Soldering and Brazing

1
Soldering and Brazing

• Soldering and Brazing are joining processes where
  parts are joined without melting the base
  metals. 
• Soldering filler metals melt below 450 C. 
• Brazing filler metals melt above 450 C. 

(De)soldering a contact from a wire

• Soldering is commonly used for electrical
  connection or mechanical joints, but brazing is
  only used for mechanical joints due to the high
  temperatures involved

2
Soldering

• A method of joining metal parts using an alloy of
  low melting point (solder) below 450 C (800 F).
• Heat is applied to the metal parts, and the
  alloy metal is pressed against the joint, melts,
  and is drawn into the joint by capillary action
  and around the materials to be joined by 'wetting
  action'.
• After the metal cools, the resulting joints are
  not as strong as the base metal, but have
  adequate strength, electrical conductivity, and
  water-tightness for many uses.

3

• Soldering and Brazing Benefits
•  Economical for complex assemblies
•  Joints require little or no finishing
•  Excellent for joining dissimilar metals
•  Little distortion, low residual stresses
•  Metallurgical bond is formed
•  Sound electrical component connections

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Soldering can be done in a number of ways

• Including passing parts over a bulk container of
  melted solder, using an infrared lamp, or by
  using a point source such as an electric
  soldering iron, a brazing torch, or a hot-air
  soldering tool.
• A flux is usually used to assist in the joining
  process.
• Flux can be manufactured as part of the solder in
  single or multi-core solder, in which case it is
  contained inside a hollow tube or multiple tubes
  that are contained inside the strand of solder.
• Flux can also be applied separately from the
  solder, often in the form of a paste.
• In some fluxless soldering, a forming gas that is
  a reducing atmosphere rich in hydrogen can also
  serve much the same purpose as traditional flux,
  and provide the benefits of traditional flux in
  re-flow ovens through which electronic parts
  placed on a circuit card are transported for a
  carefully timed period of time.

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• One application of soldering is making
  connections between electronic parts and printed
  circuit boards.
• Another is in plumbing. Joints in sheet-metal
  objects such as cans for food, roof flashing, and
  drain gutters were also traditionally soldered.
• Jewelry and small mechanical parts are often
  assembled by soldering.

Soldering can also be used as a repair technique
to patch a leak in a container or cooking vessel.
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• Soldering is distinct from welding in that the
  base materials to be joined are not melted,
  though the base metal is dissolved somewhat into
  the liquid solder much as a sugar cube into
  coffee - this dissolution process results in the
  soldered joint's mechanical and electrical
  strengths.
• A "cold solder joint" with poor properties will
  result if the base metal is not warm enough to
  melt the solder and cause this dissolution
  process to occur.

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• Due to the dissolution of the base metals into
  the solder, solder should never be reused
• Once the solder's capacity to dissolve base metal
  has been achieved, the solder will not properly
  bond with the base metal and a cold solder joint
  with a hard and brittle crystalline appearance
  will usually be the result.
• It is good practice to remove solder from a joint
  prior to resoldering - desoldering wicks or
  vacuum desoldering equipment can be used.
• Desoldering wicks contain plenty of flux that
  will lift the contamination from the copper trace
  and any device leads that are present. This will
  leave a bright, shiny, clean junction to be
  resoldered.

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• The lower melting point of solder means it can be
  melted away from the base metal, leaving it
  mostly intact through the outer layer.
• It will be "tinned" with solder.
• Flux will remain which can easily be removed by
  abrasive or chemical processes.
• This tinned layer will allow solder to flow into
  a new joint, resulting in a new joint, as well as
  making the new solder flow very quickly and
  easily.

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Common joining problems and discontinuities 

• No wetting
•  Excessive wetting
•  Flux entrapment
•  Lack of fill (voids, porosity)
•  Unsatisfactory surface appearance
• Base metal erosion

10

• Basic electronic soldering techniques
• All solder pads and device terminals must be
  clean for good wetting and heat transfer.
• The soldering iron or gun must be clean,
  otherwise components may heat up excessively due
  to poor heat transfer.
• The devices must then be mounted on the circuit
  board properly.
• One technique is to elevate the components from
  the board surface (a few millimeters) to prevent
  heating of the circuit board during circuit
  operation.
• After device insertion, the excess leads can be
  cut leaving only a length equal to the radius of
  the pad.
• Plastic mounting clips or holders are used for
  large devices to reduce mounting stresses.

