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EXECUTIVE SUMMARY
Electronic packaging has become an important element in electronic
systems and may very well be a critical pacing feature in the future. Modern
packaging technology embodies a host of materials science and technology
issues and necessitates an integrated design approach that includes packaging
considerations from the very outset. Integrated circuit performance can be
limited or enhanced by packaging features, and the designer no longer has the
latitude to ignore such considerations until the electronic portion of the
design is complete. Packaging, with all its implied materials issues, is a
basic design feature. The materials of packaging and interconnection that are
intrinsic to the design effort are the subject of this report. The ability to
incorporate these issues into microelectronics products will be a key factor
in maintaining this nation's competitive position in the world market for
advanced electronics
Electronic systems needed in the next few years will require
unprecedented packaging technology. The rapid advances in integrated circuit
chip capabilities will continue to increase demand for enhanced interconnect
capability as regards numbers of connections (pinouts), pinout configuration,
heat removal, signal rise time, signal transit time, power lead inductance,
power supply current, and environmental protection. The projected evolution
of chip parameters is presented in the format suggested by scaling theory for
three families of chips "bipolar, CMOS, and GaAs], and the concomitant
implications for packaging and interconnection are discussed. Appendix A
contains a list of many special terms used in this report. No attempt is made
to explain all terms commonly used in the industry. For individual chips,
there will be hundreds of pinouts, tens of watts, and subnanosecond rise times
in the l990s. Clearly, interconnect structures will require considerable
enhancements to translate these chip capabilities to system performance. Heat
dissipation in thousands of watts and power supply requirements of thousands
of amperes are projected at the board level. Special physical design problems
arise with very-high-frequency and very high-speed circuits. There is concern
that the United States is relying too heavily on foreign sources of packaging
and interconnection materials for high-density electronic circuitry.
The United States has lost significant market share in advanced chip
technology, and the process continues. As domestic production is lost (e.g.,
in DRAlls I, advanced packaging and interconnection strategies will be
handicapped by reluctance of foreign chip makers to supply ICs in unpackaged
or other non- star~dard forms . Printed wiring board (PWB) technology continues
to evolve with the introduction of efficient surface-mount technology, finer
1
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patterns, greater numbers of signal layers, and improved board resins for
greater dimensional control. Forty-layer boards have been achieved. PWBs
will certainly be important for many years to come, but their limitations in
very-high-density electronics will have to be addressed in terms of new design
strategies in which materials innovations will play a key role.
Packaging is now approaching a turning point at which single-chip
packages cannot be assembled directly onto conventional circuit boards without
impairment of performance. A new level of packaging, the multichip module
(MCM), is coming into prominence. MCMs consist of inorganic base layers, to
provide power, ground and decoupling capacitances, and signal interconnect
patterns fabricated of high-conductivity metals and low-dielectric-constant
organic polymer dielectrics. The individual chips are assembled on top,
either in unencapsulated form or in low-impedance single-chip packages. The
signal interconnection density achievable is very high, owing to the fineness
of the patterns. Two layers of MCMs can replace dozens of layers in
conventional Pubs. Materials support for MCM designs must be strongly
encouraged in the United States.
Materials issues emphasize both process and final property aspects of
design, and material compatibility is a critical issue. Many different
properties and compatibilities must be optimized simultaneously. Issues of
importance include the following:
· coefficient of thermal expansion
· dielectric properties
· thermal conduc t ivi ty
· electrical conductivity
· interracial chemis try
· adhesion
~ mechanical strength and toughness
· impact strength
· long-term stability
purity (including absence of radioactivity)
· vapor permeability (especially water)
· corros ion
~ metal migration
· process control and reproducibility
~ process compatibility
The engineering-design-manufacturing process sequence is somewhat difficult to
describe briefly while emphasizing materials factors. Discussing some
specific packaging and interconnection materials (see Chapter 5) can give a
flavor for the complex compromises that must be made. These systems include
epoxies for encapsulation and PWBs, ceramic materials for packages and co-
fired circuits, polyimides for dielectric layers, and more exotic materials,
such as superconductors, synthetic diamond layers, and composites.
