Experimental and modelling studies of electronic packaging interconnections formed with lead-free materials

Both the experimental and the modelling techniques have been investigated and used to investigate factors that influence the formation, quality and reliability of electronic packaging interconnections formed using solder alloys and anisotropic conductive films (ACFs). The wetting behaviours of new lead-free solders (i.e. Sn-2.8Ag-0.5Cu-1.0Bi and Sn-0.7Cu-0.3Ni) have been evaluated using the wetting balance test. This assessment ahs been performed for three soldering temperatures with three different types of fluxes. The results have been compared with the conventional Sn-Pb, Sn-Ag-Cu and Sn-Cu solders. It has been found that the wettability of the lead-free solders is not as good as that of the Sn-Pb solder. The additions of Bi into the Sn-Ag-Cu solder and Ni into the Sn-Cu solder improve the wettability that is strongly dependent on the type of the flux and the soldering temperature. In general, NC-flux is suitable for Cu-substrate whereas Ws-flux is suitable for Ni substrate, but for the Sn-2.8Ag-0.5-Cu solder on Ni substrate, good wettability has been observed with both the NC and the R-type fluxes. Computational modelling of this test has revealed that the increase in the depth and the radius of the solder bath has little effect on the wetting force, but the meniscus height decreases when the bath radius exceeds 14 mm. Dissolution of solid metals into liquid solders has been investigated through experiments and computer modelling. Microstructural studies have been carried out and the growth behaviours of the intermetallic compounds (IMCs) during wetting, solidification and isothermal ageing have been investigated. It has been found that the addition of Bi into the Sn-Ag-Cu solder reduces the consumptions of the substrates and suppresses the growth of IMCs during wetting and ageing. Similarly, the addition of Ni into the Sn-Cu solder reduces both the consumptions of the substrates and the growth of IMCs during wetting and short term ageing but enhances the growth of IMCs during long term ageing. Experimental and computer modelling techniques have been used to measure the temperature in the ACF during bonding. The temperature in an ACF joint becomes very close to the required maximum bonding temperature within the first 1 s of bonding time. The impact of this temperature on the cure process and on the ACF physical properties such as loss modulus, storage modulus, and glass transition temperature has been investigated. It has been found that the higher the bonding temperature the more the curing degree of ACF is. Rapid changes occur in the physical properties of ACF at temperatures above the glass transition point. When the ACF is cured for a long time at a high temperature, the physical properties may degrade. The adhesion strength of ACF joint increases as the curing degree increases. However, when the ACF joint undergoes a thermal aging treatment, the adhesion strength increases for the samples bonded at lower temperatures, but decreases for the samples bonded at higher temperatures. The rate of increase in the contact resistance is dramatically higher for the samples bonded at lower temperatures than for those bonded at higher temperatures. Computer modelling of the isothermal ageing of ACF joint confirms that the thermal load causes the expansion of the adhesive matrix and generates high stresses on the conductive particle. This may result in the permanent damage of the outermost conductive metallic layers as well as electrical failure. The effect of external bending loads on the electrical reliability of ACF-based interconnection has also been studied through computer modelling. The analysis reveals that ACF thickness increases at the corners of the chip-ACF joint more than that of the middle position. This causes a gap between the chip and the substrate results in the failure of the electrical interconnections

Novel fine pitch interconnection methods using metallised polymer spheres

There is an ongoing demand for electronics devices with more functionality while reducing size and cost, for example smart phones and tablet personal computers. This requirement has led to significantly higher integrated circuit input/output densities and therefore the need for off-chip interconnection pitch reduction. Flip-chip processes utilising anisotropic conductive adhesives anisotropic conductive films (ACAs/ACFs) have been successfully applied in liquid crystal display (LCD) interconnection for more than two decades. However the conflict between the need for a high particle density, to ensure sufficient the conductivity, without increasing the probability of short circuits has remained an issue since the initial utilization of ACAs/ACFs for interconnection. But this issue has become even more severe with the challenge of ultra-fine pitch interconnection. This thesis advances a potential solution to this challenge where the conductive particles typically used in ACAs are selectively deposited onto the connections ensuring conductivity without bridging. The research presented in this thesis work has been undertaken to advance the fundamental understanding of the mechanical characteristics of micro-sized metal coated polymer particles (MCPs) and their application in fine or ultra-fine pitch interconnections. This included use of a new technique based on an in-situ nanomechanical system within SEM which was utilised to study MCP fracture and failure when undergoing deformation. Different loading conditions were applied to both uncoated polymer particles and MCPs, and the in-situ system enables their observation throughout compression. The results showed that both the polymer particles and MCP display viscoelastic characteristics with clear strain-rate hardening behaviour, and that the rate of compression therefore influences the initiation of cracks and their propagation direction. Selective particle deposition using electrophoretic deposition (EPD) and magnetic deposition (MD) of Ni/Au-MCPs have been evaluated and a fine or ultra-fine pitch deposition has been demonstrated, followed by a subsequent assembly process. The MCPs were successfully positively charged using metal cations and this charging mechanism was analysed. A new theory has been proposed to explain the assembly mechanism of EPD of Ni/Au coated particles using this metal cation based charging method. The magnetic deposition experiments showed that sufficient magnetostatic interaction force between the magnetized particles and pads enables a highly selective dense deposition of particles. Successful bonding to form conductive interconnections with pre-deposited particles have been demonstrated using a thermocompression flip-chip bonder, which illustrates the applicable capability of EPD of MCPs for fine or ultra-fine pitch interconnection

