Samjid H. Mannan

A review: On the development of low melting temperature Pb-free solders

Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.

Materials and processes for implementing high-temperature liquid interconnects

This paper describes the results of a study investigating liquid solder joints at elevated temperatures (up to 200/spl deg/C). The reactions of eutectic 52In/48Sn solder, which melts at 118/spl deg/C, with various metal barrier layers is presented. The main emphasis of the research was to find a combination of solder and substrate metallization which has good adhesion strength but also remains stable during temperature cycling and high-temperature storage when the solder is molten. Intermetallic growth rates and solder-substrate adhesion strength have been measured for a range of potential barrier layers including Ni, Cr, Pt, Ti, V, Nb, Ta, and W. Of these, only Nb was found to have acceptable properties for a high-temperature barrier layer to In/Sn solder. Other aspects of liquid solder interconnections that have been studied include stability of the molten solder-underfill interface under electrical bias and retention of electrical contact during vibration and phase change. Plastic ball grid array (PBGA) devices have been assembled with Nb barrier layers and liquid solder joints and their reliability during temperature cycling (-20/spl deg/C to +180/spl deg/C) has been compared to PBGA joints with Sn95.5/Ag4/Cu0.5 solder balls.

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.

Squeegee deformation study in the stencil printing of solder pastes

We report on the results of an experimental comparison of different types of squeegee blade used in the stencil printing of solder pastes for reflow soldering in SMT, concentrating on paste heights (scooping) and printing defects. We show how our experimental results for squeegee deformation into stencil apertures lead to the construction of a model for squeegee deformation. The model takes into account the force on the squeegee due to solder paste flow and some of the non-Newtonian properties of the solder paste. An explanation is proposed for the differences in paste heights between apertures of different orientations. >

The top quark condensate

It is possible that electroweak symmetry might be broken dynamically by a condensate of top quarks. We study to quark dynamics in the presence of some new heavy gauge boson exchange, and consider the effect of making the four-fermion approximation. The gap equation in dressed ladder approximation is solved numerically and the solutions ∑t(p) are used to determine 〈tt〉 and the decay constants F±,F0, as well as the top quark mass itself. We also discuss the cut-off dependence of these results, and comment on the fine-tuning problem.

Anisotropic conducting adhesives for electronic assembly

Presents some experimental and theoretical results from research exploring the design rules and relevant process parameters in the assembly of electronic components using anisotropic conductive adhesive materials. The experimental configurations studies have geometries representative of flip‐chip and micro ball grid array chip scale packaging. Evaluates a range of materials combinations, including (random filled) adhesive materials based on both thermoplastic and thermo‐setting resin systems, combined with both glass reinforced polymer printed circuit board and silver palladium thick film on ceramic substrate materials. Also presents a summary of assembly experiments which have been conducted using a specially developed instrumented assembly system. This test rig allows the measurement of the process temperatures and pressures and their relationship with the consequent bondline thickness reduction and conductivity development. Finally summarizes the capabilities of models which have been developed of the assembly process and of the final joint properties.

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.