David C. Whalley

Thermal Interface Materials - A Review of the State of the Art

The past few decades have seen an escalation of power densities in electronic devices, and in particular in microprocessor chips. Together with the continuing trend of reduction in device dimensions this has led to dramatic increase in the thermal issues within electronic circuits. Thermal management is therefore becoming increasingly more critical and fundamental to ensuring that electronic devices operate within their specification. Although a thermal management system may make use of all modes of heat transfer to maintain temperatures within their appropriate limits and to ensure optimum performance and reliability, conductive heat transfer is typically used to spread the heat out from its point of generation and into the extended surface area of a heat sink. To minimise the contact resistance, thermal interface materials (TIMs) are introduced to the joint to fill the air gaps and are an essential part of an assembly when solid surfaces are attached together. This paper reviews the conventional interface materials and then goes on to present a comprehensive review of the emerging state-of-the-art research in the use of carbon nanotube based materials. The paper also outlines the advantages and disadvantages of each TIM category and the factors that need to be considered when selecting an interface material

A review of the impact of conductive adhesive technology on interconnection

This paper reviews the current state of conductive adhesive technology. Most work to date has been carried out with isotropically-conductive adhesives which conduct electricity in any direction. In this review, particular attention has been paid to recently-developed anisotropically-conductive adhesives which are electrically conductive along one axis only. Patents filed in this area have been surveyed and the key points relating to the technology employed are summarized. A survey of the market was carried out and is presented. Adhesive processing techniques were studied and reliability issues relating to adhesives in general and to conductive adhesives in particular investigated. Voids in the adhesive leading to reduced adhesion and stress concentration were seen to be an area of concern while the effect of moisture leading to increased joint resistance and reduced strength was concluded to be the key limiting factor in the long-term reliability of conductive adhesives.

Conduction mechanisms in anisotropic conducting adhesive assembly

This paper explores experimentally and through analytical and computational models, the mechanisms of conduction in flip chip interconnects using anisotropic conducting adhesives. A large number of assemblies were constructed with geometries in the 200-500 /spl mu/m range, and wide variations in joint resistance were found to occur both within the same assembly and between assemblies under the same experimental conditions. In order to explain the origin of these unsatisfactory connections, a series of tests to measure the contact resistance linearity of both high and low resistance joints were made. The results from these measurements show that a large number of low resistance joints are ohmic, while most joints of relatively high resistance show resistive heating. However, in some of the initially high resistance joints there is an initial ohmic behaviour which is followed by a breakdown of a dielectric or insulating film, resulting in lower resistance. In addition to linearity measurements, computational models of metallic conduction in solid and polymer core particles were constructed to help understand the conduction mechanism. These models, which are based on the finite element method, represent typical conductor particles trapped between appropriate substrate and component metallisation. The model results show that the contact area required to explain high resistances is small and that the likelihood of obtaining a high resistance through such a small area of metal-to-metal contact is small, thus giving a strong indication of the presence of high resistivity films at the joint contact surfaces.

Electroless nickel bumping of aluminum bondpads. I. Surface pretreatment and activation

Electroless nickel bumping of aluminum (Al) bond-pads followed by solder paste printing is seen as one of the lowest cost routes for the bumping of wafers prior to flip-chip assembly. However, the electroless nickel bumping of Al bondpads is not straightforward and a number of activation steps are necessary to enable the nickel deposit to form a strong, electrically conductive bond with the Al. For the electroless nickel coating of mechanical components made of aluminum, a zincate activation process has been used for many years; however, extension of these techniques to semiconductor wafers requires careful control over these pretreatments to avoid damage to the very thin bondpads. This paper reports a number of experiments designed to characterize the activation of Al bondpads to electroless nickel plating, focusing on the effects of solution exposure time and bondpad composition. In addition, the results are discussed in the context of other studies presented in the literature to provide an understanding of the mechanism of the zincate activation process applied to Al bondpads.

THE ASSEMBLY PROCESS FOR ANISOTROPIC CONDUCTIVE JOINTS — SOME NEW EXPERIMENTAL AND THEORETICAL RESULTS

This paper reports recent results from an experimental and theoretical programme addressing the issues in the assembly of joints constructed using random and ordered array anisotropic conductive adhesive materials. The paper presents initial theoretical models of the early stages of the assembly process to assist understanding of the process variables. Complementary experimental work is presented which explores the evolution of conductivity with processing conditions. Initial results are presented on the assembly of a novel ordered array material. The paper closes by drawing the results of the experimental and theoretical work together and presents suggestions for further work.

