appearance of this diagram. As you
know, I typically use LochMaster to
present strip board layouts. However,
it turned out to be tedious to get
LochMaster to position components
slightly off-center from the strip board
traces (which is sometimes necessary
to do for an SMD), so I decided to
use the EAGLE PCB layout software
for Figure 4.
The colors (or lack thereof) may
seem a little strange, but I think the
final diagram accurately presents the
necessary positioning for each of the
SMDs. It’s important to remember
that we’re not discussing any kind of
a functional circuit here — just five
independent devices soldered to the
same strip board for practice.
With that caveat in mind, let’s
discuss each of the five SMDs,
starting at the upper left corner of
the strip board. For the 0805 and the
1206 devices shown in rows 1 and 2,
the black rectangular regions on each
device do not represent a pin or tab;
those rectangles indicate the areas
on the main body of each device that
have been pre-tinned, so that the
device can be soldered at each end
by positioning the tip of a soldering
iron so that it contacts the edge of
the device and the strip board trace
at the same time.
When that junction is heated
sufficiently, a small amount of solder
will wick around and under the end
of the device, securely attaching
it to the strip board trace on which
HOLDING SMDS IN
POSITION WHILE
SOLDERING
Now that we know how to
position the devices on the strip
board, the next question is “How
do we hold them in place while
soldering?”
For the purely hand- soldered
approach that we’re discussing, my
online searching turned up two fairly
common approaches. However,
the device is placed.
PICAXE PRIMER
black rectangular regions on each of
the two SOT devices in Figure 4 do
represent pins or tabs that protrude
from the body of the device. If you
look closely at the SOT-223, you will
see that it’s placed slightly off-center
from the mid-line of trace 4.
Centering the SOT-223 on trace
4 would result in the large tab on its
left edge touching traces 2, 3, and 4,
which would necessitate cutting trace
3 at the C3 hole. Of course, this isn’t
a big deal, but the slight offset avoids
the extra work. In many (but not all)
SOT-223 devices, the large tab is also
connected to one of the pins on the
other side of the device. If the tab
happens to be connected to the
middle pin, trace 4 wouldn’t need to
be cut at position C4.
Whenever an SOT-223 device is
used, be sure to check its datasheet
to determine which pin (if any)
connects to the large tab.
The SOT- 23 device in the lower
right corner of Figure 4 may be the
most difficult to solder, due to the
relatively small size of its pins. These
devices must also be offset a little, so
that two of the three pins are fully
positioned on one trace, and that
trace must be severed at the hole
that separates the two pins on the
trace; in this case, that’s hole F5.
Just use this same approach for
the hole at F2. Again, a minimum of
copper is removed, and the two small
pins that are on trace 5 are both fully
positioned on the copper.
iron to tin one of the device’s pads
on the PCB by melting a small
amount of solder onto the pad.
Then, with your soldering iron in one
hand and the device held by a pair
of tweezers in the other, you re-heat
the solder on the pad, slide the
device into its proper position with
one pin in the melted solder, and
allow the solder to cool. The process
is a little difficult to explain clearly in
words, so here’s a link to a very
informative video tutorial: http://tan
gentsoft.net/elec/movies/tt03.html.
June 2013 17
