Introductory
Soldering Techniques
for
Electronics
Prepared by Mike Crompton (Rev. 19 Jan 2005)
Soldering Techniques for Electronics.
Soldering is a method for making reliable electrical connections.
Good soldering is better than most other mechanical connections (screws, push on
connectors, crimp connectors, nuts, bolts etc ) because:
a) Vibration or mechanical shock does not loosen it.
b) It prevents oxidation and corrosion of the soldered surfaces.
c) It produces a good electrical connection between the soldered objects.
Solder is an alloy of tin (Si) and lead (Pb). Tin melts at 232 C, lead melts at 327 C, and
surprisingly solder melts at around 189 C, a lower temperature than lead or tin.
When solder melts - it goes through 3 stages:
1. Solid, below melting temperature.
2. Plastic, where it begins to melt but is not
completely liquid or solid.
3. Liquid, above melting temperature. As the
solder cools it moves through the three
stages again in reverse order: liquid plastic - solid.
Solder that is 60% tin and 40% lead (60/40
solder) is solid below 183 C, plastic from
183 C to 191 C and liquid above 191 C.
This makes it ideal for hand soldering,
particularly if any components have to be
removed or reworked.
A special case solder is ‘Eutectic’ or Sn-63 solder. Made from 63% tin and 37% lead, it
has no plastic range and goes almost instantly from solid to liquid and vice-versa. This
solder is ideal for ‘flow’ or ‘wave’ soldering machines as it goes to a solid as soon as the
printed circuit board (PCB) leaves the ‘wave’ of solder. This eliminates fractured joints
caused by components moving during the cooling or plastic period. Eutectic solder makes
reworking by hand difficult. It solidifies too quickly allowing no time for component
removal. This leads to overheating damage due to multiple attempts at removal.
SOLDER 1S NOT A GLUE!
When hot liquid solder contacts copper it
penetrates the copper surface creating a new alloy
of solder and copper. This action (forming the new
alloy) is called the ‘wetting action’. It is this
wetting action that creates the bond that forms the
good connection between the solder and the copper.
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THE WETTING ACTION
Molten solder can only penetrate the copper if the surface is clean. Surface dirt, oil, finger
prints, water etc must be removed before soldering. Solvents such as isopropyl alcohol,
trichloroethane, freon and acetone are often used as cleaning agents. Most solvents are
volatile, toxic and flammable making them a hazard. Always take precautions when using
them.
1)
2)
3)
4)
DO NOT breathe the fumes.
DO NOT smoke when using the chemicals.
KEEP them away from your skin.
DO NOT use near open flame.
Even when cleaned with solvents a thin film of oxide remains or will reform very rapidly.
This film is removed, usually during the soldering operation, by ‘Flux’ that is contained
in the solder. In automated soldering (wave or flow soldering) the flux is applied just
before the wave.
Flux is a liquid, paste or powder containing ‘Resin’ Flux cored solder
(made from the sap of evergreen trees). When it is
heated it becomes slightly acid and removes any oxide
or corrosion film from the copper. Once it cools the
acidity disappears preventing the objects from being
‘eaten away’ with time. Most electronic solder has flux
in the centre or core and is known as flux cored solder. flux cores
Normal and low deposit fluxes are available and this
refers to the amount of flux residue left after the
soldering operation is complete. Removing flux residue
is sometimes difficult but essential for high quality Printed Circuit Boards (PCBs). To
this end special water soluble fluxes have been developed, and completed PCBs are put
through a wash and rinse cycle in a dishwasher to ensure complete removal of residues.
WORK CONSIDERATIONS
Several conditions effect good solder connections. The most obvious are:
a) Relative size (mass) of the components being soldered. Obviously this mass varies
with the size of the terminal, pad, component or the size and number of wires. The
larger the mass the larger the soldering iron tip required.
b) Surface Condition. The soldering surfaces MUST be clean. Remove heavy dirt
with a small wire brush, cloth or mild abrasive (e.g. abrasive stick, ink eraser).
Remove oils with solvents.
c) Thermal Linkage created by the contact area between the work and the soldering
iron tip. This linkage can be improved by adding a small amount of solder between
the work and the iron tip.
Apply a small amount of solder
to increase thermal linkage
from iron to component
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d) Applying the iron to the point of maximum
mass and allowing the heat to penetrate all the
parts to be soldered.
