Practically Speaking
Hands-on techniques for turning ideas into projects – by Mike Hibbett
Introduction to surface mount technology – Part 1
O
ver the years that this
magazine has been in circulation – the first issue came out in
1964 – our projects have been based on
a wide variety of electronic components
and soldering techniques. Integrated circuits were not widely available in 1964,
certainly not for hobbyists; and in the
early days we soldered components between lugs on a pegboard, often leaving
the components in the air. We certainly
used transistors, but also valves (‘tubes’
for our US friends), which were so large
they needed metal work to support them.
In those days, some projects were as much
mechanical construction as electrical!
It’s difficult to accurately define when
electronic components were first introduced to the public; it very much depends
on your definition of an ‘electronic component’. Leyden jar ‘capacitors’ probably
come first (18th century), acting as a
primitive electrical power source – but
they were important because a circuit
cannot function without a power supply.
These were followed by piles (early batteries), coils, including transformers, and
large plate capacitors, all used extensively by scientists like Faraday, Hertz and
later Marconi to create early communication technologies such as telephone, data
transmission via Morse code and radio.
The invention of vacuum tube diodes
and then triodes using thermionic emission technology (based on X-ray tube and
light bulb research) ushered in what we
would now call ‘electronics’, ultimately
leading to the birth of what we recognise
as our hobby in the late 1920s.
The Second World War was a huge
impetus to electronics. The mass production of radio, radar, electronic proximity
fuses and a whole host of other devices
spurred research that led to huge improvements in electronic component
size, quality and price.
Early kits
Fig.1. Old-school construction – (top) part of a power amplifier built without the benefit
of a PCB (photo courtesy of Alan Winstanley); (bottom) an example of a 1962 home
project – a Heathkit Visual-Aural Signal Tracer Model IT-12.
Practical Electronics | April | 2020
Jut like today, early hobbyists were expected to assemble circuits themselves,
and kits by companies like Heathkit (who
formed in the years after the Second World
War) were based on designs created from
the surplus of ex-military components
sold at low cost at the end of the war.
The Heathkit assembly manuals from
those days were works of art; their kits
include products ranging from Morse
code audio oscillators up to televisions
and oscilloscopes. Even 30 years later
in the 1970s with advanced production
technologies available, radically new
electronic products were still being offered in kit form – the Acorn System 1
computer being one example (the author
still has his.)
The mechanical structure of electronic components has changed enormously
over the years. Initially, circuits were
based around glass tubes with pins
designed for large sockets, or large component packages with long wires, these
devices were ideally suited to circuit
assembly that more reflected a mechanical assembly project than the designs
we build today. Anyone who has seen
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a 1960s Heathkit assembly manual, like
the one in Fig.1, will understand. And
the tools we needed to assemble these
projects were huge. Not to mention the
need to wear a tie like the chap in Fig.2!
(For younger readers, in those days it was
not uncommon for manual workers to
wear two or three-piece suits.)
Over the years components became
smaller, wirewrap board assembly and
eventually PCB manufacturing processes
became available. The vital pivot to the
consumer industry however was when
surface-mount components and production techniques became available.
Automation
Circuits based on valve technology could
be assembled only by hand. Even with
today’s technology, robotic point-to-point
wiring would be a harder problem to automate than driving a car. The precision
required to solder point-to-point wired
circuits would be at the millimetre level
in a three dimensional space. Automated
cars are required to navigate roads at the
centimetre level. And they are not able
to do that reliably yet.
With the introduction of surface-mount
components, component part sizes and
even the reels that they are supplied on
became standardised. That standardisation meant that the designers of machines
that will hold, dispense, pick up and
place components could be built to a
common standard – a standard used by
all component suppliers. This focused
academic and industrial research into
efficient tools to do the job, and with all
manufacturers adopting those common
standards, there was a business case to
justify the R&D investment. If the market
had been fragmented by a large number
of proprietary component size and delivery packages, automation advances
would simply not have been as financially viable, and technology improvements
not as fast.
As the accuracy of automated pick-andplace machines has improved over the
years, the size of components that can be
automatically placed has been reduced,
significantly. A 4.7kΩ 1/8-watt wire-ended
resistor that we all know and love is not
the size it is because it needs to be; as
can be seen in Fig.3, it can be a lot smaller – and this is not the smallest size an
automated pick and place machine can
work with! The wire-ended component
is this size because of the wires, nothing
else. It’s true that for specialised components, working at very high voltages or
very high wattage rating, their sizes will
be larger – but the vast majority of components placed on circuits for consumer
products work at very low voltages and
very low power ratings, and these components can be incredibly small, while
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Fig.2. Building a circuit 1965-style. Want to be on the cover – make sure you wear a tie!
(For the benefit of younger and foreign readers, the 1965 price of ‘2'6’ is 12.5p, or £0.125!)
still reliably placed by machines onto a
printed circuit board (PCB).
The rise of SMD
The use of surface-mount components
(SMD) also results in a significant reduction in the size of the PCB, with a
corresponding reduction in weight (if that
is important,) and cost, which is always
important. With less weight and closer
contact to a PCB, SMD components are
less susceptible to mechanical stresses,
assuming they have been soldered to the
PCB correctly in the first place.
