It is true to say that, until the 1990s, electronic manufacturing used the same basic technology as that introduced in the 1930s, i.e. soldering components onto a substrate and interconnecting them with discrete or printed wiring.
In the 1990s advances in component design, e.g. fine pitch SMT, and a worldwide desire to improve the environment by banning the use of lead (and thus traditional tin‐lead solders) led to the development and introduction of new joining techniques based on conductive adhesives.
This book presents state‐of‐the‐art knowledge of adhesive joining technology in various packaging applications. Each chapter is contributed by a world authority on his/her topic.
The volume is grouped into four parts. The first part (Chapter 1) gives an introduction to conductive adhesive technology whilst Part Two (Chapters 2‐11) deals with issues of material selection, manufacturing, processing, conduction mechanisms and reliability studies.
Part Three (Chapters 12‐16) focuses on applications of adhesives for surface mount, die attach, display and flip‐chip and the final part (Chapters 17‐18) covers toxicity and environmental health aspects.
Chapter 1, “Introduction to conductive adhesive joining technology” by Ken Gilleo, reviews the historical development of adhesives in general and conductive adhesives specifically. Very clear descriptions of adhesive types identify the differences between isotropic, bi‐directional and uni‐directional anisotropic, patterned anisotropic and non‐conductive adhesives.
Materials are covered in the same detailed manner and include descriptions of polymer binder, thermoplastics, thermosets, radiation curable systems and conductor fillers.
Applications for conductive adhesives are discussed next and the chapter is concluded with a review of future possibilities including intrinsically conductive polymers and polymer bonding.
Heiner Bayer contributes Chapter 2, “Cationic cure of epoxy resins and UV options for conductive adhesives”.
The advantages of UV curability are discussed and then follows a detailed description of cationic epoxy chemistry, including the polyol component and initiators.
Conductive filling, dual cure adhesive systems, pre‐assembly irradiation techniques (PASI) and adhesive properties are explained and finally application examples are given.
Chapter 3 by James E. Morris, entitled “Conduction mechanisms and microstructure development in isotropic electrically conductive adhesives”, concerns the determination of the mechanisms of electrical conduction through the metal‐epoxy matrix.
The study includes the choice of metal particulate, microstructural cure effects, ICA contact effects and percolation theory. The topics are supported by highly detailed and researched experimental data and theoretical modelling.
“Models to determine guidelines for the anisotropic conducting adhesives joining process” continues the modelling theme in Chapter 4, presented by authors Samjid Mannan, David Williams, David Whalley and Adebayo O. Ogunjimi.
The authors show how a model of fluid flow for the adhesive in an ACA can be constructed and how this model can be used to guide the choice of materials and substrates.
Model verification by experiment is explained and formulae devised and presented relating the probabilities of bridging and opens as a function of various other parameters.
Chapter 5, “Curing of isotropic electrically conductive adhesives” by Li Li and James E. Morris, describes the experiments carried out on three commercially available ICAs, all silver‐loaded epoxies.
The aim of these experiments was to assess the effectiveness of the manufacturers’ specified cure schedules and to relate the degree of cure to electrical properties.
The adhesive cure process is fully described together with the model data and experimental results.
The study of electrical properties continues in Chapter 6, “Contact reliability modelling and material behaviour of conductive adhesives under thermomechanical loads”.
In their introduction, authors Sean X. Wu, Kai X. Hu and Chao‐Pin Yeh compare the properties of ACA and ICA material and describe a micromechanics approach to the determination of polymer and particle properties under stress.
Then follows a highly detailed account of the work carried out to determine:
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1 electro‐thermal‐mechanical responses of ACA materials;
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2 the effect of process induced residual stresses; and
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3 high frequency characterisation of conductive adhesive joints under mechanical and thermo‐mechanical loading.
“Design and modelling of solder‐filled ACAs for flip‐chip and flexible circuit applications”, by authors J.K. Kivilahti and P. Savolainen, is presented in Chapter 7.
Topics include: conductive adhesives, metallurgy of microsoldering and a detailed review of solder fillers for Z‐adhesives including tin‐bismuth and bismuth.
Reliability results are given and fully discussed and the chapter concludes with a review of solder filled adhesives in flip‐chip applications.
A manufacturing review, rather than experimental modelling, is the theme of Chapter 8, “Recent advances and evaluation of anisotropically conductive adhesives for microelectronics assembly”, by A.M. Lyons and C.P. Wong.
Following a brief but informative introduction the authors review in great detail the materials and conduction mechanisms currently in use with ACAs. Highlighted topics include compliant particles, solder particles, No particles, electrical properties and component/substrate geometry effects.
ACA assembly is next dealt with under the headings Bare Die and SMT Assembly Processes, Registration and Coplanarity Concerns and Assembly Parameters.
Techniques for extremely fine pitch are fully discussed and cover epoxy coated conductive particles, magnetic field enhanced particle dispersions, adhesive flow control, particle localisation, unfilled adhesive systems and fixed conductor arrays.
A suggested table of ACA characterisation methods is given and the various methods described in greater detail are microscopy, energy dispersive X‐ray, Fourier transform infra‐red spectroscopy, differential scanning calorimetry, microdielectrometry, thermal gravimetric analysis and mechanical adhesion.
The chapter concludes with a description of a test vehicle design for testing the reliability of ACA interconnections.
Reliability issues are further explored by Johan Liu and Pontus Lundström in Chapter 9, “Manufacturability, reliability and failure mechanism in conductive adhesive joining for flip‐chip and surface mount applications”.
Following their introduction the authors mention concerns about solder joint reliability in harsh environments as a reason for considering conductive adhesives. (Since solder has been the joining medium for almost all electronic assembly for the past half century this statement should be perhaps qualified and substantiated.)