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• Heat sink the leads of sensitive devices to
  prevent heat damage.
• Apply soldering iron or gun to both terminal lead
  and copper pad to equally heat both.
• Apply solder to both lead and pad but never
  directly to the tip of soldering iron or gun.
• Direct contact will cause the molten solder to
  flow over the gun and not over the joint.
• The moment the solder melts and begins to flow,
  remove the solder supply immediately.
• Do not remove the iron yet. The remaining solder
  will then flow over the junction of the lead and
  pad, assuming both are free of dirt.
• Let the iron heat the junction until the solder
  flows and then remove the iron tip. This will
  ensure a good solid junction.
• Remove the iron from the junction and let the
  junction cool. Solder flux will remain and should
  be removed.

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• Be sure not to move the joint while it is
  cooling. Doing so will result in a fractured
  joint.
• Do not blow air onto the joint while it is
  cooling Instead, let it cool naturally, which
  will occur fairly rapidly.
• A good solder joint is smooth and shiny. The lead
  outline should be clearly visible. Clean the
  soldering iron tip before you begin on a new
  joint. It is absolutely important that the iron
  tip be free of residual flux.
• Excess solder should be removed from the tip.
  This solder on the tip is known as keeping the
  tip tinned. It aids in heat transfer to the
  joint.
• After finishing all of the joints, remove excess
  flux residue from the board using alcohol,
  acetone, or other organic solvents.
• Individual joints can be cleaned mechanically.
• The flux film fractures easily with a small pick
  and can be blown away with canned air.
• In solder formulations with water-soluble fluxes,
  sometimes pressurized carbon dioxide or distilled
  water are used to remove flux.

13

• Traditional solder for electronic joints is a
  60/40 Tin/Lead mixture with a rosin based flux
  that requires solvents to clean the boards of
  flux.
• Environmental legislation in many countries, and
  the whole of the European Community area, have
  led to a change in formulation.
• Water soluble non-rosin based fluxes have been
  increasingly used since the 1980's so that
  soldered boards can be cleaned with water or
  water based cleaners. This eliminates hazardous
  solvents from the production environment, and
  effluent.

14
Lead-free electronic soldering

• More recently environmental legislation has
  specifically targeted the wide use of lead in the
  electronics industry. The directives in Europe
  require many new electronic circuit boards to be
  lead free by 1st July 2006, mostly in the
  consumer goods industry, but in some others as
  well.
• Many new technical challenges have arisen, with
  this endeavour.

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• For instance, traditional lead free solders have
  a significantly higher melting point than lead
  based solders, which renders them unsuitable for
  use with heat sensitive electronic components and
  their plastic packaging. To overcome this problem
  solder alloys with a high silver content and no
  lead have been developed with a melting point
  slightly lower than traditional solders.
• Not using lead is also extended to components
  pins and connectors. Most of those pins were
  using copper frames, and either lead, tin, gold
  or other finishes. Tin-finishes is the most
  popular of lead-free finishes. However, this
  poses nevertheless the question of tin-whiskers.
  Somehow, the current movement brings the
  electronic industry backs to the problems solved
  40 years ago by adding lead.
• A new classification to help lead-free electronic
  manufacturers decide what kind of provisions they
  want to take against whiskers, depending upon
  their application criticity.

16
Stained glass soldering

• Historically soldering tips were copper, placed
  in braziers. One tip was used when the heat had
  transferred from the tip to the solder (and
  depleted the heat reserve) it was placed back in
  the brazier of charcoal and the next tip was
  used.
• Currently, electric soldering irons are used
  they consist of coil or ceramic heating elements,
  which retain heat differently, and warm up the
  mass differently, internal or external rheostats,
  and different power ratings - which change how
  long a bead can be run.
• Common solders for stained glass are mixtures of
  tin and lead, respectively
• 60/40 melts between 361-376F
• 50/50 melts between 368-421F
• 63/37 melts between 355-365F
• lead-free solder (useful in jewelry, eating
  containers, and other environmental uses) melts
  around 490F

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Pipe/Mechanical soldering

• Sometimes it is necessary to use solders of
  different melting points in complex jobs, to
  avoid melting an existing joint while a new joint
  is made.
• Copper pipes used for drinking water should be
  soldered with a lead-free solder, which often
  contains silver. Leaded solder is not allowed for
  most new construction, though it is easier to
  create a solid joint with that type of solder.
  The immediate risks of leaded solder are minimal,
  since minerals in municipal or well water
  supplies almost immediately coat the inside of
  the pipe, but lead will eventually find its way
  into the environment.
• Tools required for pipe soldering include a
  blowtorch (typically propane), wire brushes, a
  suitable solder alloy and an acid paste flux,
  typically based on zinc chloride. Such fluxes
  should never be used on electronics or with
  electronics tools, since they will cause
  corrosion of the delicate electronic part.