Application of adhesives in MEMS and MOEMS assembly: a review

This paper presents a review of the recent literature on the use of adhesives in MEMS packaging applications. The aim of this review has been to establish the current applications of adhesives in MEMS and MOEMS assembly and to investigate the limitations and future requirements of these materials. The review has shown that while there is a wealth of information available on the packaging of MEMS devices, there is very limited detail available within the public domain regarding the specific uses of adhesives and in particular exactly which products are in use. The paper begins with an overview of the uses of adhesives in MEMS packaging, subdivided into sections on structural adhesives, adhesives for optical applications and other applications. The paper then describes methods for adhesive dispensing and issues with adhesive use which affect the reliability of the package. The reliability of MEMS devices assembled using adhesives is a challenging issue, being more than a simple combination of electrical, mechanical and material reliability. Many failure modes in MEMS devices can be attributed to the adhesives used in the assembly; for example, thermal expansion mismatches can cause stress in the die attach, while outgassing from epoxies can cause failure of sealed devices and contamination of optical surfaces.

The effects of conducting particle distribution on the behaviour of anisotropic conducting adhesives: non-uniform conductivity and shorting between connections

This paper presents mathematical models that are intended to assist in the design of anisotropic conducting materials. Such materials are usually random distributions of hard conducting spheres in epoxy resin matrices. Conducting joints are constructed in soft conducting materials by the plastic deformation of the conductor on the printed circuit boards. An earlier paper (Williams et al., 1993) describes some of the average behaviour of the materials during joint assembly and after assembly. This work explores the effects of the random distribution of particles in the material on resistivity variation and the probability of shorting between fine-pitch interconnections.

Direct-write techniques for maskless production of microelectronics: A review of current state-of-the-art technologies

Recently, there has been growing interest in direct-write methods for the manufacturing of microelectronic products, as the entire electronics industry sector is aiming towards low cost, rapid manufacturing and shorter time-to-market, as well as reduced environmental impacts. This paper will review the main direct-write techniques, most of which have been invented or seen significant development during the last decade. These techniques include droplet-based direct writing, such as inkjet printing, filament-based direct writing, such as the Micropen and nScrypt processes, tip based direct-write methods, and laser beam direct writing. For each category, only a few examples are presented, although there are a number of specific methods and variants within each of these categories.

Electroless nickel bumping of aluminum bondpads. II. Electroless nickel plating

Electroless nickel has been used for many decades to provide a hard, corrosion resistant surface finish to engineering components. In recent years, its application has been extended to the electronics industry for the production of solderable surfaces on printed circuit boards, which utilize a further thin gold coating to prevent oxidation of the nickel surface. The recent interest in the use of flip-chip technology in electronics manufacture has required the development of low cost methods for solder bumping of semiconductor wafers. The electroless nickel process has been considered as a suitable candidate for the deposition of a solderable under bump metallization (UBM) layer onto the Al bondpads. However, the extension of existing electroless nickel plating processes to this new application requires greater understanding of the technique. In particular, the coating of the small isolated bondpads on the wafer surface introduces difficulties that make the use of many commercially available solutions impossible. This paper reports the results of a number of experiments carried out to investigate the electroless nickel bumping of Al bondpads and highlights the issues that need to be considered when selecting materials and techniques.

Some optimum processing properties for anisotropic conductive adhesives for flip chip interconnection

In terms of electrical properties, the optimum interconnect packaging option for a bare semiconductor die is to attach directly the chip to the substrate — flip chip attach. This strategy has been followed for high complexity/cost boards in the past, but recently considerable interest has grown in the possible use of this technology for lower cost substrates and processes. Anisotropic conductive adhesives offer one possible route for low-cost flip chip technology, and in this paper, we use models to determine some of the optimum material properties required, and compare them against the properties of some representative current-generation materials to identify where improvements can be made. The materials properties examined in this paper include rheology of the adhesive resin, and type, size and volume fraction of conductive filler particles. The models developed deal with the time required to process the adhesive, changes in particle densities on the pads, and probabilities of electrical opens and shorts.