What Is Soldering?

Soldering uses a filler metal with a low melting point, also known as solder, to join metal
surfaces. The solder is usually made up of an alloy consisting of tin and lead whose melting
point is around 235°C and 350°C, respectively.

But when tin and lead are mixed then the melting point of the mixture is reduced to 183°C.
The alloy is melted by using a hot iron at above 316 °C (600 °F).

As the solder cools, it creates a strong electrical and mechanical bond between the metal
surfaces. The bond allows the metal parts to achieve electrical contact while it is held in place.

Note that lead-free solders are increasingly used as an alternative to environmentally harmful
lead-based solders due to regulations.

The first step in soldering is to wear protective gear in a well-ventilated area. Next, the
soldering iron should be preheated. For cleaning the soldering tip, you can use a wet sponge.
Likewise, any residue on the workpiece surface should be wiped off.

After finishing the preparations, it’s time to heat the base metal to a working temperature
using the hot iron. Doing so will help prevent thermal shock, activate the solder, and overall
improve the quality of the joint. A good indicator that the metals are well-heated is when the
molten solder freely flows into the joint. The filler material solidifies as it cools down, making
it the best time for inspection.

The key to successful soldering is ensuring that the metals being joined are clean and free of
any oxides or other contaminants.

Desoldering

From time to time, components fail and need replacement. When these components are
mechanically held in place with solder, a process called desoldering removes the material
cleanly and safely.

A soldering iron or a heat gun can be utilised to melt the solder, allowing you to safely
remove any soldered components. To remove the liquid solder, you can use a desoldering
pump as a vacuum, or a soldering wick to absorb the molten solder.

Alternatively, you can resort to an aggressive method using compressed air that can blow off
the liquid solder.

Soldering Tools

Soldering irons are hand tools that heat the solder above its melting temperatures. They
offer a wide variety of sizes, which is great for different applications. The tip of the iron has
different types and sizes that suit a variety of projects.

Soldering guns are employed when higher temperatures require more power. A soldering
gun heats quicker and offers better flexibility as it can be operated in confined spaces, heavy
electrical connections, and metalworks.
 Soldering stations are multipurpose devices that have everything covered for minor
projects. They are more durable than regular soldering irons due to them being equipped with
sensors, fuses, alerts and temperature regulation.

Solders

Lead-based solder

Most soldering projects are typically performed using lead solder consisting of a 60-40 tin-to-
lead ratio. This solder melts in a range of 180 to 190°C and is usually the best choice for
soldering electrical connections.

Lead-free solder

As a way to mitigate the use of harmful elements, lead-free solders were developed. These
usually come as solder wire and are composed of metals with higher melting points: tin,
copper, bismuth, silver, brass, indium, and antimony.

Flux core solder

These filler metals come as paste or soldering wires that contain a flux solder core. The flux
releases a protective layer around the workpiece as it is consumed, which achieves cleaner
electronic connections and better wetting properties.

Flux

Rosin flux (alternatively called passive flux) is used for electronics as it leaves a residue that
doesn’t lead to corrosion.

Acid flux solders contain aggressive properties, which are effective in removing the oxides of
the metal surface. This leads to stronger and cleaner metal joints compared to rosin.

The type of flux can be broken down into two groups depending on its application. No-clean
flux is made with natural rosin or other synthetic materials, requiring no post-cleanup,
while water-soluble flux contains water-soluble resin that is easily removed by rinsing.

Advantages of Soldering
1. Soldering is operated at lower temperatures compared to common welding methods.
2. Most metals and non-metals can be soldered.
3. A simple process makes it easy to learn.
4. The base metal isn’t melted in the process, unlike welding techniques such as stick
welding, flux-cored welding, etc.
5. Soft soldering can be undone using a desoldering tool without damaging the base materials.
Disadvantages of Soldering
1. Weaker joints compared to other welding methods such as MIG and TIG.
2. Soldering isn’t suitable at high temperatures, as the solder has a low melting point.
3. Heavy metals aren’t suitable for soldering.
4. Melted solder might leave a toxic flux residue.
5. Improper heating may cause deformities or voids in the solder.

