(rework) of soldered joints should be performed only
when the reliability needs to be improved to avoid
causing added problems by the introduction of
additional heat.
Before we end this part of the guide, there are
some common terms utilized in electronics assembly
that will be used going forward that are important to
understand. First, a component refers to any device that
handles electricity. Many components have leads, which
are two or more rigid metal wires which stick out of the
component. Some leads are straight, some are not.
Some leads are made of round wire, others are made of
flat wire. Components with leads (Figure 7) that are
meant to be soldered through holes in the board are
known as through-hole components. We will talk more
about leads later on when we talk about soldering
specific parts.
As designers have increased the density of parts on
a PCB (printed circuit board), component sizes have
correspondingly shrunk in physical size. Components
commonly have their electrical connections to the PCB
on the underside of the component body in order to
reduce the footprint. These components are attached to
the surface of the printed wiring board with solder. Surface
mount components are usually called SMDs (surface-mount
devices — Figure 8) for short.
Also keep in mind that some components — such as
diodes — have a positive and negative connection to the
board, and so must be placed on the board with the
connections oriented properly. Components with positive
and negative connections are said to have polarity. The
positive lead is called the anode, while the negative lead is
called the cathode. Polarity can be indicated
on parts in a variety of ways, including (+)
and (-) symbols. Markings and symbols for
either the anode or cathode leads can take
many shapes and forms.
Some components will also have a
value, which is a numerical quantity given to
them, and a tolerance which is the variation
from the value allowed for that component.
Resistors and capacitors are common
components on PCBs and it is critical that
the right valued components (many of which
look exactly the same physically) are placed
in the correct location on the PCB.
Another thing to keep in mind when
placing components is orientation.
Component orientation refers to how the
component is to be positioned and soldered
to the board. Orientation is usually marked
in some way on the component’s body.
Orientation marks (Figure 9) — or symbols — include: a
notch, a dimple, a wedge, a stripe, or numbers. With multi-pin components such as ICs, these orientation symbols
indicate where “pin 1” of that component is located, so
that pin may be mated with the corresponding pad or land
on the board. Matching orientation marks may be found on
the board. These include a symbol — usually a plus —
and/or a square pad or land on the board.
Next in this series, we’ll discuss soldering through-hole
components like connectors, so stay tuned. SV
SERVO 12.2014 61
Figure 8. SMT components.
Figure 9. Proper orientation is important on certain components.
Be sure to match it to the board.
Bob Wettermann is a certified master instructor for IPC
and Nick Bruck is certified as a CIS. Contact them at
info@solder.net if you have specific soldering questions.
