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BOOKSHELF
Surface Mount Technology
Surface Mount
Technology by
Rudolph Strauss.
Published in 1994
by ButterworthHeinemann, Oxford.
Hard covers, 361
pages, 240 x 160mm,
ISBN 0-7506-1862-0,
$99.00
This book will provide informative reading for any person presently
involved with wave soldering equipment for through-hole mounting
components or anyone considering
the assembly of PC boards with surface
mounted devices (SMD's).
In the first chapter the Author asks
the same question we all ask. Why
SMD's? What is wrong with existing
components? But after a moment's
thought we realise that components
have always been getting smaller.
From valves to transistors to integrated
circuits, the package size has shrunk
dramatically.
Microprocessors have gone from
16 pin devices to 40 pins to 169
pins and so on. The trend to higher packaging density of IC's and to
higher frequency operation, requires
shorter IC interconnecting leads and
components with very short (low
inductance) leads. Thus SMD's have
come to the rescue.
In chapter 2 the Author discusses
SMD shapes including MELF's (Metal Electrode Faced Components),
chips, small outline devices (Transistors-SOT's, Integrated Circuits-SOIC's
etc) and discusses their solderability
and mechanical stability in wave
soldering machines. Chapter 3 covers soldering methods, fluxes, solder
composition, the effects of impurities
in solder baths and the properties of
soldered joints.
Chapter 4 describes wave soldering
in detail, including the relative simplicity before SMD's. Wave soldering is
a technique where the PC board, with
all components physically mounted or
glued, is passed over a wave of molten
solder. The solder is pumped through
a vertical nozzle, usually the width of
the machine and as it flows up and out
the PC board is moved through this
"wave". Other topics in this chapter
include fluxes, board preheating, the
solder wave, oxygen-free atmospheres
and the role of adhesives.
Chapter 5 covers reflow soldering.
What is the difference? In wave soldering, as in hand soldering the flux
comes first, the solder and heat come
at the same time. With reflow, the heat
comes last, the flux and solder or solder paste having already been placed
on the PC board and sometimes on the
component leads as well.
The Author points out reflow soldering is not new. Plumbers use reflow
when joining modern copper capillary
fittings. These are supplied with an
insert of flux and solder. The plumber
only needs to clean the copper pipe he
intends to use, push it into the fitting
and apply heat. The flux and solder
inside the fitting will now do their job
and make a waterproof joint.
Strauss then discusses one pass
and two pass soldering, where the
PC board is reflowed on one side
then turned over and the second side
soldered. You would think that the
components on the bottom would fall
off on the second pass and they will if
they are too heavy. Believe it or not,
the surface tension of the softened
solder holds the smaller components
in place.
One layout requirement for two
pass SMD's is to keep all the heavy
components on one side of the board.
The rest of the chapter covers various
methods of reflow, including vapour
phase, infrared, hot air or gas, laser
and impulse.
Chapter 6 details the requirements
for PC boards for SMD's. This covers
layout procedures, which should take
into account the direction of the board
through the solder machine as most
components will be more mechanically stable in one orientation.
Chapter 7 discusses the placing of
components on the PC board, from
manual methods to fully automatic
placement. Chapter 8 discusses the
methods used to clean the boards after
soldering.
Historically CFC's have been used
but now they have been phased out
due to their environmental unfriendliness; new cleaning compounds and
methods have evolved.
The final chapters cover quality control, inspection and rework. These are
interrelated, for obviously, the better
the quality control the less rework.
Inspection before soldering and rectification of faults can be far easier and
cheaper than repairing the same fault
after the solder bath.
To sum up, this is a very interesting book, written by an Author who
knows his subject - a must for the
production manager of any electronics
manufacturing company not yet into
SC
SMD. (R.J.W.)
August 1995 53
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