Practically Speaking
Hands-on techniques for turning ideas into projects – by Mike Hibbett
Introduction to surface-mount technology – Part 3
I
n the June issue we looked at
how to choose SMD (surface-mount
device) components for projects. We
started with the simplest devices; passives such as resistors and capacitors.
In this concluding article we will move
onto the more complex task of IC and
transistor selection.
First, let us dispel a common myth.
SMDs are not ‘different’ to through-hole
devices; in most cases the electronics
inside the device, the sliver of silicon on
which the circuit is built up, is identical. The difference is simply the package
they come in, and the wires that connect the sliver of Silicon to the outside
world. You might ask, ‘Why have different package sizes?’ There are a number
of reasons.
Packaging
Wire-ended components have been a
popular choice for engineers for decades
because they are a logical improvement
from earlier manufacturing technologies based around point-to-point wiring
solutions from the valve era (see Part
1). As the working voltages of circuits
lowered from hundreds of volts (using
valves) to the 12V or lower of modern
transistor circuits, the size of the circuits
inside components could be reduced;
one reason being that the circuits did
not require large track widths and spacings to avoid voltage breakdown. Just as
improvements in semiconductor technology brought lower operating voltages,
they also reduced current consumption,
at least for digital devices. A few exotic
semiconductor devices still switch high
voltages and high currents, but they are
rare, and it would be unwise to work
with them unless you really know what
you are doing!
The lowering of device voltage and
current consumption also lowered the
diameter of the wires connecting the
surface of the silicon to the pins on
the device package. Since the circuits
to which these devices are connected
to have thin tracks, then so too can the
packages in which the components are
placed. Fig.1 shows some examples – a
BC547 in the familiar TO92 package and
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also in the SMD SOT23 package. Same
device, different wires attached to it in
the packaging. As you can see in Fig.2,
the two package images in Fig.1 are not
shown to the same scale! Both of these
transistors are still in high-volume production, and both are in high demand.
So why has the much smaller and
cheaper SMD variant not rendered
the older through-hole part obsolete,
like valves? Through-hole components
still have value in the global electronics industry. Old product designs that
use through-hole components are still
working, still valuable, so why change?
Examples include railway station ticketing machines, electronic access gates
and fire alarm systems, to name a few. As
the saying goes, ‘if it ain’t broke, don’t
fit it!’ and there are many electronic systems in use globally that were designed
30 years ago, and which are still commercially valuable.
There are some specific cases where
through-hole components still dominate
in modern designs. The main example
is in high-energy systems, where transistors switching very large currents,
such as heating controllers, require thick
wires to pass the large currents. For the
majority of hobbyist projects, however,
very large current handling is rarely a
design requirement.
The demand for SMD
The move to SMD components was
driven by the consumer electronics
market, specifically audio, computing
and communication devices, where very
large currents are not a factor (excluding power amplifier circuits, of course)
and the sheer quantity of potential sales
drove research, both academically and
within industry, to improve the efficiency of electronics manufacturing, where
efficiency measures not only cost, but
also reliability, size, weight, and the
ease and speed of manufacture.
That research and innovation drove
the need to be able to place components
onto PCBs quickly. Machines already
existed that could place wire-ended
components onto a PCB, cut the leads
back and pass them through a soldering
Fig.1. Two variants of the same BC547
transistor, but at very different scales.
bath. The technology developed to place
wire-ended components into PCBs is incredible. Check out this YouTube video
to see it in practice: https://youtu.be/
ZFy0b-Jw4Ec
Although clever through-hole systems are relatively slow and error prone,
and with demand for the production
of electronics devices increasing, the
ability to move to component packages
which could be machine placed more
easily became a world-wide objective.
International standards and cooperation brought us quickly to where we are
now, with new package types – without
wires – and new production machinery
to handle them.
On a personal level I feel it was the
combination of international cooperation coupled with consumer demand
that made this move to SMD production
possible, in such a short timescale. Just
a few decades.
The general trend in IC packages can be
seen in Fig.3. The devices in the bottom
Fig.2. SMD transistor, to scale.
Practical Electronics | August | 2020
row are beyond normal hobbyist application because they demand expensive
4-layer or more PCBs, expensive CAD
design tools, and a lot of experience.
These tend to be highly specialised devices, so we are not missing much by
avoiding them.
