Table Of ContentNewnes
Electronics Assembly
Handbook
KEITH BRINDLEY
Heinemann Newnes
Heinemann Newnes
An imprint of Heinemann Professional Publishing Ltd
Halley Court, Jordan Hill, Oxford OX2 8EJ
OXFORD LONDON MELBOURNE AUCKLAND SINGAPORE
IBADAN NAIROBI GABORONE KINGSTON
First published 1990
© Keith Brindley 1990
British Library Cataloguing in Publication Data
Brindley, Keith
Newnes electronics assembly handbook.
1. Electronic equipment. Components. Assembly
I. Title
621.3815
ISBN 0 434 90203 9
Typeset in Monophoto Times New Roman by
Vision Typesetting, Manchester
Printed and bound in Great Britain by
Courier International Ltd. Tiptree, Essex
Preface
Electronics assembly, if nothing else, is about the many ways in which
components are joined together to form a circuit. There are large numbers
of types of components so, understandably, there are large numbers of
ways to join them together. In the past, types of components have been
thrust upon the manufacturer of electronics assemblies, simply as the
component manufacturer finds a new component type. However, we are
witnessing a change in this position: as electronics assemblers themselves
begin to define the requirements for new component types from the
manufacturer. This, of course, means that not only can the components be
designed to fit an existing assembly method, but that new assembly
methods can be developed for which components can be specified.
Consequently, electronics assembly is dynamic. I have tried to show in
this book how dynamic it is; and indicated the ways in which it will change
in the future.
1 Introduction
Electronics assembly, in the broad sense, is about how to bring together a
multitude of electronic components and component types to form a
working, reliable, appliance.
Of course, many of the basic decisions and problems have been overcome
by the integrated circuit manufacturers, who have combined hundreds,
thousands, even hundreds of thousands of components on to single devices
- to effectively ease the job of the electronics assembler, or at least allow the
electronics assembler to construct more complicated appliances for the
same work. Who could envisage, for example, the sale of microprocessor-
based computers in their millions if no integrated circuits existed?
Nevertheless, many problems remain for the electronics assembler.
Indeed, the use of very large scale integrated circuits has generated,
arguably, even greater numbers of problems to contend with.
This book is about printed circuit boards: their design; their manufac
ture; their assembly; their production; their component parts; their
packaging; their testing; their standardization; their quality. For printed
circuit boards of one form or another are used in all but the simplest of
electronics assemblies. In many respects, the whole topic of electronics
assembly may be viewed as just a collection of the multitude of consider
ations regarding printed circuit boards.
Bringing together varied disciplines
The art of electronics assembling requires that a number of methods of
printed circuit board manufacture be available for the purpose of
combining components and, through time, many have evolved and nearly
as many fallen by the wayside. The methods of note, however,
are:
• Through-hole circuits.
• Hybrid circuits.
• Surface mounted circuits.
Often, combinations of two or more of these methods are used to create
single assemblies.
Through-hole circuits
Through-hole circuits use printed circuit boards with holes, to hold
components with wire leads. On one or both board surfaces, or even within
2 INTRODUCTION
the board, is a conductive track which interconnects the components into
the complete circuit. Wire leads of the components are inserted through
holes in the board and track and then bonded to this track, by soldering.
The fact that component leads are inserted through holes in the board
accounts for the term through-hole circuits.
Hybrid circuits
These are a combination of film technologies to create some passive
components on to a base, and discrete component technologies to add on
the remaining passive components and the required active components,
thus forming the complete circuit.
Remaining discrete components, added after the film has been made,
may be leaded (that is, with wire leads allowing connection into circuit, in a
similar manner to through-hole printed circuits) or, more important, may
be leadless (that is, they have no wire terminations, merely metal feet which
sit on the connecting pad of the film, prior to soldering).
Two forms of film technologies exist: thin film and thick film. In thin film
assemblies passive components are manufactured by vapour deposition
and selective etching. In thick film assemblies they are manufactured by
printing, drying and firing pastes. Needless to say, each film technology has
pros and cons.