Beyond the domain of engineering, packaging and interconnection
materials are strongly affected by business, organizational, and government
policy issues. International competition presents implications in terms of
economics and national security. These interacting business and government
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issues have much in common with many other high-technology areas, but
packaging materials is a field in which the organizational problems are
important, competition is fierce, and the financial consequences of losing out
are massive. The United States has already lost consumer electronics and the
entire computer market could follow. Packaging is one of the essential
ingredients in the future U.S. position.
In certain respects, the United States is poorly positioned in regard to
materials employed for packaging and interconnection. In the sequence of raw
materials, intermediates, formulated materials, completed piece parts, and
assembled modules and systems, materials and information move from one company
to another in a highly incomplete and imperfect way. The sequence is
fragmented, and the technology is migrating to other countries, leaving U.S.
manufacturers dependent on foreign supply houses. Some form of domestic
supply chain that accomplishes the advantages of a vertically integrated
corporation is urgently needed.
A further problem exists in connection with government-funded research
and development. The most substantial source of federal money is the
Department of Defense, and many important initiatives have received timely
funding from that agency. Unfortunately, electronic packaging of military
systems has evolved along lines that differ significantly from computer,
consumer, automotive, and other electronic subareas. There are some hopeful
signs of rationalization, but much remains to be done.
In recent years, many consortia have been formed to address problems
faced by the United States in regard to international competition in
technological matters. Groups of companies and other organizations come
together to address common issues in a coordinated way and to pool resources.
A consortium specifically addressed to materials for electronic packaging and
interconnection could be an effective approach to some of the problems faced.
Coupling to system design and engineering and the involvement of first-class
engineering talent are essential features. An alternative to an independent
consortium on packaging might consist of an expanded emphasis on packaging and
interconnection by existing consortia.
The United States has enormous intellectual resources in the university
system, and it will be important to bring this potential to bear on the
critical area of electronic packaging. Coordination of efforts of the various
relevant university engineering and science departments will not be easy for
this interdisciplinary field. Coupling of university programs to industrial
design, development, and manufacturing projects in productive arrangements
will require creative management. Similarly, involvement of national
laboratories and other organizations also must be implemented. Issues of
organizational culture complicate communication among the potential
contributors. The National Science Foundation's program on engineering
research centers is a recent approach to finding a useful format, but it is
directed to support large centers. Smaller grants that encourage materials
innovations and industrial collaboration are needed.
The United States has lost significant segments of the electronics
market in recent years. The future position of the United States as a world
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power and the U. S. standard of living depend on a broadly-based response to
foreign competition. Any response that does not include aggressive materials
and packaging programs will fail to regain and sustain the U. S . position.
Both domestic comfort and national security are at stake
The committee's conclusions and recommendations regarding specific areas
· National policies. Antitrust laws still wield considerable influence
in U.S. business circles. The need for strict interpretation of the laws has
been largely eliminated by the growth in overseas competition, which will
prevent any U.S. combine from raising prices to the disadvantage of the
public. Modernization of these laws should be undertaken with emphasis on the
features that affect the competitiveness of U. S . corporations in critical world
marke ts .
The introduction of advanced packaging technology and materials in U. S .
industry could be facilitated by selectively removing antitrust restrictions
on "buying cooperatives . " Specificat ly, if U. S. companies were permitted to
aggregate demand for new products (e. g., packaging materials), U. S . producers
would be stimulated to risk their capital and develop the needed manufacturing
capability. The committee concludes that a maj or impediment to U. S .
competitiveness In computer and other electronic systems is the unwillingness
of domestic material suppliers to invest capital for research, development,
and production in long-term ventures. By aggregating market demand (as is
done in Japan), U.S. electronics manufacturers would provide a more attractive
market that would stimulate production of advanced materials and build a self-
sustaining market. This would help compensate for systemic advantages of
similar actions enjoyed by foreign materials companies.
