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By Tim Blythman
SMD
Trainer Board
Are you interested in learning to solder small surface-mount devices but
don’t want to ruin an expensive board or chip gaining those skills? Perhaps
you have no choice but to learn since so many parts made these days only
come in SMD packages. This simple SMD Trainer project is a great way to
practice soldering a variety of surface-mount devices. If done correctly,
you’ll be rewarded with a series of LEDs flashing in sequence.
S
urface-mount devices
(SMDs) are the preferred type
of parts used in most commercial equipment due to their compactness, good reliability, low cost and
widespread availability. While some
manufacturers are still producing
new through-hole parts, your choices
become a lot more limited if you can’t
handle SMDs.
We know it seems daunting initially
(it did to us, too), but you will be surprised how easily you can do it with
a bit of practice. And that’s precisely
what this board is designed for. It’s a
working circuit designed using a wide
variety of different SMD parts, allowing you to try out soldering them. This
way, you can master the techniques
and become familiar with the common
sizes and packages.
It’s designed so you can start with
the larger parts and, as you gain confidence, move onto the smaller ones.
And you can test it along the way, so
you’ll find out pretty quickly if you’ve
made a mistake and have an opportunity to correct it.
This article includes the basic
instructions for building and testing
the SMD Trainer Board, along with
a description of how it works. The
accompanying article provides considerably more detail regarding the
necessary tools and techniques.
We recommend that you look at that
article now and refer back to it later
36
if you come across anything that you
don’t fully understand. That’s especially the case if you are not experienced at soldering, or have doubts
about your ability to handle SMDs.
Assuming you have read that article (at least in part) and are starting to
get an idea of how you would go about
assembling this board, let’s move on
to describing its design.
Circuit details
The circuit of the SMD Trainer Board
is shown in Fig.1. We’ll explain how
it works before going any further. It’s
important to know what it should
do, so that you can figure out what’s
wrong if it doesn’t work initially.
There are two main parts to the circuit, the second of which depends on
the first. The first part of the circuit is
also easier to build, so you can try out
your skills on that before dialling up
the difficulty.
Common to both parts is the power
supply. Coin cell holder BAT1 is paralleled with a USB socket, CON1. Only
one of these should be fitted. We recommend the coin cell holder, as a coin
cell is less likely to deliver damaging
current in case you make a mistake
building it.
Due to the presence of a coin cell,
take care that the SMD Trainer Board
is kept out of reach of children (swallow/choke hazard). It has flashing
lights, so it will appeal to curious
eyes – but there is no reason for it to
come into a child’s hands as it is definitely not a toy.
First half
IC1 is a timer IC (a 7555). We’ve chosen this CMOS variant rather than the
bipolar transistor-based 555 to allow
the circuit to work at low voltages and
be powered by a coin cell. The supply
passes to IC1’s pin 8 (positive) and 1
(negative). Pin 4 (RESET) is held high
to allow the timer to run.
IC1 has its supply bypassed by a
100nF capacitor and a second 100nF
capacitor stabilises the internal voltage
on the CV pin (pin 5). IC1 is configured with the 100kW resistors and 1μF
capacitor in the well-known astable
oscillator configuration.
In this arrangement, the 1μF capacitor charges from the supply via the two
100kW resistors; its top is connected to
input pins 2 and 6. When pin 2 rises
above 66% of the supply voltage (about
2V), an internal flip-flop toggles and
pin 7 is connected to ground (through
a transistor inside IC1). At the same
time, pin 3 goes low.
This causes the 1μF capacitor to discharge through the lower 100kW resistor into pin 7, until the voltage on the
capacitor reaches 33% of the supply
(about 1V). The flip-flop resets, pin 3
goes high, pin 7 stops sinking current,
the capacitor begins charging again,
and the cycle repeats.
Practical Electronics | December | 2022
SMD Trainer
Fig.1: this simple circuit lets your soldering
efforts speak for themselves. IC1 is
configured as an oscillator that alternately
flashes LED11 and LED 12. IC2 is clocked
from IC1’s output and lights up each of
LED1-LED10 in turn. Power comes from
either a USB socket or coin cell holder.
With the provided component values, the oscillator frequency is around
4.8Hz with a 66% duty cycle at pin 3
(ie, pin 3 is high about 2/3 of the time).
When pin 3 is low, current is sunk
from the supply via LED12 and its
1kW series current-limiting resistor,
causing it to light. When pin 3 is high,
MOSFET Q1 is switched on by the
positive voltage at its gate, and current flows through LED11 and its series
resistor instead. Thus, these two LEDs
flash alternately.
