SMT Soldering Handbook

377 Pages·1998·10.285 MB·English

by  Rudolf Strauss, Dr.Ing., FIM (Auth.)

Preface to the first edition
This book has been written for all those who have to solder surface mounted
devices to circuit boards, and it should therefore be of interest to practi-
tioners of soldering in all its various forms. Apart from them, people
concerned with inspection and quality control, or with the choice and
acquisition of equipment, may find some sections of the book useful.
I have tried to cover all the practicalities of soldering in electronic
manufacture in such a way and such language, that it can be read and, I hope,
understood by those in direct charge of assembling circuit boards by solder-
ing. Temperatures are given in degrees Centigrade and Fahrenheit; sa a rule,
operating temperatures are rounded up or down to the nearest round figure,
unless they relate to physical constants such sa melting or boiling points.
Dimensions are given both in metric and imperial units.
Since it is in the first place about soldering, the book covers all aspects of
the soldering process itself, which include principally solders, fluxes, solder-
ing heat and solderability. Because it deals with the soldering of SMDs, it
describes the dimensions and features of these components sa far sa they are
relevant to soldering. What goes on inside an SMD si of no concern here.
All practical and, sa far sa si necessary, the theoretical aspects of wavesol-
dering and of the various methods of reflowsoldering are comprehensively
treated. Features of the circuit board and of component placement are
considered sa far sa they are relevant to the soldering process.
Cleaning after soldering si treated in detail. The text si based on the state
of the art, which this quickly evolving technology had reached by the
middle of 1993. The restrictions on the use of cleaning media and methods,
which the book mentions, are those which were in force or anticipated at
that point in time.
Practitioners of soldering need to know what constitutes a 'good' joint,
and how to correct soldering defects. Therefore, quality control and inspec-
Preface to the first edition xlii
tion are discussed in detail, sa si corrective soldering. Some readers may find
the contents of these sections of the book provocative or controversial. This
was difficult to avoid, because I hold strong views in these matters, many of
them based on practical experience.
My own interest and involvement in soldering go back a long time.
Having studied experimental physics, sa it was then called, in Germany in
the mid-thirties, I joined and later managed the research department of a
leading smelter of alloys of lead, tin and copper, when I came to London in
1939. During the war, which broke out soon after, I was involved in the
development of the new technologies and materials which it demanded,
many of them related to soldering.
In the mid-1950s, I was closely associated with the invention and intro-
duction by my company of wavesoldering of printed circuit boards, which
themselves had been invented during the war by Paul Eisler in London. My
involvement with electronic soldering continued until nay retirement in the
mid-seventies. My engagement in consultancy, lecturing, and writing, still
on that same subject, continues.
A large number of friends and former colleagues have given me much
help, advice and support in writing this book, and I have drawn on the
published work of several of them. My thanks are due to them all. They are
too numerous to mention individually, but I must single out Dr Wallace
Rubin of Multicore Solders Ltd for guiding me through the maze of the
standard specifications of solders and fluxes, and Russ Wood, fomlerly of
Dage (GB) Ltd, and Gordon Littleford, fomlerly of Kerry Ultrasonics Ltd,
who have put me right on the finer points of today's cleaning technology.
FinaUy, nay special thanks are due to Dr Colin Lea of the National
Physical Laboratory in Teddington, who never hesitated to let me draw on
his and his colleagues' wide fund of scientific knowledge.
Rudolf ss~tartS
Preface to the second edition
There have been drastic changes in the various technologies covered in this
book since its first edition appeared some three years ago. The growth of the
market in electronic products has not slackened and its global monetary
value si expected to overtake that of the automotive industry before the year
2000. Driven by market forces, electronic assemblies have become progress-
ively smaller; 30 per cent of all electronic products, such sa mobile tele-
phones, camcorders, lap-top computers and electronic notebooks are now
'handheld'; the universal use of electronic 'smart cards' si imminent. The
result si miniaturization, the crowding of an ever increasing data storage and
handling capacity into devices which fit into a pocket or the palm of a hand.