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Soldering defects

• Soldering defects are solder joints that are not
  soldered correctly.
• These defects may arise when solder temperature
  is too low.
• When the base metals are too cold, the solder
  will not flow and will "ball up", without
  creating the metallurgial bond.
• An incorrect solder type (for example,
  electronics solder for mechanical joints or vice
  versa) will lead to a weak joint.
• An incorrect or missing flux can corrode the
  metals in the joint. Without flux the joint may
  not be clean.
• A dirty or contaminated joint leads to a weak
  bond. A lack of solder on a joint will make the
  joint fail.
• An excess of solder can create a "solder bridge"
  which is a short circuit. Movement of metals
  being soldered before the solder has cooled will
  make the solder appear grainy and may cause a
  weakened joint.
• Soldering defects in electronics can lead to
  short circuits, high resistance in the joint,
  intermittent connections, components overheating,
  and damaged circuit boards. Flux left around
  integrated circuits' leads will lead to
  inter-lead leakage.
• It is a big issue on surface mount components and
  causes improper device operation as moisture
  absorption rises. In mechanical joints defects
  lead to joint failure and corrosion

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Soldering processes

• Wave soldering
• Reflow soldering
• Infrared soldering
• Induction soldering
• Ultrasonic soldering
• Dip soldering
• Furnace soldering
• Iron soldering
• Resistance soldering
• Torch soldering
• Silver soldering/Brazing

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Brazing

• Is similar to soldering but uses higher melting
  temperature alloys, based on copper, as the
  filler metal.
• "Hard soldering", or "silver soldering"
  (performed with high-temperature solder
  containing up to 40 silver) is also a form of
  brazing, and involves solders with melting points
  above 450 C. Even though the term "silver
  soldering" is more often used than silver
  brazing, it is technically incorrect.
• Since lead used in traditional solder alloys is
  toxic, much effort in industry has been directed
  to adapting soldering techniques to use lead-free
  alloys for assembly of electronic devices and for
  potable water supply piping.

21
Brazing

• Brazing is a joining process whereby a
  non-ferrous filler metal and an alloy are heated
  to melting temperature (above 450C) and
  distributed between two or more close-fitting
  parts by capillary action.
• At its liquid temperature, the molten filler
  metal interacts with a thin layer of the base
  metal, cooling to form an exceptionally strong,
  sealed joint due to grain structure interaction.
  T
• he brazed joint becomes a sandwich of different
  layers, each metallurgically linked to each
  other.
• Common brazements are about 1/3 as strong as the
  materials they join, because the metals partially
  dissolve each other at the interface, and usually
  the grain structure and joint alloy is
  uncontrolled.
• To create high-strength brazes, sometimes a
  brazement can be annealed, or cooled at a
  controlled rate, so that the joint's grain
  structure and alloying is controlled.

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• In Braze Welding or Fillet Brazing, a bead of
  filler material reinforces the joint. A
  braze-welded tee joint is shown here.
• In another common specific similar usage, brazing
  is the use of a bronze or brass filler rod coated
  with flux, together with an oxyacetylene torch,
  to join pieces of steel. The American Welding
  Society prefers to use the term Braze Welding for
  this process, as capillary attraction is not
  involved, unlike the prior silver brazing
  example.
• Braze welding takes place at the melting
  temperature of the filler (e.g., 870 C to 980 C
  for bronze alloys) which is often considerably
  lower than the melting point of the base material
  (e.g., 1600 C for mild steel).

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• A variety of alloys of metals, including silver,
  tin, zinc, copper and others are used as filler
  for brazing processes.
• There are specific brazing alloys and fluxes
  recommended, depending on which metals are to be
  joined. Metals such as aluminum can be brazed
  though aluminum requires more skill and special
  fluxes. It conducts heat much better than steel
  and is more prone to oxidation.
• Some metals, such as titanium cannot be brazed
  because they are insoluble with other metals, or
  have an oxide layer that forms too quickly at
  intersoluble temperatures.

