Soldering Equipment
How To Solder - Soldering Tutorial
Soldering is defined as "the joining of metals by a fusion of alloys which have relatively low melting
points". In other words, you use a metal that has a low melting point to adhere the surfaces to be
soldered together. Consider that soldering is more like gluing with molten metal, unlike welding
where the base metals are actually melted and combined. Soldering is also a must have skill for all
sorts of electrical and electronics work. It is also a skill that must be taught correctly and developed
with practice. This tutorial will cover the most common types of soldering required for electronics
work. This includes soldering components to printed circuit boards and soldering a spliced wire
joint. This presentation came from
Soldering Equipment
The Soldering Iron/Gun
The first thing you will need is a soldering iron, which is the heat source used to melt solder.
Irons of the 15W to 30W range are good for most electronics/printed circuit board work.
Anything higher in wattage and you risk damaging either the component or the board. If you
intend to solder heavy components and thick wire, then you will want to invest in an iron of
higher wattage (40W and above) or one of the large soldering guns. The main difference
between an iron and a gun is that an iron is pencil shaped and designed with a pinpoint heat
source for precise work, while a gun is in a familiar gun shape with a large high wattage tip
heated by flowing electrical current directly through it.
A 30W Watt Soldering Iron
A 300W Soldering Gun
For hobbyist electronics use, a soldering iron is generally the tool of choice as its small tip
and low heat capacity is suited for printed circuit board work (such as assembling kits). A
soldering gun is generally used in heavy duty soldering such as joining heavy gauge wires,
soldering brackets to a chassis or stained glass work.
You should choose a soldering iron with a 3-pronged grounding plug. The ground will help
prevent stray voltage from collecting at the soldering tip and potentially damaging sensitive
(such as CMOS) components. By their nature, soldering guns are quite "dirty" in this respect
as the heat is generated by shorting a current (often AC) through the tip made of formed
wire. Guns will have much less use in hobbyist electronics so if you have only one tool
choice, an iron is what you want. For a beginner, a 15W to 30W range is the best but be
aware that at the 15W end of that range, you may not have enough power to join wires or
larger components. As your skill increases, a 40W iron is an excellent choice as it has the
capacity for slightly larger jobs and makes joints very quickly. Be aware that it is often best
to use a more powerful iron so that you don't need to spend a lot of time heating the joint,
which can damage components.
A variation of the basic gun or iron is the soldering
station, where the soldering instrument is attached to a
variable power supply. A soldering station can
precisely control the temperature of the soldering tip
unlike a standard gun or iron where the tip temperature
will increase when idle and decrease when applying
heat to a joint. However, the price of a soldering station
is often ten to one hundred times the cost of a basic
iron and thus really isn't an option for the hobby
market. But if you plan to do very precise work, such as
surface mount, or spend 8 hours a day behind a soldering iron, then you should consider a
soldering station.
The rest of this document will assume that you are using a soldering iron as that is what the
majority of electronics work requires. The techniques for using a soldering gun are basically
the same with the only difference being that heat is only generated when the trigger is
The choice of solder is also important. There several kinds of solder available but only a few
are suitable for electronics work. Most importantly, you will only use rosin core solder. Acid
core solder is common in hardware stores and home improvement stores, but meant for
soldering copper plumbing pipes and not electronic circuits. If acid core solder is used on
electronics, the acid will destroy the traces on the printed circuit board and erode the
component leads. It can also form a conductive layer leading
to shorts.
For most printed circuit board work, a solder with a diameter
of 0.75MM to 1.0MM is desirable. Thicker solder may be used
and will allow you to solder larger joints more quickly, but will
make soldering small joints difficult and increase the likelihood
of creating solder bridges between closely spaced PCB pads.
An alloy of 60/40 (60% tin, 40% lead) is used for most
electronics work. These days, several lead-free solders are
available as well. Kester "44" Rosin Core solder has been a
staple of electronics for many years and continues to be
available. It is available in several diameters and has a noncorrosive flux.