The necessary conditions for a good conductive adhesive joint are discussed together with microstructures of various conductive adhesives and the effect of curing degree on the joint reliability.
Manufacturing and processing of ACAs is described with notes on inspection and repair followed by a substantial review of failure mechanisms covering topics such as oxidation/hydration and polymer degradation.
Quality issues regarding ACA flip‐chip joints are next discussed and in particular the effects of bonding pressure, particle size and bump geometry.
Finally, the authors present data regarding high frequency properties of materials and thermal resistance of ACA joints.
A prediction methodology for the estimation of real service life after accelerated testing is given and an appraisal of development trends in ACA joining concludes an informative and well‐illustrated chapter.
“Anisotropic conductive adhesive films for flip‐chip interconnection”, by authors Itsuo Watanabe and Kenzo Takemura, Chapter 10, is specifically devoted to the use of films as the base material for flip‐chip interconnections.
Subject matter includes principles of ACF connections, materials, approaches for very fine pitch connections and flip‐chip interconnections to organic substrates – bumped and bumpless.
Chapter 11, “Reliability of electrically conductive adhesive joints in surface mount applications” by Jannes C. Jagt, is a highly researched review of the subject matter including the data from 11 international and academic bodies and 86 other references.
After reviewing the benefits and concerns of conductive adhesives, Jagt describes the more general resins used and then overviews the reliability investigations carried out by each of the 11 bodies.
A summary of the investigations is given in Table 11.9. Several of the mechanisms reported are compared and discussed, i.e. oxidation and corrosion, crack formation, depletion of silver, creep effects, formation of an intermetallic layer and silver migration.
Although seemingly a contradiction in terms, Chapter 12 is entitled “Electrically conductive joints using non‐conductive adhesives (NCAs) in surface mount applications”. Authors Andreas Bauer and Thomas Gesang show how electrically conductive adhesive joints can be formed using non‐filled organic adhesives, i.e. without any conductive filler particles.
A literature review describes the published data on NCAs since the 1980s, followed by NCA joining and theory of contact formation.
Applications in fine pitch SMT technology include materials and manufacturing process, morphology and electrical properties of the joints, and reliability.
The results of investigations of the conducting mechanism are discussed and cover voltage and temperature characteristics, analysis of fracture surfaces and a model for the conduction mechanism.
High current and thermal properties are discussed, including a description of an experimental set‐up, effects of high current load, temperature dependence of contact resistance and the visualisation of hot spots.
Chapter 13, “Use of conductive adhesives as die‐attach for power electronics applications” by Are Bjorneklett is, as the title suggests, a specialist application of conductive adhesive.
Following the introduction, the author explains thermal conductivity of adhesives, thermal resistance and stress, cracking, delamination and thermal fatigue.
A brief review of design and optimisation of die attach completes the chapter.
The theme of power electronics attachment is continued by Outi Rusanen in Chapter 14, “Replacing solder with isotropically conductive adhesives in die‐bonding of power semiconductors”.
Following the criteria for selecting polymer and filler materials for CAs, comparisons are made between adhesive and solder die‐bonding including adhesion and mechanical properties, electrical and thermal conductivity, processing, cost, and environmental considerations.
The experimental results from replacing solder with ICAs covers the test adhesive, manufacturing the test module and the electrical, thermal and reliability effects considered.
Finally, reliability aspects cover thermal and operational cycling, elevated humidity and temperature ageing, and adhesive reliability.
Chapter 15, “Overview of conductive adhesive interconnection technologies for display applications” by Helge Kristiansen and Johan Liu, covers the field of liquid crystal displays (LCDs).
Following a description of LCDs and their use, the authors detail the application of adhesives for LCD driver connections with notes on ACA particle distribution, bonding parameters and chip on flex (TAB).
Chip on glass technology is reviewed with focus on three types of flip‐chip bonding, i.e. anisotropic conductive film, isotropic conductive adhesive and pressure connections with bumps. All the technologies are described together with 12 actual company case histories.
A discussion of reliability, failure mechanisms and testing, followed by future considerations, ends the chapter.
The final manufacturing chapter, in this case Chapter 16, “Integration of microsystems using flip‐chip technologies and adhesives” by Maja Amskov and Henrik L. Hvims, focuses on combining micro‐electromechanical systems (MEMS) with conventional microelectronics.
Different stacking processes are described, involving both conductive and non‐conductive adhesive systems.
Health, safety and the environment comprise the subjects for the final part of this book.
Chapter 17, “Adhesives and health hazards” by Helene Carlsson and Ann‐Beth Antonsson, is a review of chemical health hazards based on an actual occupational health handbook for the adhesive bonding industry.
Containing much welcomed information, it details the factors influencing chemical health hazards how different substances affect health, what do adhesives contain? and health risks associated with epoxy, acrylic and polyurethane adhesives.
Finally, Chapter 18, “Health and environment aspects of conductive adhesives – the use of lead‐based alloys compared with adhesives” by H. Westphal, commences with an overview of the effects of lead in the environment and the potential of substituting conductive adhesives.
The hazards of working with adhesives, and epoxy in particular, are described, culminating in an environmental comparison and recommendations to adhesive users.
This volume, Conductive Adhesives for Electronic Packaging, is a fine reference work for all involved in this very specialised technology. Although many of the chapters are scientific in nature there is enough practical information to interest the manufacturing and quality engineer.
All contributors are to be congratulated on their use of the many fine figures, tables and photographs that enhance the depth of the written detail.
As a minor criticism, the omission of the authors’ names from the contents pages is disappointing.