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• Although there is a popular belief that brazing
  is an inferior substitute for welding, this is
  false.
• For example, brazing brass has a strength and
  hardness near that of mild steel, and is much
  more corrosion-resistant.
• In some applications, brazing is indisputably
  superior. For example, silver brazing is the
  customary method of joining high-reliability,
  controlled-strength corrosion-resistant piping
  such as a nuclear submarine's seawater coolant
  pipes.
• Silver brazed parts can also be precisely
  machined after joining, to hide the presence of
  the joint to all but the most discerning
  observers, whereas it is nearly impossible to
  machine welds having any residual slag present
  and still hide joints.

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• In order to work properly, parts must be closely
  fitted and the base metals must be exceptionally
  clean and free of oxides for achieving the
  highest strengths for brazed joints.
• For capillary action to be effective, joint
  clearances of 0.002 to 0.006 inch (50 to 150 µm)
  are recommended. In braze-welding, where a thick
  bead is deposited, tolerances may be relaxed to
  0.5 mm.
• Cleaning of surfaces can be done in several
  ways. Whichever way is selected, it is vitally
  important to remove all grease, oils, and paint.
  For custom jobs and part work, this can often be
  done with fine sand paper or steel wool.
• In pure brazing (not braze welding), it is
  vitally important to use sufficiently fine
  abrasive. Coarse abrasive can lead to deep
  scoring that interferes with capillary action and
  final bond strength. Residual particulates from
  sanding should be thoroughly cleaned from pieces.
  
• In assembly line work, a "pickling bath" is often
  used to dissolve oxides chemically. Dilute
  sulfuric acid is often used. Pickling is also
  often employed on metals like aluminum that are
  particularly prone to oxidation.

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• In most cases, flux is required to prevent oxides
  from forming while the metal is heated. The most
  common fluxes for bronze brazing are borax-based.
  T
• he flux can be applied in a number of ways. It
  can be applied as a paste with a brush directly
  to the parts to be brazed. Commercial pastes can
  be purchased or made up from powder combined with
  water (or in some cases, alcohol). Alternatively,
  brazing rods can be heated and then dipped into
  dry flux powder to coat them in flux.
• Brazing rods can also be purchased with a coating
  of flux. In either case, the flux flows into the
  joint when the rod is applied to the heated
  joint. Using a special torch head, special flux
  powders can be blown onto the workpiece using the
  torch flame itself.
• Excess flux should be removed when the joint is
  completed. Flux left in the joint can lead to
  corrosion.
• During the brazing process, flux may char and
  adhere to the work piece. Often this is removed
  by quenching the still-hot workpiece in water (to
  loosen the flux scale), followed by wire brushing
  the remainder.

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• Brazing is different from welding, where even
  higher temperatures are used, the base material
  melts and the filler material (if used at all)
  has the same composition as the base material.
• Given two joints with the same geometry, brazed
  joints are generally not as strong as welded
  joints. Careful matching of joint geometry to the
  forces acting on the joint, however, can often
  lead to very strong brazed joints.
• The butt joint is the weakest geometry for
  tensile forces. The lap joint is much stronger,
  as it resists through shearing action rather than
  tensile pull and its surface area is much larger.
  To get joints roughly equivalent to a weld, a
  general rule of thumb is to make the overlap
  equal to 3 times the thickness of the pieces of
  metal being joined.
• The "welding" of cast iron is usually a brazing
  operation, with a filler rod made chiefly of
  nickel being used although true welding with cast
  iron rods is also available.

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• Vacuum brazing is another materials joining
  technique, one that offers extremely clean,
  superior, flux free braze joints while providing
  high integrity and strength.
• The process can be expensive because it is
  performed inside a vacuum chamber vessel however,
  the advantages are significant. For example,
  furnace operating temperatures, when using
  specialized vacuum vessels, can reach
  temperatures of 2400 C. Other high temperature
  vacuum furnaces are available ranging from
  1500 C and up at a much lesser cost.
• Temperature uniformity is maintained on the work
  piece when heating in a vacuum, greatly reducing
  residual stresses because of slow heating and
  cooling cycles.
• This, in turn, can have a significant impact on
  the thermal and mechanical properties of the
  material, thus providing unique heat treatment
  capabilities.
• One such capability is heat treating or age
  hardening the work piece while performing a
  metal-joining process, all in a single furnace
  thermal cycle.
• Reference M.J.Fletcher, Vacuum Brazing. Mills
  and Boon Limited London, 1971.