Large joints, such as soldering a bracket to a chassis using a high wattage soldering gun,
will require a separate application of brush on flux and a thick diameter solder of several
millimeters. Remember that when soldering, the flux in the solder will release fumes as it is
heated. These fumes are harmful to your eyes and lungs. Therefore, always work in a wellventilated area and avoid breathing the smoke created. Hot solder is also dangerous. It is
surprisingly easy to splash hot solder onto yourself, which is a thoroughly unpleasant
experience. Eye protection is also advised.
Preparing To Solder
Tinning the Soldering Tip
Before use, a new soldering tip, or one that is very dirty, must be tinned. "Tinning" is the
process of coating a soldering tip with a thin coat of solder. This aids in heat transfer
between the tip and the component you are soldering, and also gives the solder a base from
which to flow from.
Step 1: Warm Up the Iron
Warm up the soldering iron or gun thoroughly. Make sure that it has fully come to
temperature because you are about to melt a lot of solder on it. This is especially important
if the iron is new because it may have been packed with some kind of coating to prevent
Step 2: Prepare A Little Space
While the soldering iron is warming up, prepare a little space to work. Moisten a little sponge
and place it in the base of your soldering iron stand or in a dish close by. Lay down a piece
of cardboard in case you drip solder (you probably will) and make sure you have room to
work comfortably.
Step 3: Thoroughly Coat the Tip in Solder
Thoroughly coat the soldering tip in solder. It is very important to cover the entire tip. You
will use a considerable amount of solder during this process and it will drip, so be ready. If
you leave any part of the tip uncovered it will tend to collect flux residue and will not conduct
heat very well, so run the solder up and down the tip and completely around it to totally
cover it in molten solder.
Step 4: Clean the Soldering Tip
After you are certain that the tip is totally coated in solder, wipe the tip off on the wet sponge
to remove all the flux residue. Do this immediately so there is no time for the flux to dry out
and solidify.
Step 5: You're Done!
You have just tinned your soldering tip. This must be done anytime you replace the tip or
clean it so that the iron maintains good heat transfer.
Soldering a Printed Circuit Board (PCB)
Soldering a PCB is probably the most common soldering task an electronics hobbyist will perform.
The basic techniques are fairly easy to grasp but it is a skill that will take a little practice to master.
The best way to practice is to buy a simple electronics kit or assemble a simple circuit (such as an
LED chaser) on a perf-board. Don't buy that expensive kit or dive into a huge project after only
soldering a few joints.
Soldering components onto a PCB involves preparing the surface, placing the components, and
then soldering the joint.
Step 1: Surface Preparation:
A clean surface is very important if you want a strong, low resistance solder joint. All
surfaces to be soldered should be cleaned well. 3M Scotch Brite pads purchased from the
home improvement, industrial supply store or automotive body shop are a good choice as
they will quickly remove surface tarnish but will not abrade the PCB material. Note that you
will want industrial pads and not the kitchen cleaning pads impregnated with cleaner/soap. If
you have particularly tough deposits on your board, then a fine grade of steel wool is
acceptable but be very cautious on boards with tight tolerances as the fine steel shavings
can lodge between pads and in holes.
Once you have cleaned the board down to shiny copper you can use a solvent such as
acetone to clean any bits of the cleaning pad that may remain and to remove chemical
contamination from the surface of the board. Methyl hydrate is another good solvent and a
bit less stinky then acetone. Be aware that both these solvents can remove ink, so if your
board is silk screened, test the chemicals first before hosing down the entire board.
A few blasts with compressed air will dry out the board and remove any junk that may have
built up in the holes.
Step 2: Component Placement
After the component and board have been cleaned, you are ready to place the components
onto the board. Unless your circuit is simple and only contains a few components, you will
probably not be placing all the components onto the board and soldering them at once.
Most likely you will be soldering a few components at a time before turning the board over
and placing more. In general it is best to start with the smallest and flattest components
(resistors, ICs, signal diodes, etc.) and then work up to the larger components (capacitors,
power transistors, transformers) after the small parts are done. This keeps the board
relatively flat, making it more stable during soldering. It is also best to save sensitive
components (MOSFETs, non-socketed ICs) until the end to lessen the chance of damaging
them during assembly of the rest of the circuit.
