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
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.
Soldering and Brazing Benefits
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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
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.
• 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.
• 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.
• 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.
• 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.
Common joining problems and
discontinuities:
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No wetting
Excessive wetting
Flux entrapment
Lack of fill (voids, porosity)
Unsatisfactory surface appearance
Base metal erosion
• 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.
• 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.
• 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.
• 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.
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.
• 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. Tinfinishes is the most popular of lead-free finishes.
However, this poses nevertheless the question of tinwhiskers. 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.
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°-376°F
• 50/50: melts between 368°-421°F
• 63/37: melts between 355°-365°F
• lead-free solder (useful in jewelry, eating containers, and
other environmental uses): melts around 490°F
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.
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
Soldering processes
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Wave soldering
Reflow soldering
Infrared soldering
Induction soldering
Ultrasonic soldering
Dip soldering
Furnace soldering
Iron soldering
Resistance soldering
Torch soldering
Silver soldering/Brazing
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.
Brazing
• Brazing is a joining process whereby a non-ferrous filler
metal and an alloy are heated to melting temperature
(above 450°C;) 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.
• In Braze Welding or Fillet Brazing, a bead of
filler material reinforces the joint. A brazewelded 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• Vacuum brazing is another materials joining technique,
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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.
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.
• 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.
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.
Brazing processes
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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
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.
Riveting
This bridge is one of the
finest cantilever bridges in
the world - a gift to India
from the Purulian
engineers.
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.
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
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.
Riveted Buffer beam on a Locomotive
Manual installation of a rivet