USING THE IRON
Match the tip to the work, tip too small = too slow heating, tip too large = too fast heating
i.e. no time to work properly without damaging (overheating) components and PCB. Use
a tip that allows a 2 - 4 second work time.
The heat moves from the tip to the work. A small contact area = small heat flow, large
contact area = large heat flow. We can increase the contact area and the heat flow by
putting a little solder between the tip and the part as shown above (Thermal Linkage).
If ready to use the iron, quickly clean the tip on a damp (not wet) sponge and then ensure
the tip is “tinned” by applying a small amount of solder.
When soldering melt the solder on the wires or component leads, not on the iron.
Follow the steps below to create a good soldered connection:
1. Place the tip on the largest mass.
2. Add a small amount of solder (a bridge) to the part/tip contact area and heat the
connection. Allow sufficient time for the heat to penetrate all parts, but not long
enough to overheat any component.
3. Touch the solder to the connection at a point farthest from the tip. NOT to the
iron.
4. Remove the solder and the iron together.
1.
2.
3.
4.
5. Allow the joint to cool without disturbing (moving) any part.
6. Replace the iron in its holder.
The whole operation should take approximately 3 to 4 seconds. Use a light touch and do
NOT press hard with the iron.
WIRE CONNECTIONS
Before wire is soldered, it is prepared by stripping off some insulation and tinning the
exposed wire. This prepares it for a good solder connection. Strip the required amount of
insulation off the wire with a good quality wire stripping tool. The tool should be
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adjusted so that the strands of wire are not damaged or cut. Gently re-twist the strands if
they have splayed out during the stripping. Do not over twist them.
Tinning Procedure
a) Heat the iron and put it on a holder or on the work surface with the tip up.
b) Hold the wire in position against the tip.
c) Allow the wire to heat for a short period taking care NOT to melt the insulation.
d) Touch the solder to the wire and move it across the tip, add more solder if required.
To make a loop after tinning, - use the round nose pliers
and cut off the excess wire. The three typical loops are
180°, 270° and 360°.
Place the loop on the terminal and squeeze it closed for a
good mechanical contact, then solder. Apply the iron to
point of maximum mass. Make a solder bridge to
increase thermal linkage, and apply solder to the side
opposite the iron. Remove the iron tip with a wiping
motion.
Characteristics of a Good Connection
Inspect the finished connection. A good connection
should:
a) Be a bright silver (like a mirror)
b) Have minimum solder - the strands are still
visible
c) Have feathered edges
d) Have a smooth concave fillet
e) Have no holes, pockets or peaks
f) Have no flux between the work and the solder
g) Have no extra wire extending from the
connection
h) Have a minimum of 1 wire diameter and a
maximum of 2 wire diameters space between
the insulation and the connection.
TERMINAL CONNECTIONS
There are 5 basic terminals. They are - turret, bifurcated, perforated (or eye), hook and
cup terminals.
Turret Terminals
Turret terminals look like a post with two locations for
soldering connections. Usually wire connections are
made at the base and component connections are made
at the top. Connections to turret terminals always
come in from the side. Turret terminals are soldered
from the base up. The first connection is at the bottom,
the next is above that and so on. The insulation of the
wires should touch when there is more than l wire on
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the same section. The space between the connection and the insulation should be
approximately 1 to 2 diameters of the wire. All connections should come in from the
same direction.
Bifurcated Terminals
Bifurcated terminals can have 3 types of entry : from the top, from the bottom and from
the side. The top entry is not very typical.
Side entry - the wire comes in from the side and is wrapped around 1 fork or tang of the
terminal for 90 or 180 (2 - 90 bends). The wire must touch the terminal completely no space between the terminal and the wire.
When two wires come in from the side the hooks must go to the opposite sides. The wire
insulation should touch. The ends should not protrude beyond the edge of the terminal.
Typical side entry bifurcated
terminal wire positions
Top entry - the wire comes in from the
top. The wire end is folded and inserted
between the forks. The connection is
soldered the full length of the fork. The
wire should contact the fork for the full
length. (This entry is not very common).
Bottom entry - the wire passes through
the terminal from the bottom. The wire is wrapped round one fork of the terminal and it
contacts two sides of the fork. When two wires are connected they should connect on
opposite forks.
