sn alloys with 5 - 20% tin • A typical modern bronze is 88% Cu, 12% Sn • Used extensively in marine applications before stainless steel became readily available • Low friction, suitable for bearings and cannons!!!
• Does not strike sparks against stone (good for coal miners) • Used for piano strings (low pitch)
• Phosphor bronze 3.5 - 10% Sn and up to 1% P. (used where for conductive springs, musical instruments and bearings)
28 Pewters and similar alloys • Pewter 88-99% Sn remainder Cu. (M.P. 170-230oC)
• Britannia metal (92% Sn, 6% Sb and 2% Cu). MP 255oC. Commonly used to make articles that are plated with Ag.
• English pewter 91% Sn, 7.5% Sb and 1.5% Cu. (It used to contain Pb but this was removed in 1974 because of tableware use).
• Fields metal 32.5% Bi, 51% In, 16.5% Sn (Fuses at 62oC - less toxic than Woods metal - 50% Bi, 26.7% Pb, 13.3% Sn, and 10% Pb )
• Galistan Eutectic alloy liquid at RT (Typically 62-95% Ga, 5-22% In, 0-16% Sn).
• Rose’s Metal 50% Bi, 25-28% Pb and 22-25% Sn (M.P. 98oC used for fusible formers and links - does not contract during cooling).
• Babbitt metal (invented in 1839 - used for bearings) 29 Babbitt bearing alloys
Alloys consist of a matrix composite of small hard crystals in a softer metal
Typical bearing
30 Bell Metal
• Alloy of 78% Cu and 22% Sn • Harder and less ductile than either Cu or Sn
• Resistant to oxidation, forms protective patina (CuO/CuCO3) • Used for more than 3,000 years • Ag can be added to change the tone
31 Electroplating applications
Electronics/electrical Printed circuit boards • Etch resist and also a solderable coating, (tin with 5 - 10% lead is common to avoid whisker growth)
• Plating solutions - SnSO4 in H2SO4 (pure tin only), Sn(BF4)2 in H3BO3/HBF4, Sn(CH3SO3)2 in CH3SO3H. (Additives used to produce smooth deposits. Occluded organic matter can cause fused deposits to outgas). Copper wire
• Plating solutions - Sn(BF4)2 in HBF4, Sn(CH3SO3)2 in CH3SO3H.
(Sn(CH3SO3)2 can be made as a concentrate containing 300g/l Sn). 32 Electroplating applications Continued
• Steel sheet for “tin” cans
• Steel and copper/copper alloy cooking utensils
• Electronic connectors
• Specialist engineering components
33 Antoxidants for stannous tin Oxidation of stannous tin • Stannous tin in acid solution is oxidised in air to stannic tin • Stannic tin makes solutions turbid with the foration of alpha stannic
acid (H2SnO36H2O)
• This then slowly converts to meta-stannic acid (H2Sn5O11) which clogs filters and can be co-deposited causing deposit roughness. Antoxidants • The most widely used are poly hydroxy benzenes in concentrations as low as 0.5 g/l. the most common are: OH OH OH
OH
OH OH Catechol Resorcinol Hydroquinone
• Metallic elements of Groups IVB, VB & VIB e.g.V, W, Zr, Ta, Ti, Nb also work at concentrations between 0.025 & 5g/l. 34 Tin/tin-lead as an etch resist
• Typically 20 g/l Sn as Sn(CH3SO3)2 in 100 ml/l CH3SO3H, (SnSO4 in H2SO4 (pure tin only) or Sn(BF4)2 in H3BO3/HBF4 can be used) • Tin deposited at Ca 2 A/dm2, 20 oC with agitation (not with air sparge) • Additives used to produce a smooth matt deposit • Tin is not attacked by copper etchants used for PC manufacture. A typical etchants is sprayed (NH4)2CO3. Sometimes with Na2ClO2. • After the copper has been removed the tin or tin/lead can be fused with a flux to leave a bright solderable coating • Alternately tin can be stripped to leave a copper finish • Copper finishes can be “Solder levelled” to apply a tin coating to exposed copper features. Here boards are fluxed, immersed in molten solder at about 185oC and extracted through a blast of warm air to remove excess solder that is returned to the solder tank.
35 Medium sized printed circuit plating plant
36 Printed circuits with copper and tin finishes
37 Electronic connectors
• Reel to reel machines used where connector strips are part immersed in solutions
• Plating solutions - SnSO4 in H2SO4 (pure tin only), Sn(BF4)2 in H3BO3/HBF4, Sn(CH3SO3)2 in CH3SO3H. • Strip is cleaned electrolytically, rinsed, activated in acid, nickel* or copper plated ca 2 microns, rinsed, tin plated at about 2 A/dm2 at 20oC Ca 2 microns, rinsed and dried. • Strip can be inverted for gold plating of the mating face.
