Table Of ContentSoldering in Electronics Assembly
Soldering in Electronics Assembly
Mike Judd and Keith Brindley
UNEWNES
Newnes
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First published 1992
© Mike Judd and Keith Brindley 1992
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A catalogue record for this book is available from the British Library
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Preface
Electronics assembly is about how components are soldered onto a printed circuit
board. Soldering, as a principle in electronics asembly, is straightforward and
simple — two metals surfaces (a component termination and a connecting pad on
a printed circuit board track) are joined by the metallic bonds created by molten
solder between them. Soldered joints are supported by the solder when it
solidifies, and solder allows electrical contact between metals in the joints. These
things cannot change.
On the other hand, components are changing — they are smaller, they have
greater numbers of leads, they have different mounting requirements. Printed
circuit boards reflect this — they too are smaller, with narrower connecting tracks
and different characteristics. Consequently, as components and their printed
circuit boards change so must the ways they are soldered together change.
Many estimates of how electronics assembly will change over the coming
years have been made. While differing marginally in specific details, they all
point to the fact we are in the middle of a colossal movement away from
conventional through-hole assemblies using leaded components, towards sur-
face mounted assemblies with only leadless components. BPA (Technology &
Management) of the UK, for example, has shown how — in 1987 — less than
20% of European and North American assemblies featured any leadless surface
mounted components at all. Figure 1 shows this together with figures for 1992 and
1997. Currently some 40% of assemblies feature surface mounted components.
In 1997, however, we can expect over 60% of assemblies to be so. In terms of
■jf^y-fewM'^
13 loot
Rest of world
2 i*
90
^ South-east Asia
80
H USA
&
70
111 Europe
60 |_
tiJ Japan
50
40 |_
30 L
20 L
Figure 1 Estimates of electronics
assemblies worldwide which 10
feature leadless components of any
type, in terms of percentages of
totals (BPA, 1992) 1987 1992 1997
X PREFACE
sheer numbers of surface mounted assemblies, Figure 2 shows trends over the
current ten years. Interestingly, values for Japan in both Figure 1 and Figure 2 are
significantly higher than either Europe or North America.
1800
Japan
„ 1600
o
g 1400
Rest of world
si 1200
USA
1000 Europe
800
South-east Asia
600
400
Figure 2 Estimates of electronics
assemblies worldwide featuring 1987 1992 1997
leadless components, in terms of
total numbers (BPA)
Yet this is only half the story. While many of yesterday's and today's
assemblies are mixed ie, they feature both leaded and leadless components,
tomorrow's assemblies will tend to be totally surface mounted, with very-few
leaded components at all. Figure 3 shows trends of three common types of
assemblies from 1987 to 1997. While mixed technology types are in a process of
reducing in numbers, pure surface mounted assemblies are increasing. By the turn
of the century we might reasonably expect there to be only minimal through-hole
assembly manufacture to speak of at all.
iL _
1S 5 01
□ 1987
<4m
□ 1992
S40
9 ■ 1997
#
30 |||
III
20
III
10 111
111
Type II — leaded Type III — leaded
components alone ccoommppoonneennttss oonn tloopp,, components on top,
Figure 3 Estimates of electronics leadless components on leadless components on
assemblies worldwide of three bottom top and bottom
main asembly types, in terms of
percentages of type totals (BPA)
PREFACE xi
This trend towards surface mounted technology is just the first step in
miniaturization of products. Surface mounted components themselves are smaller
than their leaded counterparts. Ways in which surface mounted assemblies can
be manufactured allow surface mounted components to be positioned closer
together. Accurate automatic assembly of surface mounted components means
components with greater lead-counts may be used. Greater lead-counts mean
narrower connecting tracks must be used. Narrower tracks mean that components
can be more closely spaced — and so the cycle goes on! Products like personal
computers which five years ago took up three-quarters of your desktop now sit
on your lap, yet feature the same computing power. In five years time...
Finally, as products become smaller they stretch the boundaries of the very
technologies used to manufacture them. Smaller components with greater lead-
counts make soldering to printed circuit boards more and more difficult. New
soldering techniques and adaptations of older techniques are constantly in
development.