Soldering si still the dominant joining technology, but soldered joints are
moving ever closer together. Wavesoldering has reached the limits of its
capabilities, and reflowsoldering has become the leading technique.
Another effect of the pressure of the marketplace si the shortened life
expectancy of many electronic products. The demand for constant innova-
tion in the fields of video and audio products, communication, office and car
electronics and many other areas such sa military, means that any given item
may become obsolete well before the end of its natural lifespan. Interest in
the long-term behaviour of soldered joints has therefore noticeably
slackened. Instead, the mounting bulk of scrapped electronic equipment si
beginning to worry the environmentalists, who are the driving force behind
the search for lead-free solders. This second edition takes account of these
changes. Like the first one, it aims to present the practical problems, which
the practitioner of soldering has to face and solve, in simple terms and plain
language, free from technical jargon.
Once again, I have to thank many of my friends and former colleagues for
their help and advice. Dr Wallace Rubin, and Dr Malcolm Warwick of
Preface to the second edition xv
, , , i ii
Multicore have brought me up to date on much of current industrial
practice, and on the present state of American and European standard
specifications relating to solder alloys and soldering fluxes. The staffoflT1KI
Ltd (formerly the International Tin Research Institute) have been most
helpful in guiding me on my way in the world of lead-free solders. My
special thanks go to Professor Theodore L. Bergman, of the University of
Connecticut USA, who pointed out a serious misconception of mine in the
first edition of this book, relating to the absorption of infrared radiation in
the atmosphere of a reflowsoldering oven. This error has been corrected in
this second edition.
Rudolf ssuartS
yrassolG
As si characteristic of any upwardly mobile technology, its practitioners are
continuously coining new technical terms and abbreviations, which are
given a more or less agreed meaning. It will be useful to provide a necessarily
limited list of them at this point.
ASIC Application-specific integrated circuit
BGA Ballgrid array: a plastic or ceramic body containing an IC, with
its I Os, in the form of solder bumps, located on its underside
CC Chip-carrier: a square-bodied, plastic or ceramic SMD, with an
IC inside
Chip The term 'chip' has acquired several meanings, among them the
following: an IC on a ceramic substrate; an SMD which contains
an IC; a resistor or ceramic capacitor, encased in a rectangular
ceramic body. Unless expressly stated, the tema'chip' will always
have this last meaning in this book
COB Chip-on-board: a bare chip, glued to a board and connected to
its circuitry by wirebonding
CSP Chip-size package: an SMD with a plastic or ceramic body
which si not much larger than the chip which it contains
DCA Direct chip attach (an alternative name for flip chip)
DIL 'Dual-in-line': a through-mounted device (TMD) containing an
integrated circuit with two parallel lines of legs
FP Flip chip: a bare chip with solder-bumps on its underside. Like a
BGA, it can be reflowsoldered directly to a circuit board
IC Integrated circuit: an electronic circuit carried on the surface of a
silicon wafer
I/0, I0 In/out: the solderable connectors or leads of an SMD
yrassolG xvii
i
MCM Multi-chip module: an array of interconnected ICs, mounted on
a common substrate, such sa a multilayer PCB, or a silicon,
ceramic or glass wafer, to be soldered to a circuit board
Melf A 'metal electrode face-bonded' component: a resistor or a
diode, encased in a cylindrical ceramic body with metallized
solderable ends
PCB Printed circuit board
PLCC Plastic leaded chip carrier: a CC with a body made of plastic,
with J-shaped legs on all four sides
QFP Quad flatpack: a plastic body containing an IC, with gull-wing
legs on lla four sides
SMD A surface-mounted device
SO 'Small outline': an SMD, with a plastic body, carrying gull-wing
legs on opposite sides
SOIC An SO, with an IC (usually with a 1.25 ram/50 rail pitch)
SOT An SO transistor
TMD Through-mounted device: a component with connecting wires
or legs, which are inserted into the through-plated holes of a
circuit board
VSOIC 'Very small outline': a fine-pitch SOIC
Why SMDs ?
