Electronic Packaging Materials and Their Properties

120 Pages·1999·5.33 MB·English

by  Michael Pecht

The Electronic Packaging Series
Series Editor: Michael G. Pecht University of Maryland
f
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Michael Pecht and Yeun Tsun Wong
Electronic Packaging Materials and Their Properties
Michael Pecht, Rakesh Agarwal, Patrick McCluskey, Terrance Dishongh,
Sirus Javadpour, and Rahul Mahajan
Guidebook for Managing Silicon Chip Reliability
Michael Pecht, Riko Radojcic, and Gopal Rao
High Temperature Electronics
Patrick McCluskey, Thomas Podlesak, and Richard Grzybowski
Influence of Temperature on Microelectronics and System Reliability
Pradeep Lall, Michael Pecht, and Edward Hakim
Long-Term Non-Operating Reliability of Electronic Products
Michael Pecht and Judy Pecht
ELECTRONIC
PACKAGING
Materials and Their Properties
Michael G.Pecht
CALCE Electronics Packaging Research Center
University of Maryland, College Park
RakeshAgarwal
Delco Electronics, Kokoma, Indiana
Patrick McCluskey
TerranceDishongh
Sirusjavadpour
Rahul Mahajan
CALCE Bectronics Packaging Research Center
University of Maryland, College Park
CRC Press
Boca Raton London New York Washington, D.C.
Portions of the text were adapted from R.J. Hanneman, A.D. Kraus, and Michael Pecht,
editors, Physical Architecture of VLSI Systems, 1994. Adapted by permission of John
Wiley & Sons, Inc.
Library of Congress Cataloging-in-Publication Data
Electronic packaging materials and their properties / Michael G. Pecht
... [etalj.
p. cm.-- (Electronic packaging series)
Includes bibliographical references and index.
ISBN 0-8493-9625-5 (alk. paper)
1. Electronic packaging- Materials. I. Pecht, Michael.
II. Series.
TK7870.15.E4222 1998
621.381 '046—dc21 98-34479
CIP
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2 3 4 5 6 7 8 90
Table of Contents
Preface 1
1 Properties of electronic packaging materials 3
1.1 Electrical properties 3
1.2 Thermal and thermomechanical properties 9
1.3 Mechanical properties 13
1.4 Chemical properties 17
1.5 Miscellaneous properties 20
2 Zeroth-level packaging materials 21
2.1 Semiconductors . 21
2.2 Attachment materials 25
2.3 • Substrates materials 30
3 First-level packaging materials 39
3.1 Wire interconnects 39
3.2 Tape interconnects 44
3.3 Case materials 47
3.4 Lid seals 50
3.5 Leads 54
4 Second-level packaging materials 57
4.1 Reinforcement fiber materials 57
4.2 Resins 62
4.3 Laminates 66
4.4 Constraining cores 75
4.5 Flexible wiring board materials 76
4.6 Conformal coatings 84
5 Third-level packaging materials 89
5.1 B ackpanel materials 89
5.2 Connector materials 89
5.3 Cables and flex circuit materials 98
6 Summary 103
Appendix A 105
Appendix B 107
References 109
Index 113