With smaller components the soldering
process itself becomes a critical part of
the overall production process. Academic
and industrial research has focused on
this for decades, covering:
Component size standardisation
Component solder pad design
Solder mask
Solder paste
Component placement
Soldering process
Let’s cover each of those in turn. The
first two points are basically the same; if
component manufacturers agree on standard component sizes, engineers spend
less time designing new footprints for the
components they add to their PCBs. The
resulting world-wide agreements has had
a major impact beyond saving engineers
a few hours effort. Optimisation of soldering processes and general reliability
improvements has been far more effective
and quickly adopted when all companies
are able to focus on the same problem.
Even minor improvements in the design
of a solder pad shape, if it improves soldering reliability by a fraction of 1%, can
have a huge impact when every company
in the world is using the same design,
and can make the same changes. This is
the power of standardisation!
Solder mask
The solder mask is a solder-resisting film
layer applied to the PCB during manufacture. It is automatically calculated
Practical Electronics | April | 2020
Fig.3. Wire-ended components verses their SMD equivalents: exactly the same device, very different packaging.
by your CAD program based on the
standard component outlines that you
place on your PCB; it’s not something
you have to create yourself. The solder
mask is a coloured layer (you get to
choose the colour) that both protects the
copper traces from corrosion and also
acts as a repellent to solder – forcing
any solder that wants to ‘creep’ between
two tracks away, back to the pad they
are supposed to melt on. This is hugely
important as component solder pads get
closer together.
Solder paste
The solder paste layer is again a layer that
is created automatically for you based on
the components you place. It defines the
areas on your PCB that should be left exposed for the application of a thin layer
of solder paste.
While it is possible to add solder paste
to PCBs by hand, it is of course far more
preferable to be able to apply the paste
automatically, or at least by a more efficient process. The solder paste layer is a
collection of polygons that define where
the solder paste should be deposited;
PCB manufacturers take this layer and
laser cut a sheet of thin steel with these
holes. Then, using the age-old process
of screen printing, the thin sheet of steel
is placed over the PCB and a squeegee is
used – by a robot or manually – to apply
a thin coat of solder paste to the PCB.
Ink-jet-style printers are now available
to do this task, but manual application
is still common.
Component placing
Next, components need to be added to
the PCB using very fast pick-and-place
machines. I could write a whole article
on these fascinating robots, which can
place parts from reels or ‘sticks’ of parts
at up to 100,000 parts per hour and to an
accuracy of within 25µm. They typically
represent 50% of the outlay for a complete production line. For the hobbyist
side of things they come free – it’s you!
Practical Electronics | April | 2020
Soldering process
The soldering process itself has many
critical parameters. The solder paste,
consisting of tiny balls of solder in a flux
paste, must be heated to a point where
the flux melts, time allowed for the flux
to clean the surfaces, then the temperature is raised again to melt the solder,
and time allowed for the solder to flow
over the component and the PCB pad.
This process depends on many factors,
including the size of the component, it’s
shape, and how much of a ‘heat sink’
effect the PCB traces connecting to the
component have. These problems have
been studied for decades, and the results shared, driving subtle changes to
the temperatures used, the cycle times
and the specific layout of ground pads
on components. It’s wonderful that this
knowledge has been shared rather than
tied down in patents and copyrights.
While this all sounds very complicated, the process is actually very easy to
use at home. With low-cost PCB manufacture available in Asia, it’s possible
to order 10 complex double-sided PCBs
with a laser-cut solder paste stencil for
less than £30 (US$40). Solder paste can
be purchased in small quantities from the
usual electronics distributors, and while
it has a limited shelf life, for hobbyists a
small 30g tub of paste can be stored in
the fridge for one to two years – if your
partner will tolerate it!
The re-flow oven used by electronics
assembly companies can be replaced by
a simple desktop convection oven, costing around £40 (US$52). A re-flow timer
controller can be added for about $50
(US$65), less if you build one yourself.
These are simple devices that will last
for decades and are worth the investment
if you expect to be making many boards,
or would simply like to make assembling
surface-mount PCB easier.
This approach to SMD component assembly is well suited to the hobbyist, but
if you are looking to expand, perhaps
manufacture hundreds of PCBs, then there
are better alternatives to simply getting
a professional manufacturer to assemble
you boards. Automated ‘Pick and Place’
machines have been created by amateurs
and small companies that can place components on PCBs at high rates, yet the
equipment is low cost – around €1700
for a machine equipped with an AI-based
vision system to accurately place components (see: www.liteplacer.com). These
kind of machines are only suited to very
low production volumes, but they give
the benefit of enabling you to quickly produce a large number of PCBs, albeit with
a fair amount of post-production ‘touching up’ (error correction) required. They
are desktop machines, and significantly
cheaper than the £100k machines used
by regular PCB assembly companies.
The development and improvement
of these machines has been driven by
the open-source community, a group
of people keen to expand their experience while sharing it with others. It’s
through this community that small cottage industry companies have been able
to create useful, technologically advanced
machines that can help budding entrepreneurs grow their businesses.
Practical Electronics magazine has
been in print for 56 years. Ownership
may have changed; staff have left or
passed on. Over that time the magazine
has carried the electronics hobbyist tradition across the era of valves/tubes
wired to Paxalon, point-to-point wired
components through to robot-assembled
surface-mount components measuring
just a few millimetres across. It makes
us wonder what (and how) hobbyists 50
years hence will be assembling!
Summary
Next time in Practically Speaking we will
look at passive surface-mount parts, including sockets and switches, and the
challenges involved in choosing and
placing them on a PCB, both automatically and by hand.
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