■ FIGURE 1. Surface mount
compared to through-hole
components. Shown is a
14-SOIC and a 1206 SMT
package next to a TO-220
been with us for over three
decades. In this time,
standards have emerged
for device packages as
seen in Figure 2. The SOT-
23 package is used for general-purpose
diodes and transistors. Power transistors are found in the SOT-223 case
style. The Small Outline Integrated
Circuit (SOIC) package is a standard for
multi-pin ICs. Passive components such
as resistors and capacitors are often
found in case styles 805 and 1206.
This is a small sample of the packages available. For more information,
look to the device data sheets. These
are available from all manufacturers on
the Internet. Large catalogs from the
big suppliers are another place to learn
about chips and packaging; check out
Newark, Mouser, Allied, and others.
an inkjet printer will
• Bare copper clad circuit boards.
• Chemical etchant. Ferric chloride or
ammonium persulfate are both
available from most electronic
• SMT components.
• Soldering iron with fine tip. I use a
Weller WTCPT iron with a PTJ- 7
• Fine solder ( 28 gauge).
• Desoldering braid.
• Tweezers. Use a style designed for
SMT to prevent damage to the parts.
• Nail polish remover (acetone).
• Green scrubbing pad.
• Handheld magnifying glass for
inspecting the finished solder joints.
■ FIGURE 2. Typical SMT packages.
ed. When you are drawing the schematic, you are also selecting the component
package as seen in Figure 4. For
example, when you select an LM555N,
you get an eight pin DIP package. When
you select the LM555D, you get an
eight-SOIC package. In both cases, the
schematic symbol looks the same.
PCB design is an art form akin to
playing a musical instrument. It will
take time to develop your skill and you
will get better with practice. As a starting point, I offer the following tips:
Figure 3 shows a completed
circuit board using the method
outlined in this article. This particular
PCB is a prototype circuit for an audio
amplifier I am working on. It has two
integrated circuits, five diodes, three
transistors, 10 capacitors, and six
resistors. This PCB could be physically
smaller but this was not necessary for
my needs. The following materials
were used to construct the board:
The first step in constructing a
PCB is to enter the schematic into
your CAD program.
Inspiration for circuit designs may
be found in the device data sheets and
application notes. Most semiconductor
manufacturers maintain email lists to
introduce new parts to subscribers.
Many classical designs may be built
using SMT. However, I will caution you
to watch out for power dissipation: SMT
parts don’t have the surface area to get
rid of heat and can burn
up if you are not careful.
• Use larger SMT parts. The 1206 case
size is a good starting point for resistors
and capacitors. They are relatively easy
to handle and can be seen without a
magnification aid. Also, you can run a
single trace between the component
pads as shown in Figure 5. With this
function, you can build single-sided
boards that are compact with good circuit layout. Occasionally, you will need
to use this technique to get around an
important circuit trace. A low value
resistor makes a good jumper.
• Computer running a CAD program.
This circuit was drafted using Eagle
Layout Editor available as limited freeware from
works on many types of computers. It
even works on my old Pentium 233
MHz, albeit slowly. A Linux version is
available. It allows you to quickly
switch between the board and
schematic so you can see the effect
of schematic changes on the PCB.
• Transparency film available from your
local office supply store.
• Laser printer. You must use a laser
printer or copier. The toner is a
critical component to this process ...
• Read the documentation that came
with the CAD program. Work through
the design examples. Refer back to
the documentation from time to
Design layout and circuit design are closely relat-
■ FIGURE 3. Completed
SMT PCB. The large circles
are used to connect the
board to the remainder of
December 2007 69