Perforated Terminals
The perforated terminal is usually
connected from the top, but side
entry is permitted. Use a 90 or
180 hook and do NOT fill the
eye with solder.
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Hook Terminals
Connect the hook terminal the same
way as the perforated terminal. Use a
180, 270 or 360 loop. Make sure
there is sufficient insulation gap.
Cup/Pot Terminals
The cup/pot terminal requires a different technique to the other types of terminals.
First put a twist of solder into the cup. The twist should not protrude above the terminal
cup. The wire should be tinned. Heat the terminal until the solder melts, quickly insert the
tinned end of the wire and then remove the iron. The wire should be vertical in the cup
and should touch the bottom and the back of the cup.
COMPONENT TO TERMINAL CONNECTIONS
Component leads are usually solid wire and are
pre-tinned during manufacture. Putting strain
relief bends in the leads protects the components
from both vibration and thermal expansion or
contraction damage.
The leads must always remain straight as they
leave the body of the component, so stress relief
bends should start no closer than 1/16" (2mm)
from the component body. The leads should
connect to the terminals with a 180/270 loop. All
horizontally mounted components should touch
the board. The exceptions to this rule are glass
components which should be mounted 1/32" (l
mm) above the board surface.
Use a heat sink (a metallic clip with a fairly large
mass between the component body and the
soldering iron) on heat sensitive components.
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PRINTED CIRCUIT BOARDS
Printed circuit boards are made of plastic, fibrous or fibreglass materials. There are three
main types of board, single sided, double sided and multi layer. Tracks are the conductors
that run between the pads or eyelets to which the components are soldered. The pads and
eyelets on double sided and multi layered boards have plated through holes that connect
the two (or more) sides/layers together as well as accommodate the component leads.
Single sided
board
Double sided board showing
plated through hole
Multi layered
Board
Installing Components.
Components must have stress relief bends and most of them should have the leads bent or
clinched on the back (solder) side of the board. The components must be mounted from
the top (component) side of the board. Correct polarity must be observed on polarized
items. Always clip leads to their correct length before soldering.
Install colour coded components so they can be read them from left to right or from top to
bottom, and components with printed information so the information can be read after
installation. Obviously polarity takes precedence over readability.
All horizontally mounted components should touch the board, except glass components
which should have the required space (1 mm) between the component and the board.
COMPONENT TO PCB CONNECTIONS
There are 6 basic types of connections:
a) Straight Through. This is the simplest connection. The lead or wire is inserted
through the board, trimmed and soldered from the bottom (solder) side of the
board - never the component side. The solder should flow through the hole to
the component side and form a fillet around the lead. On the solder side the lead
should protrude approximately 1mm and be covered with solder but the outline
should still be visible.
b) Bent or Semi clinched. The lead is inserted through the board and bent over
slightly. The lead is cut close to the board and then soldered. The bent lead should
not be more than 2 mm above the board. Whenever possible bend the lead in the
direction of the track. The soldering requirements from a) above apply.
c) Fully Clinched. The lead is inserted through the board and bent over
completely. It is cut to a maximum length equal to the radius of the pad, and a
minimum of ½ the radius. When possible the lead should contact the board in the
direction of the track. The soldering requirements from a) above apply.
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d) Spaded. The lead is inserted through the board and is swaged (pressed flat).
This holds the component to the board so that it doesn't move when it is
soldered. Usually a special tool is required. The soldering requirements from a)
above apply.
e) Surface Mounting. The component lead, pre-tinned, rests on a pad which
has also been pre-tinned. To solder - hold the lead against the pad and apply
light pressure. Touch the top of the lead with the iron until the solder on the
pad melts, slide the iron along the lead and remove. Release the pressure
when the joint is cooled.
f) Offset Pad Mounting. The lead is passed through the hole and bent back for
a surface mount. This is often used when a pad is offset from the mounting
hole. Bend the lead in the direction of the track or pad and use the same
soldering requirements as the surface mount in e) above.
General rules/standards for mounting and soldering components are
shown in pictorial form on page 11. The key to each component’s
non-conformance is on page 12. At right is a cross section of a
typical component to PCB solder joint.
