Assembly of planar array components using anisotropic conducting adhesives: a benchmark study. Part I - experiment

This paper presents new results from an experimental and theoretical program to evaluate relevant process parameters in the assembly of a 500 m pitch area array component using anisotropic conductive adhesive (ACA) materials. This experimental configuration has features of micro ball grid array ( BGA), chip scale packaging (CSP), and also flip-chip and conventional ball grid array (BGA) package structures. A range of materials combinations have been evaluated, including (random filled) adhesive materials based on both thermoplastic and thermosetting resin systems, combined with both organic and thick-film on ceramic substrate materials. The ACA’s used have all been applied as films, and hence are also known as anisotropic conducting films (ACF). The test assemblies have been constructed using a specially developed instrumented assembly system which allows the measurement of the process temperatures and pressures and the consequent bondline thickness reduction and conductivity development. The effects of the process parameters on the resulting properties, particularly conductivity and yield, are reported. A complementary paper [1] indicates the results of computational fluid dynamics (CFD) models of the early stages of the assembly process which allow the extrapolation of the present results to finer pitch geometries

Electrical method of monitoring percolation and abrasion of conducting spheres due to shear flow of a dense suspension in a narrow gap

This letter describes a method for studying the behavior of rigid particles in a dense suspension when they are forced into contact during flow within a narrow gap. The particles form transient percolating networks spanning the boundary walls, and will be crushed together. The method involves measuring the dc electrical resistance across the gap. The suspension e.g., solder paste consists of electrically conducting particles suspended in an insulating fluid. The electrical resistance drops when the particles are in contact with each other and the walls, and the insulating films on the surface of the conductors have been broken through. The results show a dramatic change in behavior as the ratio of gap to particle diameter is varied

Observations of solder paste reflow by means of electrical measurements

This paper presents a method for exploring the changes occurring at the surfaces of solder particles during the reflow soldering process. The method involves measuring changes in electrical resistance of a sample of paste as a function of test voltage. The results are used to estimate the size and nature of electrical contact spots between the particles, and how these depend on temperature and time. The activation energy of the process responsible for increasing the size of contact spots is deduced for RA and RMA type fluxes and it is shown that sintering is not the dominant mechanism for increasing contact size. These results, together with a programme of CFD studies are expected to help improve solder paste formulations

Assembly of planar array components using anisotropic conducting adhesives: a benchmark study. Part II - theory

This paper presents new results from an experimental and theoretical program to evaluate relevant process parameters in the assembly of a 500 m pitch area array component using anisotropic conductive adhesive (ACA) materials. This experimental configuration has features of micro ball grid array ( BGA) chip scale packaging (CSP) and also flip-chip and conventional ball grid array (BGA) package structures. The paper presents the results of computational fluid dynamics (CFD) models of the early stages of the assembly process when the adhesive is squeezed out between the device and the substrate. Experimental results on the assembly trials are presented in an accompanying paper [1] and have been previously reported at the Adhesives in Electronics Conference, Gothenburg, 1996 [2]. The CFD models outlined here show how those results might be expected to change for smaller pitches

Electroless nickel bumping of aluminium bondpads. Part 2 - 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

Conduction mechanisms in anisotropic conducting adhesive assembly

This paper explores both experimentally and through analytical and computational models, the mechanisms of conduction in flip-chip interconnections made using anisotropic conducting adhesives. A large number of assemblies have been constructed with geometries in the range of 200–500 m, and wide variations in their joint resistance were observed to occur both within the same assembly and between assemblies under the same experimental conditions. In order to attempt to explain the origin of these unsatisfactory connections, a series of experiments to measure the linearity of the contact resistance of both high and low resistance joints was made. The results from these measurements show that the large number of low resistance joints are ohmic, while most of the joints of relatively high resistance show resistive heating. In addition to the linearity measurements, computational models of metallic conduction in solid and polymer cored particles have been constructed to help understand the mechanism of conduction. These models, which are based on the finite element (FE) method, represent typical conductor particles trapped between appropriate substrate and component metallization. The results from the models show that the contact area required to explain the 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 contact surfaces of the joints