Foreign companies also enjoy low-interest loans that encourage greater
patience in market development. Some form of selective capital encouragement
should be considered as a part of a broadly-based U.S. strategic plan to
revive, sustain, and create critically-important base technologies. This
could be done at the state level as well as on the federal level. It is
difficult to overstate the need for actions that will encourage the long-
range research and development necessary to provide the technology for future
industries. Low-interest money is needed. Tax credits for research and
development are an alternative approach that should be considered.
In an era of emphasis on short-term financial results, it is important
to provide incentives for long-term investment in technologies that will build
wealth for the nation. Although it has not been discussed widely, the much-
admired U. S . corporate research and development laboratories are increasingly
under pressure to shorten their sights and turn over quick profits. The
current frenzy for mergers, acquisitions, and other forms of corporate
churning is destabilizing and destroying the climate for long-range technical
development. Some form of legislation that would discourage these practices
seems desirable Legislative solutions by their nature are long term. Thus,
it is all the more important that a national strategiic program be undertaken
to find remedies for our present malaise . The problems do not show s igns of
self-cure.
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The committee discussed some changes in regard to patent procedures that
could enhance U.S. competitiveness in the packaging materials area.
Legislation would not be required for a system that gives priority on Patent
Office and Patent Court dockets for submissions in the packaging area. (This
already has been done for high-temperature superconductivity.) The ad hoc
nature of this approach is a negative aspect. Legislation to allow the
formation of patent pools and licensing arrangements by removing antitrust
constraints for a period of 10 to 20 years has been proposed. As U.S.
corporations become owned by and allied with foreign companies, legislative
and procedural efforts to favor U.S. firms become awkward and difficult to
administer.
· User consortium. The committee exhibited considerable enthusiasm for
an industry-led consortium of packaging-materials users, with the objective to
develop a stronger U.S. base of packaging-materials suppliers. This
consortium would develop materials requirements and materials application
technology, and would cultivate domestic sources of supply. The analogy with
Sematech, the U.S.-based consortium aimed at integrated circuit process
equipment, is strong, and the term "Sepatech" was coined for purposes of
discussion among committee members. Table 1 illustrates the analogy.
Table 1 Analogy of U.S.-Based User Consortia
Parameter
Intended to motivate
Sematech "Sepatech" _
Equipment suppliers to
integrated circuit
manufacturers
Set up by: Chip makers
Activity:
Vehicles for activity:
Full-scale manufactur-
ing by:
Prototype and build
leading-edge parts,
working with equipment
suppliers to integrated
circuit manufacturers
Memories (advanced DRAM
and SRAM)
Chemical suppliers,
compounders, and other
materials suppliers
System companies using
packaging and inter-
connection technology
Perform materials and
process evaluation,
build some demonstration
systems, and establish
sources of supply
Advanced work stations
US Memories, Inc., System houses employing
employing the apparatus, materials and processes
materials, and processes
developed
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The consortium approach offers an increasingly acceptable way for
manufacturers to pool their resources and aggregate their markets at a
precompetitive stage. The approach places responsibility and control in the
hands of the user industry and, therefore, ensures development along relevant
paths. Financing must be substantially industrial ("earnest money"), but
government investment can significantly encourage and facilitate establishment
of the combine. Indeed, government funding at some level is probably
essential and justified.
Consortia are controversial, and there is a lot to be learned in their
organization, strategic objectives, and operation. Clearly, success depends
on the participating organizations, which must provide quality people at all
levels and viable long-term career paths for individuals. The committee is
aware that consortia are subject to diverse forms of criticism, but the
consortium approach is a positive mechanism to relieve the problems of U.S.
industrial fragmentation. There are not many readily implementable
alternative approaches.
