The Soldering Process
Author: Torre Viola
An Overview
Soldering is a process used to make durable and reliable electrical connections in circuits. This
document describes the process for hobby grade soldering which is used to make a prototype
after the circuit has successfully worked on a breadboard. The reason a hobbyist would use a
soldered circuit over one in a breadboard is that soldered circuits are more compact, and
therefore will take up less room on their projects. The soldered circuit can also be roughly
handled without fear of breaking the electrical connection. Someone building an RC Car, for
example, would want to use a soldered circuit so that they do not need to enlarge the car so it
could hold the circuit and do not need to worry about the movement of the car loosening the
electrical connections. The soldering process is fast and simple, involving only three
components.
The Components
There are three main components necessary for soldering: the connection that is being soldered,
the soldering iron, and the solder. The connection is the most important of the three components
because it influences which types of soldering iron and solder the hobbyist will use.
The Connection
The connection is the two or more things being soldered together. In electrical engineering, the
technical term for an electrical connection is a node. The two common types of nodes that are
soldered are pad to pin connections and wire to wire connections. These connections are shown
in Figure 1 below:
Pin to Pad Connection
Wire to Wire Connection1
Figure 1: Types of Soldering Connections
Pad to pin connections are found when soldering pin headers or wires to boards, such as
microcontrollers, prototype boards, or IC chips. Wire to wire connections are found when
soldering two wires together to make a longer wire, soldering a wire to a passive component like
a resistor, or soldering a wire to a pin header. The type of connection being soldered influences
which type of soldering iron, especially which soldering iron tip, should be used.
The Soldering Iron
The soldering iron has a long, skinny metal cylinder with a pointed tip on the end, and a rubber
handle that is held like a pencil. The purpose of the soldering iron is to transfer heat to the
connection to melt the solder. When the iron is turned on, the metal cylinder and tip heat up to
around 625 degrees. Some high-end soldering irons have dials that allow the user to change heat
settings. If variable heat settings are possible, the setting is determined by the specific board or
chip being soldered to or the type of insulator on the wire. High heat might ruin a board, chip, or
insulator, so a lower setting must be user. High heat does work faster than low heat; however, the
normal 625 degree soldering iron is often sufficient for hobby level projects. The aspect of the
soldering iron that does differ from project to project is the tip.
The tip of the soldering iron is generally used to conduct the heat. The tip can often be changed
to a bigger or smaller tip depending on the needs of the engineer. Smaller tips are used for finer
connections. For example, bigger tips are used when the hobbyist wants to be quick because a
bigger tip conducts more heat to the connection and the solder. On the other hand, when
soldering to pin headers, hobbyists will opt for a smaller soldering tip so that they do not
accidently create shorts between the pin headers. A short is a connection made between two parts
of a circuit, but the term is generally used to describe a connection that was made accidentally.
The Solder
Solder is a thread-like lead or tin based metal alloy that has a low melting point with respect to
other metals. Solder is generally packaged in spools and solder thread circumferences vary to
accommodate different node sizes. The size of the solder is directly proportional to the size of
the soldering iron tip for each project. When soldering pad to pin connections, hobbyists use
solder that has small circumferences to avoid touching the solder to multiple pins so the pins do
not short. When soldering two wires together, larger solder is generally used because it is faster
and there is no fear of shorting.
Preparation
There are two types of wires: stranded wires and single stranded wires. The preparations for each
type are shown in Figure 2 on the next page:
Single Stranded Wire Preparation
Stranded Wire Preparation
Figure 2: Wire to Wire Connection Preparation
To prepare to solder two single stranded wires together, the ends of the wires are formed into
hooks and hooked together to ensure a strong connection. To prepare to solder two stranded
wires together, the ends of the strands are separated, intertwined, and twisted together, also
ensuring a strong connection. Stranded wires and single stranded wire can also be soldered
together, by first soldering the strands of the stranded wire together to make a single stranded
wire. Then they are prepared like two single stranded wires.
There are also two types of pin to pad connections: through hole connections and surface mount
connections. These preparations are shown in Figure 3:
Through Hole Preparation
Surface Mount Preparation
Figure 3: Pin to Pad Connection Preparations
To solder through hole connections, the wire or pin header is placed through the hole. The pad
and the pin do not need to touch. To solder surface mount connections, the wire or the pin header
must touch the pad. The preparation for surface mount soldering is shown by the red wire.
Soldering the Connections
The two components are heated up at the same time with the soldering iron. In the through hole
pin to pad connections, the soldering iron must touch the pad and the pin simultaneously. In the
other connections, the soldering iron needs to only touch one component because the metals in
the components will conduct heat to each other. While the soldering iron is touching the two
metal pieces, the solder is melted onto the connection by touching the solder to the hot
connection point. The solder is not touched to the soldering iron, only to the metal pieces that are
being joined.
The reason that the solder must not touch the soldering iron is that if the solder is touched to the
soldering iron instead of the node, the connection might be a cold joint. A cold joint is a joint
where the solder is physically connecting the two parts of the node, but is not electrically
connecting them, which means no current will flow through the node and that circuit will be
open at that point. Good examples of finished through hole and stranded wire solder jobs are
shown in Figure 1 in the components section. Good examples of soldered single stranded and
surface mount connections are shown in the figures below.
Figure 4: Soldered Single Stranded Wire Connection
Figure 5: Soldered Surface Mount Connection
References
1
http://www.dansmc.com/soldering.htm
Images without references are original content.