This first part of the circuit is built
from larger SMD parts, like those we
usually include in our projects when
through-hole parts are unsuitable.
It can operate independently of the
remainder of the circuit, and can be
built and tested as the first part of a
two-part challenge.
other chips, but is left disconnected
in this case.
Each of the ten outputs noted above
has a 1kW series resistor and LED connected to its output. Thus, a clock signal at pin 14 causes the LEDs to light
up in order, one at a time.
The components around IC2 have
a variety of sizes to present a more
interesting challenge; IC2 is also in a
smaller SMD package than IC1. See
Table 1 for more details.
Placement and order
Our recommended assembly order for
most through-hole designs is for a few
reasons. Working by component type,
for example, starting with resistors,
then diodes, capacitors and then ICs,
makes it easier to keep track of what
step you are up to.
For the most part, this order is dictated by the component heights. Components that are close to the PCB are
placed first as they don’t restrict the
placement of taller parts. Also, this
means that the PCB can be turned
upside down without the throughhole components falling out; they are
held on the PCB by the work surface.
Working with SMD parts has similar motivations, but there is much
less need to invert the PCB, so no real
chance of parts falling out. Also, most
SMD parts have a low profile.
Therefore, in SMD assembly
the primary consideration will
be to place the more difficult-to-
Second half
A horizontal line on the PCB divides
it neatly into two distinct parts; part
two is below this line.
IC2, a 4017-type decade counter,
is the heart of the second part of the
circuit. It is powered from the same
supply as IC1, connected to its pin 16
(positive supply) and pin 8 (negative
supply). Its supply is also bypassed by
a 100nF capacitor for stability.
IC2 has ten outputs at pins 3, 2,
4, 7, 10, 1, 5, 6, 9 and 11. These
are driven high, one at a time, in
response to a clock signal applied to
pin 14. This signal comes from pin 3
of IC1 mentioned above. Pins 13 and
15 are pulled low to allow normal
counting operation. Pin 12 is a carry
output, which can be cascaded to
This is the SMD Trainer board that we put together (shown at approximately
166% actual size). If you’re having trouble making out the M0603/0201 LEDs, it
might be because they’re not fitted! We couldn’t solder these by hand, and won’t
pretend that it’s easy to do so.
Practical Electronics | December | 2022
37
The SMD Trainer
is designed to function
without all components installed,
making testing your SMD work easy.
access or difficult-to-solder parts first,
so that they aren’t impeded by parts
fitted later.
With this in mind, the best way to
construct hybrid circuits (that have
both through-hole and SMD parts)
is to fit the SMD parts first. Whether
they are on the same side or not, the
taller through-hole parts will be a
greater impediment to construction
if they are fitted before the smaller
SMD parts.
This also means that the process of
placing ICs last is no longer appropriate. Nowadays, ICs tend to be more
rugged and less prone to damage from
static, which was usually the motivation to fit them as late as possible.
In SMD designs (or at least those
intended to be hand-soldered), the
ICs typically have finer leads and
are harder to work with. So it makes
sense to do them first and then work
on their surrounding passive components, which are often larger.
Assembling the SMD Trainer
Refer now to the PCB overlay diagrams, Fig.2 and Fig.3, which show
which components go where. The SMD
Trainer PCB is double-sided, measures
70.5 x 40mm, coded 29106211 and
available from the PE PCB Service as
just a board or a kit.
We recommend starting with the
USB socket if you will be fitting it. The
leads are not too small, but they are not
very accessible. Fortunately, this part
has locking pins on the underside that
go into holes in the PCB. So positioning the part correctly is easy.
Place flux on all the pads for the
USB socket and press the part down.
For this application, only the two outer
pads of the five are needed to supply
power; hence they are the only ones
that are extended. You can add more
flux to the top of the pads too.
Clean the iron’s tip, apply a small
amount of solder and press the iron
against the PCB pad. If the solder
doesn’t run onto the lead, bring it
closer, until it is touching if necessary.
Repeat for the other outer pad.
With this connector, make sure
you don’t touch the iron against the
USB socket shell when making these
power connections. The tight angle
here is what makes this tricky. If you
form a bridge, apply heat to all the
pins to remove the part and tidy both
the socket and PCB with solder braid.
For the larger pads that secure the
USB socket mechanically, simply
apply the iron, add some solder until a
tidy fillet forms, then remove the iron.