While new through-hole devices have
been supplemented by surface-mount
devices and continue to be available
in both package types, some new devices are only available in SMD. This
is because surface mounting does provide some unique benefits. Shorter
wire lengths means devices can run
at higher frequencies, which can be
beneficial for high-frequency radio
parts, or fast microcontrollers. Some
devices are designed for specific uses
– like smart-phones or personal computers – so it makes no sense to offer
the parts in older package formats. On
the other hand, for a voltage regulator
it may make sense to offer the part in
both case styles, but for something like
a camera chip or a Flash memory device,
the target applications do not include
old-style manufacturing techniques.
So, as interesting ICs come onto the
market, our choice as hobbyists is to
either learn new soldering techniques
or rely on the likes of Adafruit or Sparkfun to produce a small, simple ‘plug-in’
PCB, holding nothing other than the IC
and its necessary components. If you are
not tight for space in your project design,
this plug-in PCB approach is perfectly
reasonable – and there are many fareast suppliers making these ‘break-out’
boards available on eBay for this exact
purpose. Fig.4 shows a typical board
available from Sparkfun. Just the IC and
two capacitors, but with a 0.1-inch SIL
header strip allowing it to be placed
onto a regular breadboard. Two mounting holes enable it to be fixed within a
project enclosure.
Whatever the package, some things
never change. Whether you are buying
a chip or a PCB, you will still face the
same issue – is this device right for your
design, and will it work with your other
Fig.3. IC package trends from classic DIP to assorted SMD varieties (image: Microchip).
components easily? To understand this,
you are going to have to delve into the
manufacturer’s datasheet.
Datasheets
Datasheets follow a fairly standard format,
containing similar information even across
different types of device technologies.
This information can be summarised as:
Functional overview
Device variants listing
Absolute maximum values
Detail of operation
Reference application circuit
AC/DC characteristics
Temperature – always has an impact
Device packaging specifications
Shipping packaging specifications
Some of these sections are obvious. Let’s
dive into the sections that are less so.
Device variants
This section details the different manufactured variants of the device that are
available. For microcontrollers this can
be memory size, I/O pin count, and even
the maximum operational frequency. It’s
not uncommon for some of the features
listed in the datasheet to be missing on
smaller pin count package variants, so take
care when choosing your package. Each
device variant will have a unique order
code, which will be listed in the datasheet.
Absolute Maximum values
Fig.4. SMD breakout boards are the
hobbyists new friend!.
Practical Electronics | August | 2020
These parameters list the voltages, frequencies and temperature range outside
of which the device is almost certain to
be destroyed, or not function. A 5V microcontroller may have an absolute maximum
working voltage of 6.5V. Never design
your device to the values in this section
of the datasheet.
AC/DC characteristics
In this section the manufacturer provides examples of what the operational
parameters will be like when the device
is in use. This will include current consumption and maximum operating speed,
usually given at different voltages and
temperatures. This section is critical for
understanding, for example, what size of
battery or power supply you will need to
power your circuit.
Functional overview
This is the meat of the datasheet; it’s the
section you need to be fully familiar with
before designing the part into your circuit.
How long this section is will depend on
the complexity of the part. We’ve listed a
cross section of device datasheets at the
end of this article, ranging from a seven-page transistor datasheet, through a
40-page LM358 op amp datasheet up to a
600-page (!) microcontroller specification.
20 years ago, you would have needed
to be intimately familiar with every page
of that 600-page document, but in recent
times the industry has moved to making
the designer’s job simpler. Datasheets
typically provide an example application
circuit, which invariably becomes the
starting point for designers. For simple
circuits this often forms the complete
electrical design for your project. It’s also
not uncommon for a recommended PCB
layout to be provided, especially for power,
audio or RF ICs. When a recommended
PCB layout is provided always follow it
as closely as possible; in our experience
the term ‘mandatory’ would have been
better than ‘recommended’.
More complicated ICs will have separate application notes that provide the
design of complete reference products,
completely free to copy into professional
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Fig.5. Manually placed SMD components and manual soldering.
Fig.6. Organisation is crucial when assembling PCBs with SMD parts.
designs. It is your responsibility to ensure the circuit works in
your actual application, but often 90% of the design work is
done for you, including source code for the software to make
use of the IC’s complex peripherals, again with example product designs, free to use.
