Conductive Adhesives for Electronics Packaging

Citation

Sinnadurai, N. (2000), "Conductive Adhesives for Electronics Packaging", Microelectronics International, Vol. 17 No. 2, pp. 34-35. https://doi.org/10.1108/mi.2000.17.2.34.1

>The book aims for the important slot in electronics assembly, namely joining technology. The major strengths of the book are the eminent leaders of published work in the industry who have contributed chapters and the scientific and practical depth of their contributions. The book comprises an assembly of separate contributions in 18 chapters. Dotted around in the book are also some useful assessments and comparisons with solder joining.

Chapter 1 is by a well known figure from one of the industrial materials suppliers. This chapter is very much a scene‐setting instinctive input (there are no references) from the extensive knowledge of the contributor, dealing with the history and development of adhesive types, largely for surface mount technology (SMT) rather than die attach. While he does mention elastomeric (“zebra”) connectors it is not attributed to the inventor Dr Len Buchoff. He risks perpetuating the myth of silver migration. The chapter is also useful for awareness of common terminology.

The subsequent chapters delve to varying extents into the materials, technology and processes. Chapter 2 provides a very useful grounding in the types and chemistries of adhesive resins, their curing and catalysts. Included are the benefits of dual cure, often overlooked by others, a useful explanation of photochemistry and a range of useful references – notably none from the Journal of Adhesion Science. Chapter 3 is an impressive tutorial by an academic authority in the field. It contains insight into particle materials, sizes and shapes and the conduction mechanisms (subsequent to critical loading) together with a full portfolio of pictures, graphics and informative measurements. Some 31 references are given with interpretations thereof. The science and practical application of anisotropic conducting adhesives (ACAs) are dealt with in Chapter 4 which includes useful suggestions backed by equations and helpful diagrams. The authors discuss and illustrate how opens and bridging occur. A modest range of references are provided, and probably reflects the state‐of‐art of ACAs.

Chapter 6 explores cure reactions, showing the Arrhenius dependence and consequential effect on pot life, the percentage conversion into cure and hazards of inadequate curing. Various curing profiles are provided and comparisons given for different adhesives. The reliability drift with ageing in damp heat shows the benefit obtained from full curing. Some 20 references are given. Reliability is modelled and examined in Chapter 6, the first part being dedicated entirely to theoretical modelling, making me wonder when the authors would join the real world. They do so, practically, in the second part of the chapter. Measurements are fed into finite element analyses (FEA) of stress and strain development in joints. They show that residual stresses reduce with low temperature (longer) curing. They also practically characterised the joints at high frequencies, with test vehicles to show changes in inductance and resistance during stress ageing. The authors made use of a good range of 35 references.

Solder filled adhesives are discussed in Chapter 7, which commences and proceeds as if the rest of the book does not exist, including a quite general introduction. The authors examine reactive and non‐reactive wetting. They show that reactive bonding requires a defined range of particle sizes to establish the microstructure. Deterioration occurs by decomposition of the microstructure and supersaturation of liquefied filler particles. The development of intermetallics can hamper conduction with Sn based non Pb solders. The authors explain that non‐reactive bonding with SnBi relies on sporadic fusing of Sn coated particles. Damp heat ageing affects non‐metallurgical bonds, resulting in weakening. The poorest results are obtained with joints to Ni coated conductors. The authors point out that ACA processing needs to be speeded up. A comprehensive range of 55 references has been examined. Chapter 8 also has a (repeated) general Introduction. The authors address recent advances including non‐solder based fillers which are more compliant. They also examine solder‐filled which is dealt with in chapter 7, but chapter 8 has clearer explanations and illustrations. The authors also address ACA assembly, including extremely fine pitch. They give useful explanations and insights into electrical performance and characterisation methods. They describe reliability test vehicles but provide no results. The chapter is supported by 46 comprehensive references.

Manufacturability, reliability and failure mechanisms are dealt with in Chapter 9 which is another stand‐alone chapter. The chapter re‐addresses the relationship between curing and reliability. There are a number of useful flow charts. The authors include some characterisation of joint degradation, explanation of particle oxidation and also of polymer degradation in damp heat. There is some repetition of chapter 8. There are useful micro‐sections illustrating mechanical stress and useful tables on electrical resistances for different types of circuit board. Reliability prediction is qualitative and simplistic. The chapter is supported by a good range of 33 references.

Chapter 10 also addresses ACA for flip‐chip. There is significant overlap with Chapter 8. It is a useful insight to the commitment to ACA and its production by a Japanese electronics major corporation, with Japanese electronics in the vanguard of exploiting ACA. Reliability of electrically conductive adhesives (ECA) is addressed in Chapter 11, which starts with yet another general introduction. The author cites the benefits of Au versus Ag fillers and deals with materials and processes. Silicones are mentioned for the first time. Table 11.2 and Figure 11.2 give very useful test results. Reliability examination includes thermal cycling and steady damp heat. The author then summarises a number of reliability studies by various organisations. He summarises the failure mechanisms as creep, intermetallics and silver migration. The summary Table 11.9 is useful. While there are 86 references, many of them have been summarised rather than critically analysed. Chapter 12 moves to non conducting adhesives (NCAs) used for conductive joints by bulk electrical breakdown. The introduction is concise and relevant. The literature review is informative, and the chapter proceeds through theory and application, morphology and electrical properties. The reliability discussion provides a useful comparison with PbSn. The chapter goes on to deal with conduction mechanisms. It is concise, logical and informative – a good chapter, with 38 relevant references.

Chapter 13 deals with adhesives joints for power devices, again preceded by a general introduction. The chapter usefully analyses heat flow for different structures, and fracture stress which can vary according to the hot devices joined to substrates. Optimisation of joint design is a valuable part of the chapter. Ten references are provided. Chapter 14 on solder replacement gets straight to the point (good). The author compares adhesive with solder bonding and provides useful tables. The discussions cover conductivity, processing pros and cons (versus PbSn), costs and environmental impact. Reliability studies (duplicated elsewhere) show that thermal resistance is stable with cycling but not with damp heat. Au plated conductors give better stability than Ni plated conductors. The section on structural analyses is well illustrated. The author uses only 12 references. Chapter 15 is an overview of the use of ECA for Displays. It provides a useful introduction on LCDs (liquid crystal displays) flat panels for those unfamiliar with them. The use of ACA and the opportunities for UV curing through the transparent glass are nicely pointed out. A useful list of products is tabulated. The authors describe a specific NCA product developed at SINTEF, and also provide a short tutorial on chip‐on‐glass (COG). A host of commercial applications, all of them Japanese, are summarised. This highlights the commitment of Japanese electronics industry to finding solutions to adhesive bonding and taking them into applications, while other are still researching solutions. Chapter 16 is another special, this time a short overview of the use of adhesives in integrating microelectronics with MEMS (micro‐electro‐mechanical systems). The authors describe how adhesives are used to create stacked chips for compactness.

Chapters 17 and 18 separately extract the Swedish view and the Danish view of environmental, health and toxic aspects of the use of adhesives. They are useful overviews which include precautions to be taken and environmental consequences when using adhesives. The chapters could have been editorially combined.

Overall the book is a valuable addition to the bookshelf as a set of reference papers by notable authors who give insight to solutions. There are interesting and useful gems throughout the book. The reader will find them recurring in different chapters and should search more than one chapter for a different angle on the same issue.