Preface
The effectiveness with which an electronic system performs its
electrical functions, as well as the reliability and cost of the system, are
strongly determined not only by the electrical design, but also by the
packaging materials. Electronic packaging refers to the packaging of
integrated circuit (1C) chips (dies), their interconnections for signal and
power transmission and heat dissipation. Packaging is also required for
electromagnetic interference (EMI) shielding. In electronic systems,
packaging materials may also serve as electrical conductors or insulators,
provide structure and form, provide thermal paths and protect the circuits
from environmental factors such as moisture, contamination, hostile
chemicals and radiation. As the speed and power of electronics increase, the
heat dissipation problems and the signal delay caused by the capacitive effect
of the dielectric material become even greater issues that need resolving.
The solution involves the devising of innovative packaging schemes and the
continuing search for more advanced materials.
The level of packaging or packaging architecture is often used to
classify materials and the required material characteristics for effective
performance over time. The chip, component, printed wiring board, and
assembly level packaging are referred to as the zeroth, first, second and third
levels of packaging, respectively (the fourth and fifth levels of packaging
being the electronic module formation by the integration of the backpanel
and power supply with an outer housing and the system formation by
integration of electronic modules, e.g., peripherals). In general, each level
has unique material properties requirements. The actual applications of
materials in electronic packaging include interconnections, printed circuit
boards, substrates, encapsulants, interlayer dielectrics, die attach materials,
electrical contacts, connectors, thermal interface materials, heat sinks,
solders, brazes, lids and housings.
The dependence of material properties on orientation, with respect
to material axes, is often indicated by referring to materials as isotropic,
orthotropic, or anisotropic. The materials of concern here are mostly
isotropic, exhibiting the same behavior along all directions. However,
semiconductors and composite materials are often orthotropic and exhibit
mutually independent material properties along three mutually perpendicular
axes. An orthotropic property of a semiconductor is generally referred to
with respect to crystallographic structure. Composite materials are generally
referred to with respect to the microstructure of the material architecture.
In this book, materials employed in the zeroth-level packaging are
covered in Chapter 2, the first-level packaging materials in Chapter 3, the
second-level packaging materials in Chapter 4, and the third-level packaging
materials in Chapter 5. Prior to these chapters is an overview of the key
electrical, thermal and thermomechanical, mechanical, chemical, and
miscellaneous properties and their significance in electronic packaging.

1 PROPERTIES OF ELECTRONIC PACKAGING MATERIALS
1.1 Electrical Properties
Signal processing is critical for the operation of an electronic
system, and materials, along with their architectures, play an important role
in the propagation of signals, especially for circuits operating at high speeds
and at high electrical frequency. The electrical properties of major
importance in material selection include the dielectric constant, loss tangent,
dielectric strength, volumetric resistivity, surface resistance, and arc
resistance. These electrical properties are defined and discussed with
standard test methods (refer to Table 1) wherever applicable. Some of these
properties exhibit subtle differences and some are known by more than one
term.
Dielectric constant, e. The dielectric constant of an insulating material is
the ratio of the measured capacitance with the dielectric material between
two electrodes to the capacitance with a vacuum or free space between the
electrodes. The dielectric constant is a dimensionless number also referred to
as the relative permittivity. Table 2 lists dielectric constants of some
electronic. Test method ASTM D150 is used to measure the dielectric
constant.
To account for the electrical power loss to an insulating material
subject to a sinusoidally time-varying applied potential, a complex number
called permittivity is defined as
e = e'-e" (I)
where the imaginary part £," is the electrical power loss factor, and the real
part, e', is the dielectric constant of the insulatoivmaterial.
Most materials possess a dielectric constant that depends on the
frequency of the applied electromagnetic field. The dependence on electrical
frequency is due to the orientational (defined by reorientation of inherent
dipoles in the material), ionic (defined by displacement of ions of molecules),
and electronic polarization (defined by the shift in the electronic cloud of
atoms) of the material. Figure 1A shows the typical trends of the dielectric
constant as a function of frequency for a material with all three types of
polarizations.
3
4 ELECTRONIC PACKAGING MATERIALS AND THEIR PROPERTIES
Frequency (Hz)
Figure 1A : Diagram of variation in dielectric constant as a function of frequency for a material
with orientational, ionic and electronic polarization (Zaky, 1970).
The dielectric constant typically increases with decreased
temperature. Because water possesses a rather high dielectric constant
(greater than most electronic materials), the dielectric constant of a material
increases as the absorbed moisture increases.
The delay of signal propagation through a conductor depends on the
dielectric constant of the insulating material. The propagation delay
measured in nanoseconds through one foot of conductor wire for a micro-
strip is
t = 1.017Vo.475e'+0.67 (2)
pd
and for a strip-line is
t = 1.017VF (3)
pd
Equations (2) and (3) show that the propagation delay varies directly with the
dielectric constant. For example, a reduction of dielectric constant by a
factor of 0.5 reduces 30% of the effective conductor length for the signal for
a strip-line and slightly less for a micro-strip. To achieve smaller
propagation delays in high-speed applications, low dielectric constant
materials are sandwiched between signal and ground planes. Because a
semiconductor material generally has a higher dielectric constant than the
mounting platform material, a signal conductor should leave the surface of an
integrated circuit as soon as possible. A high dielectric constant material is
desirable between the ground and the power plane to smooth small inductive
spikes in power.