Surface mounted circuits
Surface mounted assemblies are essentially a marriage of printed and
hybrid circuit methods. The same principle on which through-hole circuits
are based is used: that of mounting discrete components on to a printed
circuit board then soldering. However, where through-hole printed circuit
components are mounted with their wire leads inserted through holes in the
board, the hybrid principle of simply positioning leadless components in
place prior to soldering is used to complete the surface mounted circuit.
Applications
Through-hole printed circuit boards are, without doubt, the electronics
assembly medium of the late twentieth century. They have allowed vast
reductions in size and increases of circuit complexity, since their wide
spread acceptance about twenty years ago. However, printed circuit boards
in this through-hole form, using wire leaded components, are beginning to
reach a natural limit to miniaturization and complexity. Certain techniques
can extend the limit somewhat, but only at significant cost.
There is an axiom which says that through-hole printed circuit boards -
originally designed to hold components together - now just hold them
apart.
Surface mounted assemblies, on the other hand, look set to greatly
exceed this limit without too much extra cost - in certain cases surface
APPLICATIONS 3
mounted assemblies will be even cheaper than their through-hole counter
part. For example, surface mounted assemblies allow significant individual
component complexity - the latest high-density surface mounted in
tegrated circuits feature around 400 connection pins. Size reductions also
result from the surface mounting of components on both sides of the base.
With these in mind, one can easily forecast that surface mounted assemblies
will be the electronics assembly medium of the early twenty-first century.
Previously, though, although the potential of surface mounted as
semblies has long been appreciated, actual assembly has been a tricky job
which has held back the widespread use of surface mounting as a
production tool. The key factor is the soldering process used. Advances in
soldering technique have, however, finally released surface mounting.
On the subject of soldering in electronics assembly, there is a firm
division into two entirely different categories of mass soldering processes,
called:
• CS processes, where components are applied to the printed circuit
board first, followed by application of molten solder.
• SC processes, where solder is applied first, in the form of solder paste (or
sometimes solder preforms). Following this, the components are added,
and heat is applied to melt the solder.
Although these names are new, they are justifiable in that they describe
the two soldering processes far better than existing terms. Often, for
example, CS processes are given the title wave soldering processes, but this is
incorrect - wave soldering is a distinct process not a generic name. Similarly
incorrect is the commonly used misnomer for SC soldering processes- reflow.
The term reflow describes, once again, a single process which happens to be
an SC soldering process; like wave soldering it is not a generic name. The
two processes, and all their derivatives, are described fully.
Already, the use of surface mounted assemblies shows significant
growth, and has been well documented. Figure 1.1 shows the worldwide
trend upwards of surface mounted technology as a percentage of all
100
es
bli Through-hole assembly
m
e
s
s
a
of
e
g
a
nt
e
c
er
P
Figure 1.1 Worldwide trend of
usage of surface mount and
through-hole technologies as
percentages of total
4 INTRODUCTION
al
ot
t
of
e
g
a
nt
e
c
er
p
a
s
a
s
nt
e
n
o
p
m
o
c
d
e
nt
u
Figure 1.2 Use of surface o
m
mounted components as e
percentages of totals, for various ac
countries around the world urf
S
electronics assemblies, over the ten years since its first use. Figure 1.2 shows
the current period, forecasting the use of surface mounted technologies in
various parts of the world, in terms of percentages of discrete surface
mounted components. Given these illustrations, it is fairly obvious that
surface mounted assemblies will, very soon, form the larger part of all
electronics assemblies.
Standardization
There are three main levels of standards: national; regional; international.
National standards are those promoted by a designated organization
within a country, for instance BSI within the UK. Regional standards
usually apply, say, transcontinental^. International standards apply
intercontinentally.
We should be, and are, working towards worldwide international
standards. International standards are the Esperanto of standardization:
where at national levels (and even regional levels) standards of different
bodies may be significantly different; at international standard level we all
speak the same language.