• * A Woods nickel strike is required to ensure adhesion of nickel or copper plate and prevent dewetting during soldering.
38 Reel - reel plating
Connector strip (cathode) Weir
Anode Anode
Plating solution
39 Reel - reel plating Continued
40 Tin for electronic component leads
• Typically 20 g/l Sn as SnSO4 in 100 ml/l H2SO4 • Tin plated at 2 A/dm2,at 20oC, mechanical agitation • Additives used to produce smooth bright deposits • Additive occluded in the deposit, can interfere with soldering by outgassing or by “apparent non-wetting”
Outgassing
Apparent non wetting 41 Dewetting • Poor adhesion between any of the plated layers can result in dewetting. • Component lead appears to wet, then with extended time (several seconds) at soldering temperature the surface dewets. • Test - 2 x 5 secs in flowing solder will identify any potential dewetting caused by defective plated deposits
Tin plate Connector after plating
Connector substrate Cu or Ni undercoat
Dewetting after soldering Undercoat removed
42 Solder bead Connector substrate Tin plating of steel for cans
• Traditionally Sn(C6H5SO3)2 in C6H5SO3H (Ferrostan) used with an ethyloxylated napthol sulphonic acid grain refining additive
• Today Sn(CH3SO3)2 in CH3SO3H with a nonionic surfactant grain refining additive is becoming more popular. • Solutions contain ca 30 g/l tin, 20 g/l free acid, 1 g/l antoxidant & 1-5 g/l additive & run at 3 - 8 A/dm2 at 20 - 40oC • Continuous steel strip (0.375 - 0.975 mm thick) processed at speeds up to 600m/min is: • Cleaned electrolytically, rinsed, pickled in acid (hydrochloric) • Plated with 0.40 - 0.76 microns of Sn, rinsed & dried. • Tin flow melted in tower (resistance or induction heating)
• Dichromate conversion coating ca 1.3 mg/m2 (cathodic to give 4.5 mg/m2) • Oil applied (e.g. acetyl tributyl citrate) to protect passivated tin surface 43 Strip steel tin plating plant
First half
Second half
Up to 180,000 amps used at 24 v. A plant can process up to 250,000 tons of steel/yr 44 Tin plating plant images
45 Corrosion potentials (Seawater electrolyte)
Tin plate -0.65 Low alloy steels -0.85 (cf Zinc -1.25) Max potentials: Dry storage 0.5 V, Normal storage .25 V, High humidity/marine 0.15V 46 Tin plating of copper wire
• Copper wire is plated in specially built machines
• Solutions used were Sn(BF4)2 in H3BO3/HBF4. Now largely replaced with Sn(CH3SO3)2 in CH3SO3H with a nonionic surfactant grain refining additive • Very high CDs used to deposit ca 3 microns of tin • Several strands processed simultaneously • Plated wire is then passed through diamond dies to reduce its diameter (The tin has to be smooth without any nodules to avoid wire jamming in the dies). • Tin plate to pass a simple porosity test (immersion in solution of (NH4)S2O8 in dilute NH4OH, a blue colour indicates porosity) 47 Wire plating
Plating cell (solution fed from a sump Anodes
Wires
Plating solution Cleaner Rinse Drier Wires Dies
Cleaner Plating solution sump sump 48 Tin plating of wire
Steuler wire plating machine
49 Plating tin from stannate solutions
Sodium or potassium stannate in sodium or potassium hydroxide used for engineering parts to be flame soldered
Potassium stannate (preferred - higher solubility) is K2SnO3 + 2- that dissociates to 2K + (SnO3) How is tin deposited when 2K+ migrates to the cathode? + 0 2K + 2e 2K + 2H2O 2KOH + 2[H] o 4[H] + K2SnO3 Sn + 2KOH + H2O
2- At the anode (SnO3) - 2e SnO3 + 2H2O H2SnO3 + [O]
0 2[O] + Sn SnO2 + H2O H2SnO3 (Stannate tin solutions are for parts that need to be flame soldered as no additives are required). 50 Glass manufacture
Traditional methods
• Pour molten glass onto steel, allow to cool, cut to size, then grind and polish to the desired finish • Sheet glass pulled vertically from molten glass. Difficult to attain uniform thickness
51 Float glass process overview
• Molten glass fed onto molten tin at defined rate • Tin has high SG and is immiscible with molten glass • Tin tank is 3 - 4 m wide, 50 m long and 0.06 m deep The tin volume is 12,000 litres (88.44 tonnes)
• Atmosphere of N2/H2 to prevent tin from oxidising • Glass flows over the molten tin • Glass temp. falls from Ca 1000oC to 600oC over tank length • Glass is annealed as it slowly cools below its melting point • Solid glass pulled off tank by rollers & fed onto a conveyor