MOUNTING AND SOLDERING I.C. PACKAGES
Dual In-line Packages (DIPs)
DIPs are very sensitive to heat and also have a number of leads
(legs) that are close together. This means a large amount of heat can
be concentrated in one area. The technique used for soldering can
reduce the effect of heat, ensuring a functioning I.C. when mounted.
One technique follows:
1. Carefully insert the DIP into the PCB taking care not to bend
any legs under the body of the chip.
2. Ensure the DIP is level and then clinch two diagonally opposite leads to hold the
chip in place.
3. Trim the remaining leads to approximately 1 mm above the board.
4. Solder one leg, ensuring the solder flows through to the top of the board and forms
a fillet.
5. Solder another leg at the opposite end and side from the first.
6. Go back and solder the leg next to the one you soldered first.
7. Alternate ends/sides until all legs are soldered.
The usual soldering standards apply. It is often preferred to solder a chip holder to the
PCB. These holders are not heat sensitive and are usually more robust than a chip. They
allow you to ‘plug in’ the chip and make chip replacement very simple.
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Can type ICs and single Transistors
Transistor and can type IC's are mounted in a different manner to DIPs. They can be
mounted with or without an insulating spacer between them and the PCB. The leads
should be spread outward slightly, stress relief bends are optional if no spacer is used, but
mandatory if a spacer is used. Take care to ensure that all leads (legs) are correctly
located in the mounting holes. Do not force the component too close to the board as this
will stress the legs. Clinch the leads making sure the can is upright. Heat damage can be
drastically reduced by once again soldering alternate legs.
Vertical Mounting (Axial lead Components – a lead at each end of the component)
It is often the case that axial lead components are mounted vertically instead of
horizontally. This can be to reduce the overall size of a PCB or because the
mounting holes are too close together to accommodate the component. Vertical
components should never touch the board or have insulation material in the
mounting hole. They should have sufficient space (approximately 2-4 mm) between
themselves and the board to allow a heat sink to be used. This space will also
provide mandatory stress relief when the lead/component expands and contracts due
to heat/cold and allow access to the lead if removing the component.
During forming the upper lead must remain straight for several mm as it exits the
component body. This prevents stress to the internal connection of the component.
Colour codes should read from top to bottom, and the component should be vertical
when finally soldered. All normal soldering requirements apply.
Vertical mounting of axial components
Approx
2-4 mm
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KEY TO PCB RESISTOR MOUNTING DIAGRAM
Examples A, B, C and D are all correct.
Example E: Component is mounted at an angle and is therefore NOT touching the
board. All horizontal components, except glass bodied, should be mounted flush to the
board unless specified otherwise by Drawing or Engineering Change Order. (ECO). See
example A for correct mounting.
Example F: Leads are cut too short and do not protrude through the pad/eyelet on the
solder side of the PCB. All non-clinched leads must protrude 1 to 3 mm. See example B.
Example G: The distance between pads/eyelet’s is insufficient to accommodate the
component resulting in the leads being formed too close to the body of the resistor. This
can cause damage to the internal connection of the lead, and does not give sufficient
stress relief. See example C for correct lead forming.
Example H: The colour coding is at the bottom making it difficult to see if components
are close together. Although this does not present any physical problem, it does not
improve ‘inspectability’ if colour codes are not in the same orientation. See example D.
Example I: Component is not touching the board. See example A.
Example J: Leads are clipped too long and can be accidentally bent over to produce a
short circuit to adjacent pads/tracks. See example B.
Example K: Leads poorly formed and show evidence of multiple bends. Strong
possibility that the leads have suffered metal fatigue and will fracture. See example A.
Example L: Component is offset from center giving insufficient room to correctly form
the lead on the closer side. Possibility of internal stress/damage as in example G. See
example A.
Example M: Leads clipped too long and will probably produce a short circuit or solder
bridge to adjacent pads/tracks. Clinched leads should be clipped off at a point between
half and full radius of the pad and should not ‘overhang’. See example A.
Example N: Component not vertical, and upper lead is formed too close to body of
resistor. Possibility of internal stress/damage as in example G. See example D.
Example O: Component mounted flush to board giving zero stress relief. All vertical
components should be mounted proud of the board by sufficient distance to allow for a
heat sink and for expansion/contraction stress relief, unless specified otherwise by
drawing or ECO. On temperature induced contraction, the lead will ‘pull out’ of the
component. Also, the colour code is at the bottom. See example D.