Our job in writing this book is to show all this. While looking at the principle
of soldering, which has remained fixed since man first used molten metal to join
two other metals (first known use of solder is Roman pipework, where sections
of lead pipe were joined by melting their ends together), we also show how the
techniques to fulfill the soldering process can change and, indeed, are changing
with the requirements placed by components and printed circuit board assemblies
on them.
More than anything else these days (and in the future), state of component and
printed circuit board technologies means soldering has to be clean and precise.
In the distant past, when components were huge and printed circuit board tracks
were wide, soldering was often a case of throwing sufficient molten solder at a
board to make an adequate joint. Joints were initially made by hand. This is no
longer the case. With minute component terminations and tracks, too much solder
can be devastating. On the other hand, too little solder will not make a joint at all.
Even if economic considerations were not important, joints can no longer be
made by hand because hand soldering cannot guarantee adequate results with
such small joints. Control of soldering systems has to be precise and closely
monitored. Soldering and, inevitably, its control must be performed by machines.
While it's impossible to summarize all we have to say, in this short preface, it
is possible to make a list of points which our book, hopefully, spotlights. Overleaf
we present the 10 rules of machine soldering.
Acknowledgements
We'd like to thank a number of people who have helped us in major ways, as we
have researched and written this book. In no particular order of priority, gratitude
goes to:
# William Down; Electrovert USA
# Don Elliott; Electrovert, Canada
# Peter Grundy; Siemens, UK
# Karen Moore-Watts, DEK Printing Machines, UK
# Alan Keyte, Motorola, UK
# Andrew Nicholson, Alpha Metals, UK
# Fred Thorns, Fry's Metals, UK
# Phil Fulker; Hollis Europe
# Norman Hodson; Dynapert UK
# Richard Hart; ex Multicore Solders
# Colin Lea; National Physical Laboratory, UK
# Alan Roberts and Claude Legault
# many people within the Electovert group worldwide.
Finally, special thanks go to Bob Willis of Electronic Presentation Services in the
UK, for help and guidance throughout.
This book is dedicated to a respected man, considered by many to be a guru in
electronics soldering. Paul Bud, Technical Director of Electrovert is sadly
missed. He was a kind man and a gentleman, always willing to listen and pass on
his wide knowledge of soldering, communicating in many languages. He was an
inspiration to us all.
In his memory, we hope the book proves helpful to all who read it.
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10 rules of machine soldering
1 machine soldering, correctly controlled, produces highest quality
joints at lowest cost
2 machine soldering is a process and, like all processes, produces
consistent results if properly controlled
3 touch-up of faulty soldered joints is costly, unreliable and unneces-
sary
4 anything that reduces solder joint defects is cost effective
5 design, handling, assembly and maintenance are all parts of the
soldering process and must be properly controlled
6 solderability of printed circuit boards and components accounts for
60% of all faulty soldered joints
7 never use parts which fail solderability testing: the ultimate cost is too
high
8 soldering problems are solved by process control — not more inspec-
tion and touch-up
9 everyone concerned with the soldering process must be formally and
properly trained
10 zero-defect soldering is the lowest cost soldering.
1 Soldering process
This chapter could be seen as just another introduction to soldering — it does,
after all, explain in a fairly basic manner all important aspects of soldering in
electronics assemblies. In this light many people may be tempted to skip it.
However, this would be a mistake as it also explains philosophy behind the
complete book. Further, it discusses main features and illustrates fundamental
premises upon which we have based our text. Finally it also serves as a guide
indicating where relevant and important information is to be found within the rest
of the book. We strongly advise this chapter is looked at bearing all this in mind.
Soldering, in principle though not in practice, is a reasonably straightforward
process, used in the electronics industry to bond components together, forming
one or more electrical connections. From this description, it's easy to see that
soldering serves two functions:
# mechanical support — holding components of an assembly together
# electrical support — forming required electrical connections within a circuit.
Most components in an assembly use the mechanical support of soldered joints
alone to give adequate fixing into the assembly. A few isolated components
(notably, larger, heavier components) may require additional mechanical sup-
port, in the form of straps, nuts, bolts and so on. Where possible, however, such
large components should be designed out (that is, care should be taken when
designing a circuit not to use such components) of an assembly to keep extra
procedures and cost to a minimum.