A generous amount of solder here will
result in a secure connection.
Using a similar procedure, place IC1
and Q1, ensuring that they are oriented
correctly. Then solder the resistors and
capacitors in place. Note that there are
two different values of each – refer to
our photos.
The LEDs are also polarised, and
must be fitted with their cathodes to
the left towards the resistors.
If you wish to fit the cell holder
instead of the USB socket, do so now.
It’s usually easier to fit parts on one
side of the board at a time, but this will
allow you to test out the first part of the
circuit that you have just assembled.
Flip the PCB over and put some flux
on the two smaller outer pads. Leave
the large inner pad clear, as the PCB
pad itself becomes the negative terminal and doesn’t need soldering.
Also ensure that the holder opening
is towards the edge of the PCB, so that
you can easily insert the cell. Position
the holder roughly in place and add
some flux to the top of the leads.
Note that, unlike the USB socket,
there is nothing to lock this part in
place on the PCB.
You will probably need to turn up
the temperature on the iron slightly (if
it’s adjustable) and load some solder
onto the tip; a bit more than for the
smaller parts. Use tweezers to keep
the cell holder in place and touch the
iron to the pad.
Give it some time to heat up; remembering that it is all one piece of metal,
so it is unlikely to be damaged by too
much heat. You should see the flux
smoke and the solder flow. Remove
the iron and give the part (and solder)
a few seconds to cool before releasing
the tweezers.
The first joint doesn’t need to be
perfect; the main thing is that the
Figs.2 and 3: start by fitting the components in the top half of the PCB, which forms the alternate flasher, lighting LED11 and
LED12. These components are larger SMDs that are generally not too hard to solder. Once you have those working, you can
move onto the more challenging parts below, which form an LED chaser. With IC2 and its bypass capacitor in place, fit LED1,
LED6 and their series resistors, then move onto the smaller parts, testing it at each step to ensure your soldering is good.
38
Practical Electronics | December | 2022
part is accurately positioned and held
firmly in place.
The second pad can be approached
like the larger pads on the USB socket.
Apply the iron, feed in the solder until
a good fillet is formed, then remove the
iron. Give it a few seconds to solidify before returning to the first pad to
make it tidy. You can touch it up by
applying the iron and solder in the
same fashion.
Initial testing
The first part of the circuit should now
work. You can test it by fitting the button cell or applying power from a USB
source. If using the button cell, ensure
the polarity is correct. You should see
LED11 and LED12 flicker alternately.
If one LED is stuck on, then IC1 is
not oscillating, and you should check
it and the components around it. If
only one LED is flashing, the other
might not be soldered correctly; this
could include either of the 1kW resistors or Q1.
You might also see what appears to
be the two LEDs on at the same time.
In that case, they are probably flashing
faster than the eye can see. One possible reason for this is that the 1μF timing capacitor has been mixed up with
one of the 100nF capacitors.
At this point, it’s best to verify that
this part of the circuit works correctly.
Otherwise, if the second part doesn’t
work, it will be harder to determine
the problem.
Remainder of the circuit
You’ll note that the components in the
lower half of the PCB are fairly well
spread out. This is a luxury that won’t
be present in all SMD designs.
With the amount of space present
on the SMD Trainer Board, it’s certainly possible to install these components in just about any order. But
we recommend starting with IC2 and
its capacitor, followed by the LEDs in
Parts List – SMD Trainer
1 double-sided PCB coded 29106211, 71 x 40mm, available from the PE
PCB Service
1 mini-USB socket (CON1) OR
1 SMD coin cell holder (BAT1) [BAT-HLD-001; Digi-Key, Mouser etc]
Semiconductors
1 7555 CMOS timer IC, SOIC-8 (IC1)
1 4017B decade counter IC, SSOP-16 (IC2)
1 2N7002 N-channel MOSFET, SOT-23 (Q1)
4 M3216/1206 size LEDs, any colour (LED1, LED6, LED11, LED12)
2 M2012/0805 size LEDs, any colour (LED2, LED7)
2 M1608/0603 size LEDs, any colour (LED3, LED8)
2 M1005/0402 size LEDs, any colour (LED4, LED9)
2 M0603/0201 size LEDs, any colour (LED5, LED10)
Capacitors (all SMD X7R 10V+ ceramic)
1 1μF M3216/1206 size
SMD Trainer kit
3 100nF M3216/1206 size
We can sell you just the PCB or we have
Resistors (all SMD 1% or 5%)
a limited run of a kit of parts to build the
2 100kW M3216/1206 size
SMD Trainer Board with everything except
4 1kW M3216/1206 size
the coin cell (CR2032 type), which is
2 1kW M2012/0805 size
widely available. Altronics also has a kit
2 1kW M1608/0603 size
for this project, code K2001, at around £15
2 1kW M1005/0402 size
PLUS p&p – see: www.altronics.com.au
2 1kW M0603/0201 size
order of size from largest to smallest.