Electroless nickel bumping of aluminium bondpads. Part 1 - surface pre-treatment and activation

Electroless nickel bumping of aluminum (Al) bondpads 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

A comparative study of the interfacial reaction between electroless Ni-P coatings and molten tin

A comparative study of the reaction characteristics between molten tin and both as-plated and heat-treated Ni-P coatings was carried out, with a specific focus on the stability of the Ni3P intermetallic layer and its effects on the subsequent reaction. It was found that a continuous layer of Ni3P may be formed on both types of Ni-P during the interfacial reaction, despite the fact that heat-treated Ni-P is a two-phase mixture of Ni3P and Ni. The Ni3P formed on the heat-treated Ni-P was thinner than that on as-plated Ni-P. A mass conservation analysis of P revealed that no or limited P was lost into the molten tin when the Ni3P layer was thin, whereas a significant loss of P took place as the Ni3P thickness increased. It is proposed that the Ni3P phase is stable and it may not undergo chemical decomposition during the interfacial reaction. The loss of P to the molten tin observed in the present study is most likely due to the crumbling of Ni3P particles into the liquid phase, as a result of the enhanced mass transport due to use of thin copper wire substrates rather than a planar surface. Finally, the results show that the Ni3P phase cannot act 2 as an effective barrier layer to the attack of molten tin toward the substrate. Defects in the Ni3P were found to allow localised penetration of molten tin

Solder paste reflow modeling for flip chip assembly

Solder paste printing and reflow can provide low cost techniques producing the solder bumps on flip chips. Solder paste consists of a dense suspension of solder particles in a fluid medium (vehicle) that acts as an oxide reducing agent (flux) during reflow, cleaning the metal surfaces of oxides. This paper reports on optical observations of paste behaviour at the small length scales associated with flip chip solder joints, and attempts to model the process using Computational Fluid Dynamics (CFD). Comparison of optical observations and CFD modelling show that the behaviour of the solder cannot be described simply by surface tension and viscous flow effects and it is deduced that oxides are still present on the solder surfaces during the early stages of reflow. The implications for paste heating method and solder volume are discussed, and a preliminary CFD model (based on FIDAP) incorporating the effect of the oxide layers is presented

The effect of temperature ramp rate on flip-chip joint quality and reliability using anisotropically conductive adhesive on FR-4 substrate

In this work, the effect of temperature ramp rate on flip-chip anisotropically conductive adhesive joint quality and reliability has been studied. The experiments were performed on bumped and unbumped die. They were assembled onto bare ITO-glass and FR-4 substrates. The reason for using the transparent glass substrate is that the particle deformation and settlement can be visualised without destroying the assembled module. The temperature ramp rates studied ranged between 8.1 and 65.7°C/s. The experiments show that the best joint quality is obtained when a slow temperature ramp rate is applied to unbumped dies. A good joint is achieved when many particles have been entrapped on the die pad and when there is a significant degree of particle deformation. A large degree of deformation of particles results in a large contact area for the electrical conduction path. When a high temperature ramp rate is applied, there is a risk that the adhesive is already cured before full compression is reached. This will prevent the particles in the adhesive from contacting the bonding surface. When assembling bumped die, the temperature ramp rate does not seem to have a significant influence on the result. The joint quality evaluation has been performed using Scanning Electron Microscopy (SEM) and Optical Microscopy (OM). Furthermore, temperature cycling between -40 to +125 °C, 1000 cycles, has been performed to characterise the joint reliability under the optimum temperature ramp rate conditions. The electrical resistance has been measured continuously. A theoretical simulation of the influence of the temperature ramp rate on the adhesive joint quality has been performed using the same test module conditions as for the experimental work. The results coincide with the experimental results, particularly in the range of low bonding pressure value