Example P: Leads are ‘pulled tight’ giving no stress relief resulting in same
temperature induced problem as example O above. See example A.
Example Q: Everything is correct except that the colour coding is reversed. Although
this does not present any physical problem, it does not improve ‘inspectability’ if colour
codes are not in the same orientation. See example A.
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SURFACE MOUNT PCBs
Many of today’s PCBs are made up of completely surface mounted components. These
are extremely small and can reduce the size of a PCB or increase the ‘population’ of the
PCB by a considerable amount. The pictures below show the size of some components
(resistors at left, capacitors middle and transistors at right) compared to a typical ½ watt
through mount resistor.
As the name implies, surface mount components are not mounted in ‘through holes’ but
soldered directly to pads on the surface of the board. Their size and accuracy of
placement require a completely different technique to the through mount technology.
Most commercially produced
surface mount PCBs use
special ‘pick and place’
computer controlled robotic
machines to choose the right
components and place them in
the correct position on the
board. Prior to placement a
special paste (a combination of
flux/solder/glue) has been
applied to the mounting pads
or to the mounting surface of
the component. This ensures
the components remain in place until soldered.
The actual soldering process can also differ greatly from through mount soldering. Hot
air is often used to heat the area of the mounting pad until the solder melts and forms a
fillet with the pre-tinned component mounting area.
Amateur or hobby surface mount production can be accomplished by far less
sophisticated methods. The solder/flux paste can be applied by a hypodermic syringe,
components can be placed using tweezers and soldering can actually be accomplished
with a small tipped soldering iron or even a toaster oven. (Yes, a toaster oven).
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STATIC ELECTRICITY
Many components require special protection against static electricity. During Winter
months or in dry environments static charges of 40,000 Volts are not unusual. These
voltages can easily damage static sensitive components, and although many components
have internal protective devices built in they still require special precautions. Anti-static
lab coats and mats, special anti-static floors, wrist straps, anti-static packaging and leather
soled shoes are some of the precautionary measures taken to prevent static damage. If
you work at a static protected work station, here are some of the procedures you must
follow:
1)
2)
3)
ALWAYS wear a wrist strap.
NEVER wear the wrist strap over clothing - be sure it is on the skin.
ALWAYS ‘discharge’ your hands before you begin work by placing them on
the anti-static work table mat.
4) ALWAYS wear leather soled shoes.
5) ALWAYS button up your anti-static lab coat.
6) NEVER slide components on any surface.
7) NEVER have plastic, styro-foam cups or any unnecessary objects on your
work bench.
8) NEVER comb your hair at the work station.
9) ALWAYS pick up devices only by the body – try not to touch the leads.
10) ALWAYS transport static sensitive components in static shielding packages,
containers or bags that protect the device from static electricity.
11) ALWAYS keep other components, assemblies and work sheets in their
packages as much as possible.
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Test your knowledge of solder/soldering
Answer True (T) or False (F)
1.
2.
3.
4.
Solder works like a glue.
Solder is an alloy of tin and lead.
Wetting action creates a new alloy.
The "plastic" range is the temperature range between solder starting to melt
and becoming completely liquid.
5. Moving a joint when it is in the plastic range is acceptable.
6. Eutectic solder has a very small plastic range.
7. If a copper surface looks clean it is ready for soldering.
8. Flux removes the oxides.
9. One size soldering iron is good for all jobs.
10. A solder bridge is extra flux.
11. A large mass needs a small tip.
12. Do not tin a soldering iron.
13. You should clean the work with solvent after soldering.
14. "60/40" solder is 60% lead and 40% tin.
15. When soldering the connection, put the solder on the iron tip.
Answer the questions with complete sentences.
1.
2.
3.
4.
5.
6.
What is flux?
What does flux do?
What is flux cored solder?
Why is it important when hand soldering to use solder with a plastic range?
What determines the size or wattage of soldering iron to be used?
When soldering, where do you put the soldering iron tip and where do you put
the solder?
7. What is the ideal time to hold the iron on a connection?
8. What chemicals can you use for cleaning solder joints?
9. What is ‘tinning’?
10. What does a solder bridge do?
Prepared by Mike Crompton
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