This will allow you to power up the
circuit at any time after you have any
of the larger LEDs fitted, and check
that it is working.
Start with IC2. Apply flux and position the part. We’ve been quite generous with the length of the pads here,
for two reasons.
First, we have seen SOP variants of
this part being available with various
body widths. So this pad configuration
offers the flexibility to accept a range
of compatible parts. Second, it makes
it easier to solder.
Clean the tip of the iron and add a
tiny amount of fresh solder to it. Hold
IC2 with the tweezers and apply the
iron to the PCB pad only. You should
see the solder flow onto the lead and
form a joint strong enough to hold the
part in place.
There’s a set of TQFP pads located on the underside of the PCB. This is for you
to practice soldering, and does not have any electrical connection to the circuit.
Practical Electronics | December | 2022
Check that the leads are aligned
and solder the remaining pins in this
fashion. These tiny parts do not need
much solder, so you might find that
you only need to occasionally add solder to your iron.
Check for bridges and rectify as
needed. Follow with the remaining
100nF capacitor. LED1 and LED6 are
M3216/1206 sized parts, so you should
be comfortable fitting them and their
respective 1kW resistors. Note that all
cathodes are on the side away from IC2.
And test again
Our design is incrementally functional,
so you can power and test the partially
completed design at just about any
time. You should see LED11 and LED12
continue to alternate as before; if they
do not, then you might have a short circuit that is shunting power away from
IC1 and its components.
LED1 through to LED10 should
flicker on and off in turn when fitted.
If you get nothing at all, check that
IC2 is fitted correctly, with the correct
orientation and no bridges. Individual
LEDs not flashing are probably a sign
that a single LED or its resistor are not
fully soldered.
Completion
Take your time and work through the
differently-sized LEDs and resistors in
turn. Don’t be disappointed if you can’t
solder the M1005/0402 or M0603/0201
parts by hand. We have not used anything smaller than M1608/0603 in any
of our designs, and even we find anything smaller than M1005 challenging.
39
Table 1 – common passive SMD component sizes
Metric
M3216
M2012
M1608
M1005
M0603
M0402
Length
3.2mm
2.0mm
1.6mm
1.0mm
0.6mm
0.4mm
Width
1.6mm
1.2mm
0.8mm
0.5mm
0.3mm
0.2mm
Imperial
1206
0805
0603
0402
0201
01005
Length
0.12in
0.08in
0.06in
0.04in
0.02in
0.01in
Width
0.06in
0.05in
0.03in
0.02in
0.01in
0.005in
Anything that tiny is not intended to
be soldered by hand. The smaller LEDs
often have exposed pads only on the
underside, making it very difficult to
transfer heat where it is needed.
There are some tricks you can use,
such as applying a small amount of
solder to the pads and trying to conduct heat through the PCB trace radiating out from the lead. Or try your
hand at reflowing solder using hot air
or infrared.
We published a DIY Solder Reflow
Oven design in the April and May 2021
issues. It is also possible to successfully reflow a board with ‘tools’ such
as electric frypans and clothes irons!
Cleaning
Once you are satisfied with your progress, clean up any residual flux and
allow the board to dry fully. Although
the board doesn’t do anything incredibly useful, it is still a handy reference
Further reading
We have, of course, written articles in the past about surface-mount technology,
devices and construction. You will find these useful:
● How to Solder Surface-mount Devices
July 2010
● PIC n’ Mix: Building circuits with SMDs – Parts 1 and 2
February and March 2019
● A DIY Reflow Oven Controller for modern soldering
April and May 2021
This M0603-sized component, shown
on a fingertip, measures a miniscule
0.6 x 0.3mm, making it easy to lose.
tool and will remind you of the tricks
and techniques you learned when
constructing it.
Complete kit
We offer either just the PCB or while
stocks last, a complete kit of parts
(go to the PE PCB Service) – see the
parts list.
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
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Practical Electronics | December | 2022
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