Anisotropic Conductive Adhesives for Interdigitated Back Contact (IBC) Silicon Solar Cells

The current manufacturing process for solar panels using interdigitated back contact (IBC) silicon solar cells involves a multi-step metallization and interconnection process in which a substantial amount of silver is used. This work focuses on a new process using conductive adhesives (CA) which would increase efficiency and lower cost through a one-step metallization and interconnection process that combines with encapsulation using little silver and only requiring metal patterning on the back sheet or back glass. It would also not require direct metallization of the silicon, which would result in fewer defects, while increasing voltage and therefore efficiency. Silver-coated Poly(Methyl Methacrylate) Microsphere (AgMS) and indium powder are the primary materials used as the conductive particles in an ethyl vinyl acetate (EVA)/toluene adhesive. The CA is prepared by mixing the components in toluene. The resulting mixture is used to produce 300μm thick CA sheets using a universal applicator, cut into pieces, and pressed between a piece of glass with coplanar Ag electrodes and a silicon wafer at varying temperatures and pressures. This yields ~3 Ωcm2 for both the AgMS and indium fillers. Significantly lower values are required for the target application, and possible new approaches in attaining lower resistivity are discussed

A Novel Contact Resistance Model of Anisotropic Conductive Film for FPD Packaging

In this research, a novel contact resistance model for the flat panel display (FPD) packaging based on the within layer parallel and between layers series resistance concepts was proposed. The FJ2530 anisotropic conductive films (ACF) by Sony Inc. containing the currently smallest 3micron conductive particles was used to conduct the experiments to verify the accuracy of the proposed model. Calculated resistance of the chip-on-glass (COG) packaging by the proposed model is 0.163\Omega. It is found that the gold bump with 0.162\Omega resistance play the major role of the overall resistance. Although the predicted resistance by the proposed model is only one third of the experimentally measured value, it has been three-fold improvement compared to the existing models.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

Parasitic Effects Reduction for Wafer-Level Packaging of RF-Mems

In RF-MEMS packaging, next to the protection of movable structures, optimization of package electrical performance plays a very important role. In this work, a wafer-level packaging process has been investigated and optimized in order to minimize electrical parasitic effects. The RF-MEMS package concept used is based on a wafer-level bonding of a capping silicon substrate to an RF-MEMS wafer. The capping silicon substrate resistivity, substrate thickness and the geometry of through-substrate electrical interconnect vias have been optimized using finite-element electromagnetic simulations (Ansoft HFSS). Test structures for electrical characterization have been designed and after their fabrication, measurement results will be compared with simulations.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

ACCELERATED AGING OF MWCNT FILLED ELECTRICALLY CONDUCTIVE ADHESIVES

Electrically conductive adhesives (ECA) are discussed and studied with everincreasing interest as an environmentally friendly alternative to solder interconnection in microelectronics circuit packaging. They are used to attach surface mount devices (SMD), Integrated Circuits (IC) and Flip chips in electronic assembly. The use of ECAs brings some benefits like flexibility, mild processing conditions and process simplicity. Multi walled carbon nanotubes (MWCNT) are used instead of metal fillers because of their novel properties such as light weight, high aspect ratio, corrosion resistant, reduced processing temperature, lead free, good electrical conduction and mechanical strength. The purpose of the present work is to investigate the aging behavior of MWCNT filled adhesives based on anhydride cured epoxy systems and their dependence on loading. Composites with different loadings of MWNT in epoxy and epoxy: heloxy are prepared and then stencil printed onto different surface finished boards like gold, silver and tin to prepare contact resistance samples and onto aluminum oxide boards to prepare volume resistivity samples. These samples are kept at room temperature for about 90 days and then placed in a temperature chamber to observe the behavior of these samples after accelerated aging. The readings are taken for as prepared samples, after 45 days, after 90 days and after accelerated aging. The results are summarized and different trends are observed for different loadings of MWNT, different combinations of epoxy: heloxy and for different surface finished boards

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 AND MECHANICAL PROPERTIES OF MWCNT FILLED CONDUCTIVE ADHESIVES ON LEAD FREE SURFACE FINISHED PCB\u27s.

Electrically conductive adhesives (ECA) are an alternative to tin/lead solders for attaching Surface Mount Devices (SMD) in electronic assemblies. ECAs are mixtures of a polymer binder (for adhesion) and conductive filler (for electrical conductivity). They bring more conductivity, higher strength, less weight and longer durability than metal alloys. ECAs can offer numerous advantages such as fewer processing steps, lower processing temperature and fine pitch capability. Multi walled carbon nanotubes (MWCNT) were used as conductive fillers in this research because of their novel electronic and mechanical properties. The high aspect ratio of the nanotubes makes it possible to percolate at low loadings to obtain good electrical and mechanical properties. Replacing the metal filler with CNTs in the adhesive made the ECA light weight, corrosion resistant, reduced processing temperature, lead free, electrically conductive and high mechanical strength. The MWCNTs at different loadings were mixed with epoxy and epoxy: heloxy to form a composite mixture. Different loadings, additives and mixing methods were used to obtain good electrical and mechanical properties and pot life. Pressure dispensing, screen and stencil printing were the processing techniques used for making the samples. The volume resistivity, contact resistance, die shear and lap shear tests were conducted on different surface finished Printed Circuit Boards (PCB) like silver, tin and Electro less Nickel Immersion Gold (ENIG). The results are summarized and compared with traditional methods