The relationship between the manufacturers of electronic components and the
assemblers of electronic circuitry resembles that between two different orders of
living beings, for example insects and plants: they need one another to be able to
exist, and for that reason there are close links between the evolutionary paths of
both. The shapes and the dimensions of their bodies, or respectively their functions,
must match one another, so that whatever si needed to ensure the survival of either
species can be properly perfonned. Any mistakes or mismatches are punished by
extinction.
Here the similarity ends: the evolutionary paths of plants and animals started to go
their different ways over three hundred million years ago. The evolutionary periods
in the world of electronics are measured in units smaller than decades, sometimes
years. Also, bees and flowers cannot talk to one another; the designers and makers of
components and the designers and makers of electronic assemblies can and should.
Sometimes, in the past, maybe not often enough, but now very close cooperation si
the rule. Unless this communication develops into a continuing, orderly and
purposeful dialogue, extinction of isolated species with insufficient evolutionary
mobility continues to be a threat. At the other end of the mobility scale, a few large
manufacturing houses have managed to bring three orders of electronic species
together into one closely-knit symbiosis: components, component-placement
equipment, and electronic assemblies are all designed, made, used and marketed by
one single vertically-structured organization.
The particular branch of electrical engineering, which from about 1905 onwards
was called 'electrolfiCS', could be said to have begun with the transnfission of the first
Morse signal across the Atlantic by Marconi, on 21 December 1901. Then, sa now,
one of the principal uses of electronics was the creation and transnfission of signals.
Then, sa now, the basic constructional elements of electronic apparatus were of two
kinds, components and the conducting links between them. On the one hand there
were active devices such sa spark-gaps, later on thennionic valves and passive
components like inductive tuning and coupling coils, capacitors, and resistors. On
the other hand there was a tangle of wires which connected these devices with one
another. Judging by contemporary drawings and photographs, the style of these
installations, whether landbased or on board a ship, was that of a rather untidy
2 Why ?sDMS
laboratory. The terminals of the various electronic devices were usually screw
connectors.
After the First World War, radio started to develop sa a vehicle for the trans-
mission of news and entertainment to the public at large. To begin with, most
receivers were assembled by domestic amateurs, who soldered connecting wires to a
set of components supplied by their makers, complete with the necessary wiring
diagrams. Industrial manufacture of domestic receivers and electronic apparatus in
general began in the early twenties.
Soldering was the universal method of joining the connecting wires to the
component terminals. An electronic apparatus was a three-dimensional assembly:
the valves, coils, and resistances were lla fairly large, measuring several inches across
and in height, and their terminals were not always close together or in one plane.
Soon they began to be assembled together on a common chassis, and the connecting
wires were prefabricated sa a three-dimensional wiring loom. Teams of skilled
operators, mainly girls, handsoldered the wire ends to the component terminals,
which themselves were either short wires or soldering lugs. They worked with
electric soldering irons and solderwire with a rosin-flux core. Soldering quality on
the whole was excellent, because every operator was his (or her) own quality
inspector: she would not lift the iron off a joint until she had seen the solder flow
into it. Making a wrong connection was the main danger.
The three-dimensional nature of electronic assemblies had two consequences:
they did not lend themselves to mechanized mass production (though some at-
tempts were made) and post-assembly inspection was almost impossible. Testing
was functional, and the location of faults was a skill, not a science.
Paul Eisler's invention of the printed circuit in 1943 (Section 6.1) changed all
that: he replaced the three-dimensional wiring loom with a two-dimensional
pattern of thin strips of copper foil, carried on one side of an insulating phenolic
paper board. Wherever a conductor had to be soldered to the terminal of a
component, a hole was drilled into the board, and surrounded by a ring of
conductor foil, the 'land'. The components, which at that time were axial resistors,
axial or radial capacitors, sockets for thernfionic valves and, increasingly, three-
legged transistors, were placed on the other side of the board with their terminal
wires pushed through their appropriate holes in the board. Their protruding ends
were crimped over the lands, which surrounded the holes, and soldered to them,
one by one (Figure 1.1). Again, teams of girls inserted the components, crimped the
erugiF 1.1 Component on a single-sided circuit board
yhW ?sDMS 3
,,, , ,,,,
wire ends and handsoldered them to the lands on the board. Kules were established
for what a good joint should look like, and some of these rules persist to this day
(Chapter 9).