· Military packaging. The packaging of integrated circuits for U.S.
military systems must be hermetic, which has caused military electronics
systems to lag considerably in terms of overall capabilities. The committee
urges that alternative means be found to allow military hardware to move into
the mainstream of electronic packaging, while still preserving reliability
over long periods in difficult environments. Some study programs are under
way in specific areas (e.g., silicone gel coatings), but a broadly-based
action group should confront the issue and initiate studies that will lead to
needed change. Although it is possible to obtain high levels of performance
under hermetic constraints, the committee believes that pursuit of other
options is a promising approach that should be encouraged.
~ Industry-national laboratory-university coupling. Although systems
manufacturers have been very active in work on packaging and interconnection
materials and structures, relatively little activity in this area is evident
in universities and the national laboratories. The committee feels that
interconnection and packaging are regarded as insufficiently exciting by many
members of the academic community. Some means is needed to stimulate
university work in the area. The National Science Foundation's Engineering
Research Center program would be well suited for establishing a close coupling
of industrial scientists and engineers as an explicit part of the basic
format. A program of smaller grants, similarly structured, could be highly
useful. Also, coupling of national laboratory talent in the area should be
pursued. Clearly, new and innovative means must be found to focus more of the
considerable U.S. technical power on the issues electronic packaging.
· Specific materials for support. The following specific materials and
process areas are recommended for support in connection with electronic
packaging and interconnection. In any such list, there is the danger of
important omissions, statement of the obvious, and possible emphasis on areas
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that lie close to the interests of the committee members. The following list
had general support:
1. Present mainline materials should be pursued to bring U.S.
capability up to a position of world leadership. Briefly, this includes
copper, gold, and aluminum metallization with progress on high reliability in
high current density; alumina and aluminum nitride, and other ceramic
processing for preparation of advanced interconnect details; further
development of epoxies for encapsulation, board resins, and adhesives; and
development of polyimides and other high-temperature polymers for substrates,
interlayer dielectrics, and other uses. Benzocyclobutanes are a very
promising class of low dielectric constant polymers that are beginning to
appear in electronic products.
2. Glass-ceramic substrate compositions that can be co-fired with
copper are viewed as an area of the highest priority. Much work has been
done, but the problem has not been solved.
3. Low-dielectric-constant materials and interlayers are becoming
increasingly important because signal transit time limits circuit performance.
Polymeric materials are available with electric per~ittivity as low as 2, but
substantial development is needed to bring the many other properties and
process variables into useful ranges. Ceramics seem less likely for low
permittivity.
4. High-thermal-conductivity materials for packaging are needed.
Composites offer advantages in this area. Thermoelectric cooling offers an
alternative approach.
5. Materials for high-reliability encapsulation are extremely
important. Hermetic structures are expensive and not entirely reliable.
Silicone gels are currently under study, with promising initial results.
6. New solder compositions that are creep resistant would be valuable.
Solder substitutes (e . g. , anisotropically conducting polymers and ceramics)
can be used for low- temperature assembly.
7. Composite materials can be employed to engineer combinations of
properties not achievable with homogeneous substances.
8. Materials amenable to environmentally benign processing (e.g., dry
processing, aqueous-based systems) will become increasingly important.
Circuit-board cleaning is ~ major source of solvent loss to the atmosphere,
which leads to ozone depletion and other ills.
9. High-temperature superconducting oxides are exciting candidates for
interconnects. Chemical stability, electrical contacts, and other problems
remain to be solved . (Bear in mind that copper, aluminum, and other metals
also are considerably better conductors at liquid nitrogen temperature than at
room temperature.)
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10. Synthetic diamond films offer great promise for mechanical
protection, chemical isolation, and electrical i nsulatior~. The thermal
conductivity is uniquely high.
11. The ability to design materials with thermal expansion tailored to
the appl ication is emerging and should be extremely important for interconnect
s true tures
Representative terms from entire chapter:
integrated circuit