Bend the leads as necessary and insert the component through the proper holes on the
board. To hold the part in place while you are soldering, you may want to bend the leads on
the bottom of the board at a 45 degree angle. This works well for parts with long leads such
as resistors. Components with short leads such as IC sockets can be held in place with a
little masking tape or you can bend the leads down to clamp onto the PC board pads.
In the image below, a resistor is ready to solder and is held in place by slightly bent leads.
Step 3: Apply Heat
Apply a very small amount of solder to the tip of the iron. This helps conduct the heat to the
component and board, but it is not the solder that will make up the joint. To heat the joint you
will lay the tip of the iron so that it rests against both the component lead and the board. It is
critical that you heat the lead and the board, otherwise the solder will simply pool and refuse to
stick to the unheated item. The small amount of solder you applied to the tip before heating the
joint will help make contact between the board and the lead. It normally takes a second or two
to get the joint hot enough to solder, but larger components and thicker pads/traces will absorb
more heat and can increase this time.
If you see the area under the pad starting to bubble, stop heating and remove the soldering
iron because you are overheating the pad and it is in danger of lifting. Let it cool, then carefully
heat it again for much less time.
Step 4: Apply Solder to the Joint
Once the component lead and solder pad has heated up, you are ready to apply solder.
Touch the tip of the strand of solder to the component lead and solder pad, but not the tip of
the iron. If everything is hot enough, the solder should flow freely around the lead and pad.
You will see the flux melt liquefy as well, bubble around the joint (this is part of its cleaning
action), flow out and release smoke. Continue to add solder to the joint until the pad is
completely coated and the solder forms a small mound with slightly concave sides. If it
starts to ball up, you have used too much solder or the pad on the board is not hot enough.
Once the surface of the pad is completely coated, you can stop adding solder and remove the
soldering iron (in that order). Don't move the joint for a few seconds as the solder needs time to cool
and solidify. If you do move the joint, you will get what's called a "cold joint". This is recognized by
it's characteristic dull and grainy appearance. Many cold joints can be fixed by reheating and
applying a small amount of solder, then being allowed to cool without being disturbed.
Step 5: Inspect the Joint and Cleanup
Once the joint is made you should inspect it. Check for cold joints (described a little above
and at length below), shorts with adjacent pads or poor flow. If the joint checks out, move on
to the next. To trim the lead, use a small set of side cutters and cut at the top of the solder
After you have made all the solder joints, it is good practice to clean all the excess flux residue from the
board. Some fluxes are hydroscopic (they absorb water) and can slowly absorb enough water to become
slightly conductive. This can be a significant issue in a hostile environment such as an automotive
application. Most fluxes will clean up easily using methyl hydrate and a rag but some will require a stronger
solvent. Use the appropriate solvent to remove the flux, then blow the board dry with compressed air.
Cold Solder Joints
A "cold solder joint" can occur when not enough heat is applied to the component, board, or both.
Another common cause is a component moving before the solder has completely cooled and
solidified. A cold joint is brittle and prone to physical failure. It is also generally a very high
resistance connection which can affect the operation of the circuit or cause it to fail completely.
Cold joints can often be recognized by a characteristic grainy, dull gray color, but this is not always
the case. A cold joint can often appear as a ball of solder sitting on the pad and surrounding the
component lead. Additionally you may notice cracks in the solder and the joint may even move.
Below is the shocking image of every example of a bad solder joint you will ever see. It appears
that this FM transmitter kit was assembled using the technique of "apply solder to iron then drip
onto joint". If your joints are looking like this, then stop and practice after rereading this page. Note
that not a single of of these joints is acceptable, but amazingly, the circuit worked.
Most cold solder joints can be easily fixed. Generally all that is required is to reheat the joint and apply a
little more solder. If there is already too much solder on the joint, then the joint will have to be desoldered
and then soldered again. This is done by first removing the old solder with a desoldering tool or simply by
heating it up and flicking it off with the iron. Once the old solder is off, you can resolder the joint, making
sure to heat it thoroughly and keep it still as it cools.
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