Design Guidelines for Surface Mount Technology

313 Pages·1990·8.136 MB·English

by  John Traister (Auth.)

Design Guidelines for
Surface Mount Technolo
John E. Traister
Bentonville, Virginia
ACADEMIC PRESS, INC.
Har court Brace Jovanovich, Publishers
San Diego New York Berkeley Boston
London Sydney Tokyo Toronto
This book is printed on acid-free paper, (oo
Copyright © 1990 by Academic Press, Inc.
All Rights Reserved.
No part of this publication may be reproduced or transmitted in any form or
by any means, electronic or mechanical ,including photocopy, recording, or
any information storage and retrieval system ,without permission in writing
from the publisher.
Academic Press, Inc.
San Diego, California 92101
United Kingdom Edition published by
Academic Press Limited
24-28 Oval Road, London NW1 7DX
Library of Congress Cataloging-in-Publication Data
Traister, John E.
Design guidelines for surface mount technology / John E. Traister.
p. cm.
Includes index.
ISBN 0-12-697400-4 (alk. paper)
1. Printed circuits-Design and construction. 2. Surface mount
technology. I. Title.
TK7868.P7T73 1989
621.381'531--dc20 89-6788
CIP
Printed in the United States of America
89 90 91 92 9 8 7 6 5 4 3 2 1
PREFACE
There is a growing trend in the electronics industry away from
mounting components on printed circuit boards by inserting the
component leads through holes in the boards and then soldering. The
trend is to replace this method with a process called surface mount
technology, where the component leads, or terminals, are soldered to
the top surface of the boards. This trend has accelerated recently due
to the rise in the use of high density packages that have greatly reduced
terminal spacings. These reduced spacings make through-hole insertion
undesirable; hence the move to surface mounting, which makes the
boards easier to build, increases their reliability, and cuts labor and
manufacturing costs at the same time.
Design Guidelines for Surface Mount Technology was developed
and written to address the needs of those using this new technology.
Starting with the basics—component selection, space planning,
materials and processes—before the mechanics of surface mounted
design will give the PC designer/engineer the total concept needed to
ensure a manufacturable design. The author's aim is to provide data
of the greatest importance from the vast amount of available material
and to arrange this material in a way to be helpful in solving problems
and difficulties likely to be encountered in the daily work of those
involved in surface mounted technology.
The treatment of some subjects has necessarily been brief, but
subjects of greater importance have been dealt with more fully. The
various tables and charts have been selected with great care, and only
those that are most likly to be consulted have been included. The
numerous rules and equations are stated as simply and concisely as
xiii
xiv CONTENTS
possible, and their applications are clearly illustrated by th efull
solution of many examples.
Care has been taken to arrange the chapters in a convenient and
logical manner, and a very full index further increases the facility with
which any given subject may be located.
I am indeed grateful to the many manufacturers who supplied
reference material for use in this book. Names and addresses of these
manufacturers appear in Appendix I. I am especially indebted to
NuGrafix Group, Inc. of Los Gatos, California, Signetics of Sunnyvale,
California, and Universal Instruments Corporation of Binghamton,
New York, for use of illustrations, charts, and tables of their products.
A special word of thanks is due John Karns of Martinsburg, West
Virginia, who provided many of the drawings for this book. Ruby
Updike was responsible for much of the research, typing, and much-
needed encouragement.
JOHN E. TRAISTER
CHAPTER 1
SMD ESSENTIALS
S
urface mount technology embodies a totally new automated circuit as­
sembly process, using a new generation of electronic components: sur­
face mounted devices (SMDs). Smaller than conventional components,
SMDs are placed onto the surface of the substrate, not through it like leaded
components. And from this, the fundamental difference between SMD as­
sembly and conventional through-hole component assembly arises; SMD
component positioning is relative, not absolute.
When a through-hole (leaded) component is inserted into a printed circuit
board (PCB), either the leads go through the holes, or they don't. An SMD,
however, is placed onto the substrate surface; its position is only relative to the
solder lands. Placement accuracy is therefore influenced by variations in the
substrate track pattern, component size, and placement machine accuracy.
Other factors influence the layout of SMD substrates. For example, will the
board be a mixed-print (a combination of through-hole components and SMDs)
or an all-SMD design? Will SMDs be on one side of the substrate or both? And
there are process considerations like what type of machine will place the com­
ponents and how will they be soldered?
Designing with SMD
SMD Technology is penetrating rapidly into all areas of modern electronic
equipment manufacture; in professional, industrial, and consumer applica­
tions. Boards are made with conventional print-and-etch PCBs, multilayer
2 SMD ESSENTIALS
boards with thick film ceram­
ic substrates, and with a host
of new materials specially
developed for SMD as­
sembly.
However, before substrate
layout can be attempted,
footprints for all components
must be defined. Such a foot­ r x^y χ
print will include the com­
bination of patterns for the
copper solder lands, the sol­ (a)
der resist, and possibly, the
solder paste. So the design of
a substrate breaks down into t
two distinct areas: the SMD
footprint definition, and the
layout and track routing for i(b)
SMDs on the substrate.
Substrate Configurations
SMD substrate assembly
configurations are classified
as:
Type I — Total surface
Fig. 1-1: (a) Type 1—total surface
mount (all-SMD); sub­
mount substrates; (b) Type
strates with no through-
IIA—mixed print (double-sided)
hole components at all.
substrate; (c) Type IIB—mixed
SMDs of all types (SM in­
print (underside attachment) subs­
tegrated circuits, discrete
trate.