• Glass finally cut to size 52 Float glass process
53 Part of a float glass plant
54 Plating on plastics
• Stannous tin usually as SnCl2 reduces gold and other precious metals. - 0 - •SnCL2 + 2(KAuCN2) SnCL4 + Au + 2KCN + 2CN 0 •SnCL2 + PdCl2 SnCl4 + Pd - Also SnCL4 + 7H2O H2SnO3.6H2O + 4CL
Alpha stannic acid Stannic acid removed [H] [H] Pd [H] Pd [H] Pd [H] Non conductive surface Activated surface
Copper deposition (autocatalytic once started)
2+ - 0 - Cu + 2HCHO + 4OH Cu + 2HC00 + 2[H] + H2O [H] catalyst 55 Organo tin compounds
• In most organo tin compounds tin is quadravalent • Behaves like C, Si, Ge & Pb with 4 electrons in outer shell • C-Sn bond is weaker than C-C but still relatively stable and most organo tin compounds are non-polar
• Many organic tin compounds are made from SnCl4 e.g. SnCl4 +4CH3MgCl Sn(CH3)4 + 4MgCl2 • Sn-Sn bonds can also be synthesised
e.g. (CH3)3SnNa + (CH3)3SnCl (CH3)SnSn(CH3)3 + NaCl
• A few divalent tin compounds exist (Polystannanes (SnR2)n)
56 Organo-tin compounds Examples of organo-tin compounds in use
CH3CH2CH2CH2 CH3CH2CH2CH2
CH3CH2CH2CH2 Sn O Sn CH3CH2CH2CH2 Sn CH3CH2CH2CH2 CH CH CH CH OH 3 2 2 2 Triphenyl-tin hydroxide Bis-tributyl tin oxide (Fungicide & pesticide) (Wood preservative)
(C4H9)2Sn(SC12H25)2 (dibutyl tin bis(lauryl mercaptide) (PVC stabiliser)
Sn[OOCC7H15] (Stannous bis (2-ethoxylate)) (Urethane reaction catalyst) 57 Analysis methods for tin Volumetric Stannous tin • Titration of stannous tin in acid in an inert atmosphere with standard iodine solution & iodine indicator 2+ Sn + 2I2 SnI4 Stannic tin Titration with iodine after all of stannic tin has been reduced to stannous tin by dissolution of iron or zinc in acid solution in a non oxidising atmosphere (bicarbonate valve). Gravimetric Precipitated as tin cyclotetramethylenedithiocarbamate - poor accuracy
58 Bicarbonate valve
H2/CO2 bubbles out
NaHCO3 solution sucked in during cooling & reacts with acid to make
CO2
Sn4+ reduced to Sn2+ with Fe (fine wire) +HCl
Heat 59 Instrumental analysis methods
• Inductively coupled plasma emission spectroscopy
• Spark & glow discharge emission spectroscopy
• X-ray fluorescence spectroscopy
• (Atomic emission & B-ray backscatter have been superseded by the above).
60 Inductively coupled plasma
Dilute aqueous sample plus reference samples required. Detection limits in parts per billion - ppb 61 Spark & glow discharge emission spectroscopy
62 X-ray fluorescence spectroscopy • Can be used on most types of sample • Hand held instruments available • X-rays cause electrons to be emitted from atoms • Holes left by electrons filled from higher orbitals & radiation emitted characteristic of the atom • Energy emitted can be analysed as wavelength dispersive or energy dispersive. (Wavelength is the most accurate as each element has its own specific spectral line). • Energy intensity is related to content of element in the material
63 X-ray fluorescence spectroscopy Continued Energy dispersive Wavelength dispersive (greater accuracy & precision)
Hand held XRF
64 Recycling of tin
• The amount of tin recycled used is in the order of 8% • Tin from tin cans is removed by making the cans anodic in a warm aqueous solution of Ca 15% NaOH. (Warm NaOh with electrolysis will remove any lacquers used from the tin plated surface)
• A solution of Na2ClO2 in NaOH without electrolysis can also be used to remove tin from steel.
o • A warm solution (50 C) of 10% NaOH with 10% Na2S2O3 will remove tin from copper • Solder from printed circuit boards can be recovered by heat treatment by fusing - difficult because of capillary action. Hot air can be used to blow off the solder • Cost of tin is Ca $22,000/tonne (Ca £13.60/Kg)
65 Tin What would we do/have done without it? No bronze age Wait for iron age??? No tin-lead solder for plumbing or electronics Indium based solders (toxic, cost)? Welding? Brazing? Compression joints? No tin soldered food cans Protective varnishes? Sealed bottles? Al/plastic cans? No pre-tinned wire Flux varnishes? Copper conversion coatings? No plating on plastics inc double sided and multilayer PCBs Vacuum deposition? Copper based plating activators? Soldered rivets in PCBs? No Float glass Continue with vertical drawing 66