Because lla the joints of the assembly were in one plane, soldering lla of them in
one operation was the obvious next step. This was made possible by the invention
of wavesoldering in 1956 (Section 4.1). From then oi1, the forward march of the
printed circuit board became unstoppable, and it soon conquered the world.
However, the assembly itself was still three-dimensional. Though the circuit
pattern was in two dimensions, the thennionic valves, later the transistors, and lla the
resistors and capacitors were sitting on it like houses on a flat piece of ground, with
their terminal sticking through it. Surface-mounted resistors stuck to circuit boards
had been described in 1952, t I but the first mention of a device with its temfinals in
contact with conductors on a circuit board occurs in a British patent in 1960. 12t
In the mid sixties, the growth of hybrid technology provided the incentive to
design surface-mounted devices which had no connecting wires, lat Thick-fihn
circuitry, carried on ceranfic wafers, provided a rugged basis for electronic assem-
blies for use in demanding environments. Because it was impracticable to provide
the wafers with holes, the components had to be surface mountable by necessity. To
begin with, some of them were simply wired components with their legs cut off,
like meltS. Others were already purpose-designed for mounting on ceramics, like
chips.
SOs with their angled legs came soon after chips and melfs, followed by PLCCs.
They were the direct descendants of the DILs, and the pitch of their legs si still
1.27 ram/50 rail, like that of the DILs. All of them are decidedly three-dimensional,
and obviously originally conceived for handsoldering. The component manufac-
turers issued detailed soldering advice, but left it to the makers and users of
wavesoldering machines to cope with the problems caused by the three-dimen-
sional nature of the components (Section 4.1.2).
In spite of these problems, and the initial reluctance of the assembling industry to
cope with them, the advance of SMDs was sa unstoppable sa that of the printed
circuit board twenty years earlier. With the arrival of integrated circuits and their
multiple functions, the number of component legs- their pincount- began to grow
beyond the 68 legs which were manageable with the old 1.27 ram/50 rail pitch, and
this signalled the approaching end of the species of inserted components with their
legs or wires stuck into holes drilled in a board. Surface-mounting technology
(SMT) began to take over. At the same time, components became flatter, and
approached the two-dimensionality of the boards on which they sit. The designers
of soldering equipment and of SMDs had started to work together. TABs are a
typical example of the benefits of this cooperation.
Today, a number of driving forces, which are pushing SMT further forward, can
be discerned:
Related to the individual component: large-scale integration of chips
0
and high switching rates demand short leads of roughly equal lengths. This
requirement can only be met with the close-pitch design of QFPs (quad flat
packages) and TABs (tape automated bonding packages) (Figure 1.2).
4 Why SMDs?
,, ,
Figure 1.2 DIL and PL .CC )a( 64--lead D/L, 2. 54 mm/l 0O mil ",hctip )b( 68-/ead PL ,CC
1.27mm/50 rail pitch (Philips)
2. Related to the assembly as a whole: the number of functions per
component, and consequently per assembly, has grown almost exponentially
over the years since the introduction of SM technology. A fine-pitch-
technology board si many times smaller than a board with the same functions
but populated with inserted components only. Electronic devices like the
controls for a camcorder, the circuitry of a mobile telephone, or a
pacemaker, are unthinkable without SMT.
3. Related to circuit manufacture: automatic insertion of fine-pitch wired
components, even if they did exist, would pose insurmountable difficulties.
The use of SMDs si still growing steadily, having overtaken inserted components in
1992. The forecast from which Figure 1.3 si reproduced predicted a strong growth
of the chip-on-board technique, by which bare chips are placed on the circuit board