semiconductors and pas­
sive devices) can be mounted either on one side, or both sides of the substrate
as shown in Fig. 1-1 (a).
Type IIA — Double-sided mixed-print; substrates with both through-hole
components and SMDs of all types on the top, and smaller SMDs (transistors
and passives) on the bottom. See Fig. l-l(b).
Type IIB — Underside attachment mixed-print; the top of the substrate is
dedicated exclusively to through-hole components, with smalle rSMDs
(transistor and passives) on the bottom as shown i nFig. l-l(c).
DESIGN GUIDELINES FOR SMT 3
Although the all-SMD substrate will ultimately be the cheapest and smal­
lest variation as there are no through-hole components, it's the mixed-print
substrate that many manufacturers will be looking to in the immediate future,
for this technique enjoys most of the advantages of SMD assembly, and over­
comes the problem of non-availability of some components in surface mounted
form.
The underside attachment variation of the mixed-print (type IIB — which
can be thought of as a conventional through-hole assembly with SMDs on the
solder side) has the added advantages of only requiring a single-sided print-
and-etch PCB, and using the established wave soldering technique. The all-
SMD and mixed-print assembly with SMDs on both sides require reflow or
combination wave/reflow soldering, and in mos tcases, a double-sided or mul­
tilayer substrate.
The relatively small size of most SMD assemblies compared with equivalent
through-hole designs means that circuits can often be repeated several times on
a single substrate. This multiple-circuit substrate technique, shown in Fig. 1-2,
further increases production efficiency.
II II II II II II
=[• II : 1 II 1 II
h
Ζ 1
Js •
. Si L
'——11 1f— 1ι 1r——.' . II 11 . ^——JI 11 11 1i— ' .
41 ii I II : I II
L
Js • 5 ι
. si L
m:
II II II II
II II
Fig. 1-2: Multiple-circuit substrate.
Mixed Prints
The possibility of using a partitioned design should be investigated when
considering the mixed-print substrate option. For this, part of the circuit would
be an all-SMD substrate, and the remainder a conventional through-hole PCB
or mixed-print substrate. This allows the circuit to be broken down into, for
example, high and low power sections, or high and low frequency sections.
4 SMD ESSENTIALS
Automated SMD Placement Machines
The selection of automated SMD placement machines for manufacturing
requirements is an issue reaching far beyond the scope of this book. However,
as a guide, the four main placement techniques are outlined as follows:
In-line placement — a system with a series of dedicated pick-and-place
units, each placing a single SMD in a pre-set position on the substrate. Gener­
ally used for small circuits with few components. See Fig. l-3(a).
Sequential placement — a single pick-and-place unit sequentially places
SMDs onto the substrate. The substrate is positioned below the pick-and-place
unit using a computer controlled X-Y moving table (a "software programma­
ble" machine). See Fig. l-3(b).
(c) .(d)
Fig. 1-3: SMD Placement machines: (a) in-line
placement; (b) sequential placement; (c) simulta­
neous placement; (d) sequential/simultaneous
placement.
DESIGN GUIDELINES FOR SMT 5
Simultaneous placement — places all SMDs in a single operation. A
placement module (or station), with a number of pick-and-place units, takes an
array of SMDs from the packaging medium and simultaneously places them on
the substrate. The pick-and-place units are guided to their substrate location by
a program plate (a "hardware programmable" machine), or by software con­
trolled X-Y movement of substrate and/or pick-and-place units. See Fig. 1-
3(c).
Sequential/simultaneous placement — a complete array of SMDs is
transferred in a single operation, but the pick-and-place units within each
placement module can place all devices simultaneously, or individually (se­
quentially). Positioning of the SMDs is software controlled by moving the sub­
strate on an X-Y moving table, by X-Y movement of the pick-and-place units,
or by a combination of both. See Fig. l-3(d).
All four techniques, although differing in detail, use the same two basic
steps; picking the SMD from the packaging medium (tape, magazine or hop­
per), and placing it on the substrate. In all cases, the exact location of each
SMD must be programmed into the automated placement machine.
Soldering Techniques
The SMD populated substrate is soldered by conventional wave soldering,
reflow soldering, or a combination of both wave and reflow soldering. These
techniques are covered at length in Chapter 4, but briefly, they can be described
as follows:
Wave soldering — the conventional method of soldering through-hole
component assemblies where the substrate passes over a wave (or more often,
two waves) of molten solder. This technique is favored for mixed print as­
semblies with through-hole components on the top of the substrate, and SMDs
on the bottom.
Reflow soldering — a technique originally developed for thick-film hybrid
circuits using a solder paste or cream (a suspension of fine solder particles in a
sticky resin-flux base) applied to the substrate which, after component place­
ment, is heated causing the solder to melt and coalesce. This method is pre­
dominantly used for Type I (all-SMD) assemblies.
Combination wave/reflow soldering — a sequential process using both
the foregoing techniques to overcome the problems of soldering a double-sided
mixed-print substrate, with SMDs and through-hole components on the top,
and SMDs only on the bottom (Type IIB).
Footprint Definition
An SMD footprint, as shown in Fig. 1-4, consists of:
• a pattern for the (copper) solder lands,
6 SMD ESSENTIALS
• a pattern for the solder resist,
• if applicable, a pattern for the solder cream.
Fig. 1-4: Component lead, solder lead, solder re­
sist and solder cream "footprint."
The Design for the footprint can be represented as a set of nominal coordi­
nates and dimensions. In practice, the actual coordinates of each pattern will be
distributed around these nominal values due to positioning and processing tol­
erances. Therefore the coordinates are stochastic; the actual values from a
probability distribution, with a mean value (the nominal value) and astandard
deviation.