What Is Brazing?
Brazing joins metal surfaces together with a filler metal which has a low melting point. The
process uses capillary action wherein the homogenous liquid flow of the filler material bonds
with the base metals.

A unique quality in the brazing process is that it keeps the mechanical properties of the metals
which are useful in applications such as silver brazing or other similar metals.

Brazing Process

One of the most crucial steps in the metal joining process is the cleaning of the base metal
surfaces. Emery cloth or wire brush are both great tools to remove contaminants.

Having calculated joint gaps for the liquid filler metal to achieve surface tension with the
workpiece, the brazing operation begins with properly positioning the assembly. A torch is
normally used to slowly heat the workpiece’s metal surface and filler metal into its brazing
temperature.

As the filler metal liquefies, capillary action lets it pass through the tight spaces, thus forming
a bond between the surface of the base metals.

The brazed joints are formed as it cools down with the assembly.

Filler metal requirements

1. Once the molten flux and filler metal solidify, the brazed joint should possess the
expected mechanical properties.
2. Brazing temperatures must efficiently achieve a proper liquid flow from the molten braze
alloy into the joints.
3. Filler metals must achieve proper wetting conditions in order to create strong bonds.
Different Methods of Brazing
Brazing uses different heating methods to suit a variety of purposes and applications. Heat
can either be applied directly to a joint (localised) or to the whole workpiece (diffuse
heating).

Localised Heating Techniques

Torch brazing – Combusted fuel gas is formed by burning acetylene, propane, or hydrogen
with oxygen to heat and melt the filler metal. Flux is required while using this technique in
order to protect the joint, which requires post-cleanup later on. Torch brazing is mainly used
for small production assemblies where metal weight is unequal. The process is often
performed with gas welding equipment.

Induction brazing – High-frequency alternating current is supplied into a coil to achieve

brazing temperature, which heats the workpiece and melts the filler material.
 Resistance brazing – Heat is generated from the electrical resistance of the brazing alloy,
which is perfect for highly conductive metals. This heating technique is best suited for creating
simple joints between metals.

Diffuse Heating Techniques

Furnace brazing – Gas firing or heating elements are used to bring the furnace to the desired
temperature. The brazing filler metal is applied to the surfaces to be joined and then the entire
assembly is placed into the furnace and brought to brazing temperature. Furnace brazing
allows accuracy in controlling the heating and cooling cycles of the metals. The process is often
performed in a vacuum to protect the braze alloy from atmospheric conditions. This also
negates the need for flux protection.

Dip brazing – The workpiece or assembly is immersed in a bath of molten filler metal
(molten metal bath brazing) or molten salt (chemical bath dip brazing). Brazing flux is
applied to the parts to prevent oxidation. The assembly can be removed once the molten
brazing filler metal has solidified.

Advantages
1. Can join dissimilar metals, unlike most welding methods.
2. High production rates.
3. Consumes less power than welding.
4. Produces cleaner joints compared to most welding processes.
5. Base metals don’t melt, keeping their shape and mechanical properties.
Disadvantages
1. Weaker results compared to welded joints.
2. Cannot join components operated at high temperatures.
3. Requires tight, uniform joint gaps to achieve capillary action.
4. Unclean or contaminated metals may cause leaky joints.

Difference between Brazing and Soldering

The following table compares between brazing and soldering processes.

Aspect Brazing Soldering

Temperature Higher temperature (above 450°C) Lower temperature (below 450°C)
Filler Material Filler metal with a high melting point Solder with a lower melting point
Joint Strength Stronger joints Weaker joints
Skill Required Skilled labour is often required Requires less skill
Disassembly Joints are not easily dismantled Joints can be easily dismantled
Cost Generally more expensive Economical
Applications Used for heavier or thicker parts Typically for smaller joints