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Advantages over welding

• The lower temperature of brazing and
  brass-welding is less likely to distort the work
  piece or induce thermal stresses. For example,
  when large iron castings crack, it is almost
  always impractical to repair them with welding.
  In order to weld cast-iron without recracking it
  from thermal stress, the work piece must be
  hot-soaked to 1600 F. When a large (more than
  fifty kilograms (100 lb)) casting cracks in an
  industrial setting, heat-soaking it for welding
  is almost always impractical. Often the casting
  only needs to be watertight, or take mild
  mechanical stress. Brazing is the premium,
  preferred repair method in these cases.
• The lower temperature associated with brazing vs.
  welding can increase joining speed and reduce
  fuel gas consumption.
• Brazing can be easier for beginners to learn than
  welding.
• For thin workpieces (e.g., sheet metal or
  thin-walled pipe) brazing is less likely to
  result in burn-through.

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• Brazing can also be a cheap and effective
  technique for mass production. Components can be
  assembled with preformed plugs of filler material
  positioned at joints and then heated in a furnace
  or passed through heating stations on an assembly
  line. The heated filler then flows into the
  joints by capillary action.
• Braze-welded joints generally have smooth
  attractive beads that do not require additional
  grinding or finishing.
• The most common filler materials are gold in
  colour, but fillers that more closely match the
  color of the base materials can be used if
  appearance is important.

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Possible problems

• A brazing operation may cause defects in the base
  metal, especially if it is in stress. This can be
  due either to the material not being properly
  annealed before brazing, or to thermal expansion
  stress during heating.
• An example of this is the silver brazing of
  copper-nickel alloys, where even moderate stress
  in the base material causes intergranular
  penetration by molten filler material during
  brazing, resulting in cracking at the joint.
• Any flux residues left after brazing must be
  thoroughly removed otherwise, severe corrosion
  may eventually occur.

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Brazing processes

• Block Brazing
• Diffusion Brazing
• Dip Brazing
• Exothermic Brazing
• Flow Brazing
• Furnace Brazing
• Induction Brazing
• Infrared Brazing
• Resistance Brazing
• Torch Brazing
• Twin Carbon Arc Brazing
• Vacuum Brazing

33
A rivet is a permanent mechanical fastener.
Before being installed a rivet consists of a
smooth cylindrical shaft with a head on one end.
The end opposite the head is called the
buck-tail. On installation the rivet is placed in
a punched or pre-drilled hole, and the tail is
upset, or bucked (i.e. deformed), so that it
expands to about 1.5 times the original shaft
diameter, holding the rivet in place.
To distinguish between the two ends of the rivet,
the original head is called the factory head and
the deformed end is called the shop head or
buck-tail. Because there is effectively a head on
each end of an installed rivet, it can support
tension loads (loads parallel to the axis of the
shaft) however, it is much more capable of
supporting shear loads (loads perpendicular to
the axis of the shaft). Bolts and screws are
better suited for tension applications. Fastenings
used in traditional wooden boat building, like
copper nails and clinch bolts, work on the same
principle as the rivet but were in use long
before the term rivet came about and, where they
are remembered, are usually classified among the
nails and bolts respectively.
34
Riveting
35
Howrah bridge, links the city of Howrah to its
twin city, Kolkata (Calcutta). On 14 June 1965 it
was renamed Rabindra Setu, after Rabindranath
Tagore the first Indian Nobel laureate. However
it is still popularly known as the Howrah Bridge.
This bridge is one of the finest cantilever
bridges in the world - a gift to India from the
Purulian engineers.
36
The bridge is 705 metres long and 30 metres wide.
More than 26,500 MT of high-tensile steel went
into this unique bridge supported by two piers,
each nearly 90 meters above the road. An
engineering marvel, it expands as much as a metre
during a summer day. This is constructed entirely
by riveting, without nuts or bolts
37
Riveted Truss over Orange River. This river is
the longest river in South Africa. It rises in
the Drakensberg mountains in Lesotho, flowing
westwards through South Africa to the Atlantic
Ocean. The river forms part of the international
borders between South Africa and Namibia and
between South Africa and Lesotho, as well as
several provincial borders within South Africa.
38
Riveted Buffer beam on a Locomotive
39
Manual installation of a rivet