On the other hand, all components may use solder as electrical support to form
required electrical connections. No other method has yet been devised to take the
place of solder in all assemblies to the same level of performance, cost and ease
of operation.
Time on its side
Solder in one form or another has been around for a long time. The Romans are
known to have used solder to form joints in their plumbing systems and, indeed,
the word plumbing refers to the use of lead (from the Latin plumbum meaning
lead) as a jointing compound. Nowadays, of course, pure lead plumbing is no
longer considered, instead solder — which is an alloy of mostly tin and lead —
is used. It's interesting to note that certain countries are already in the process of
altering legislation to prevent lead being used in any plumbing where drinking
water is present.
2 SOLDERING PROCESS
Solder has been adopted by the electronics industry as the best method of
making joints within assemblies and, for many years, this jointing process was
undertaken manually — hand soldering. Inevitably hand soldering is a slow,
laborious, time-consuming and hence expensive process as each component must
be soldered into position to the printed circuit board individually. Quality and
repeatability of joints depend almost totally on the individual operator. This
clearly makes cost of large-scale electronics assembly production uneconomical
and unsatisfactory. Generally, therefore, hand soldering of electronics assemblies
is undertaken only in development and prototype stages, although there remains
a situation in which small-volume production of electronics assemblies is
economical. Hand soldering is used regularly in rework stages of manufacture,
where assemblies need to be partially disassembled for repair and service
purposes. Because of this situation we need to consider hand soldering and so it
is discussed in Chapter 5.
For the last 40 years or so, various methods of automating soldering processes
have been developed. It's easy to see how vitally important automated soldering
of assemblies has become, by remembering the space programme of getting a man
on the moon could not have been achieved without it.
Paul Bud, to whom this book is dedicated, related an interesting fact about
NASA printed circuit boards used in spacecraft. Had NASA's assemblies been
hand soldered, joints would have held more solder with the result that assemblies
Component
Component lead
Solder fillet
•.lV^Vl* «.*■.*«.*;■.*■. V,
Printed circuit board
base material ■'ft!
Figure 1.1 Soldered joint formed n
between a component lead and Copper track Plated through-hole 111 , ..
copper track on a plated through-
hole printed circuit board
TIME ON ITS SIDE 3
would have been 10% heavier. So many assemblies are used in spacecraft that
with this additional weight, the three astronauts (totalling, say, 250 kg) could not
have been carried!
Until just recently, however — say, the last ten years' — soldering of really
only one type of electronics assembly was in the main undertaken. This assembly
type, a typical joint of which is illustrated in Figure 1.1, is commonly called the
through-hole printed circuit board, simply because the components feature
leads which are inserted through holes in the printed circuit board. Copper track
interconnections exist between holes which make up the circuit of an assembly
and component leads are soldered to the copper, so that mechanical and electrical
support is provided with each solder joint. Note that, in this ideal joint, solder has
been drawn inside the hole during the soldering operation — this occurs by
capillary action. Solder between the copper track and the component lead is called
the fillet.
Emergence of a different type of electronics assembly — surface mounted
assemblies or SMAs — however, is altering the face of soldering processes used.
So much is surface mount technology (SMT) changing soldering that, at
present, somewhere around 50% of all electronics assemblies use at least a few
surface mounted components (SMCs — also sometimes called surface mounted
devices or SMDs). In 1980 the figure was, to all practical purposes outside of
Japan, zero. This change in assembly technology has only been possible with
developments bringing a parallel change in soldering technology. Changes in
component technology, too, are pushing many companies along the surface
mount route — a growing number of components simply cannot be obtained in
through-hole forms and are only available as surface mounted components.
Figure 1.2 shows a typical surface mounted assembly joint, in which a surface
mounted component is soldered to a printed circuit board. Difference between
this and the through-hole assembly joint of Figure 1.1 is immediately apparent —
Surface mounted
Solder Terminal component
fillet
Figure 1.2 Soldered joints formed Copper track Printed circuit board
between component terminals and base material
copper track on a surface mount
assembly printed circuit board