What is the purpose of standardization? Is it to ensure all manufacturers
make functionally identical equipment? Surely that would imply that little,
if any, difference occurs between the equipment made by different
manufacturers. No, the real meaning of standardization is to ensure
quality.
Standardization of processes and capabilities mean that component
parts are of high quality, and that assemblers are of high manufacturing
competence. The result is a product of assured high quality. Everyone
benefits: component manufacturer; electronics assembly producer; user.
Standardization is essential to the just-in-time (JIT) philosophy of
manufacture. Implicit in the use of JIT is the requirement that component
BOOK LAYOUT AND DESCRIPTION 5
parts do not need to be tested as they arrive at the production line. Instead,
the supplier ensures that the parts all function correctly by testing either
each component as it is made, or the supply manufacturing processes.
Book layout and description
My intention in writing this book has been to clarify the numerous aspects
of electronics assembling. Book layout, therefore, broadly follows the main
aspects of electronics assembly: through-hole printed circuit boards and
assembly; surface mounted printed circuit boards and assembly; assembly
packaging; soldering; testing; standardization, and a chapter is devoted to
each. In addition, a chapter describes the Ministry of Defence and how to
set about selling to it.
The large topic of standards has also been tackled, in depth, as an
appendix. Worldwide standards and publications are first listed in an
alphabetical index, which may be then cross-referred with following tables
giving fuller descriptions of their contents.
A further appendix lists names and addresses of worldwide organiz
ations of importance to electronics assembly.
A full glossary of all the terms used completes the book.
2 Electronics assembly: the
PCB
The printed circuit board
In principle, a printed circuit board (PCB), sometimes called a printed
wiring board (PWB), or simply printed board, comprises: a base, which is a
thin board of insulating material supporting all the components which
make up a circuit; conducting tracks, usually copper, on one or both sides
of the base making up the interconnections between components. Compo
nent connecting leads are electrically connected in some form of permanent
or semipermanent way, usually by soldering to the lands, sometimes called
pads - the areas of track specially designated for component connection
purposes. Lands may have holes drilled or punched through the
board to facilitate component mounting. If lanlds do have holes for
component mounting, the board is known as a through-hole printed circuit
board.
To clarify, the term printed is somewhat misleading, as tracks are not
printed directly on to the board.It refers instead to just one stage within the
whole printed circuit board manufacturing process, where the conducting
track layout, sometimes called pattern or image, may be produced using
some form of printing technique.
Printed circuit boards can be made in one of two main ways. First, in an
additive process, the conductive track may be added to the surface of the
base material. There are a number of ways in which this can be done, all of
which are covered later in this chapter. Second, in a subtractive process, the
base material is supplied with its whole surface covered with a conductive
layer, the track pattern is defined, and the excess conductive material is
removed, leaving the required track. Sometimes, both processes may be
combined to produce printed circuit boards with more than one layer of
conductive track.
Printed circuit boards have been around as an electronics assembly
technique in one form or another for fifty years. Nevertheless, considering
the early nature of valve-based electronic assembly it is easy to see why
printed circuit boards did not start to be refined and gain significant use
until the 1960s when transistor circuits became more and more popular.
Valve circuits have always been built by the direct positioning and
manual soldering of individual connecting leads between the required
circuit parts. Initially, transistor circuits were constructed in a similar way,
but the advantages of the printed circuit board in this respect became
quickly apparent.
THE PRINTED CIRCUIT BOARD 7
Plate 2.1 Typical through-hole
printed circuit board, loaded
with leaded components (Ultra
violet Products)
Among other things, electronics assembly using a printed circuit board
means that:
• Circuits are repeatable. After a prototype has been finalized, all
following circuits will be exact copies, unlike the situation in the
previous interconnection assembly method.
• Insertion of components may be automated.
• Soldering of components may be automated.
• Testing of final product may be automated.
• Products may be manufactured in large quantities, more cheaply.
• Reliability increases.