A final point on datasheets: some components are ‘generic’,
meaning they are produced by different manufacturers, so be
careful to read the actual manufacturer’s datasheet for the part
you are using. Voltage regulators, 555 timers and 358 op amps
are just a few examples of devices supplied by different manufactures. Subtle differences in, for example, peak working
voltage or quiescent current consumption, are normal and can
impact your design, so remember to look at the correct manufacturer’s datasheet.
assemble several identical PCBs. Parts are assembled in ‘batches’ of components to minimise the clutter on the workbench;
each component placed in a container, clearly marked. Tweezers are used to pick a component from each container, placed
onto the PCB and soldered. If a component is dropped, don’t
waste time looking for it – pick another!
With surface-mount work, where you need a visual aid to do
the work we find the best process is to lay the PCB flat under
a microscope or magnifying glass, and keep the PCB fixed in
place with masking tape or blue-tack. When assembling several boards of the same design we typically resort to building a
simple jig out of balsa wood. The jig is stuck in place and the
PCB dropped into the recess. This works very well and is cheap
and easy to do. The walls of the jig are glued in place with super-glue, so a jig can be manufactured in minutes.
PCB assembly with SMDs
In general, when creating a PCB with SMD parts you will be
using a CAD program. For example, you might use popular (and
free) applications such as EagleCAD or KiCAD to design your
board. These tools come with standard component footprints,
but do be aware that there are a number of different pad sizes
available for a given component size, such as 0805 resistors and
capacitors. There are ISO standards that define large, medium
and small footprints – relative terms, of course! A component
supplier will always have their own specific design recommendations within their datasheet.
For high-component-density designs, and to minimise the
risk of soldering failures, a designer will normally follow the
component supplier’s recommendations. They will discuss the
proposed design layout with the PCB assembly company they
intend to use, to ensure the design makes best use of the machines that will be used to solder the parts. As hobbyists, we
are not required to focus on the high-volume manufacturability
of our designs; we want to make them as easy to hand solder as
possible. With this in mind, look to use a component footprint
that leaves exposed copper on the pad for easy access with a
soldering iron. Fig.5 shows a board designed by the author that
was hand assembled. Notice the large amount of solder on the
pads of C5; this part took just a few seconds to hand solder.
Fig.5 highlights another important point with board designs
using SMD component placement. The capacitors surrounding
the IC are close to it, as they are required to be. They could have
been a lot closer, but if they were, it would have made soldering
to the IC pins difficult without causing shorts. Even with this
design the IC had to be hand placed first, so that the capacitors
did not restrict access to it with a soldering iron.
Organising your components during assembly is critical.
Only resistors come with a clear value indicated, capacitors in
the main do not. In Fig.6 you can see the author preparing to
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Advanced selection
As you move from hobbyist projects into professional circuit
design and product creation, additional requirements come into
play. It’s important to understand if the part you are selecting
meets the standards of the countries you are going to be selling
in – for example, is the device lead free (ROHS compliant), is it
available in reels, and if so, will those reels fit on your manufacturer’s machines? All of this information is available in the
datasheet or documents the datasheet refers too.
Reels of components are always supplied in airtight wrappers.
If an SMD component is exposed to air, moisture can creep into
the package and potentially destroy the device when rapidly
heated in a production SMD oven. The degree of sensitivity to
moisture ingression is related to the device package design and
is always stated on the datasheet. As a rule of thumb, we never
open sealed component packs destined for our manufacturer!
In summary
The industry trend is to continue to make small, close-pin
pitched ICs that will continue to pose challenges for hobbyists.
Thankfully, the global hobbyist market is large enough that it
is commercially viable for electronics manufacturers to supply
break-out boards for a wide variety of exotic ICs, so we hobbyists will not be excluded from taking advantage of the latest
advances in IC technologies. You can expect to see more use
of break-out boards in Practical Electronics articles, including
the current Pic n’ Mix series.
References to datasheets
https://infocenter.nordicsemi.com/pdf/nRF52833_PS_v1.2.pdf
https://www.mouser.com/datasheet/2/149/BC547-190204.pdf
http://ww1.microchip.com/downloads/en/devicedoc/22008e.pdf
https://www.ti.com/lit/ds/symlink/lm358-n.pdf
Practical Electronics | August | 2020
