Lead-tin solder has been widely used as interconnection material in electronics packaging for a long time. In response to environmental legislation, the lead-tin alloys are being replaced with lead-free alloys and electrically conductive adhesives in consumer electronics. Lead-free solder usually require higher reflow temperatures than the traditional lead-tin alloys, which can cause die crack and board warpage in assembly process, thereby impacting the assembly yields. The high tin content in lead-free solder forms tin whiskers, which has the potential to cause short circuits failure. Conductive adhesives are an alternative to solder reflow processing, however, conductive adhesives require up to 80 wt% metal filler to ensure electrical and thermal conductivity. The high loading content degrades the mechanical properties of the polymer matrix and reduces the reliability and assembly yields when compared to soldered assemblies. Carbon nanotubes (CNTs) have ultra high aspect ratio as well as many novel properties. The high aspect ratio of CNTs makes them easy to form percolation at low loading and together with other novel properties make it possible to provide electrical and thermal conductivity for the polymer matrix while maintaining or even reinforcing the mechanical properties. Replacing the metal particles with CNTs in conductive adhesive compositions has the potential benefits of being lead free, low process temperature, corrosion resistant, electrically/thermally conductive, high mechanical strength and lightweight. In this paper, multiwall nanotubes (MWNTs) with different dimensions are mixed with epoxy. The relationships among MWNTs dimension, volume resistivity and thermal conductivity of the composite are characterized. Different loadings of CNTs, additives and mixing methods were used to achieve satisfying electrical and mechanical properties and pot life. Different assembly technologies such as pressure dispensing, screen and stencil printing are used to simplify the processing method and raise the assembly yields. Contact resistance, volume resistivity, high frequency performance, thermal conductivity and mechanical properties were measured and compared with metal filled conductive adhesive and traditional solder paste.

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Electrical conductive adhesives (ECAs) containing silver filler and polyaniline co-filler were characterized for their electro-mechanical properties. Polyaniline is a conductive polymer and has a moderate conductivity in between those of the silver and epoxy. Incorporation of polyaniline (μm sized) in silver-epoxy facilitated the electrical conduction in ECAs and reduced the percolation threshold- a minimum volume of filler necessary to initiate the conduction. It also prevented the localization of charge carriers due to aggregation of silver filler particles. ‘Bridging effect’ was observed due to addition polyaniline in which the polyaniline enhanced the tunneling of electrons over the silver filler particles. We have investigated the polyaniline co-fillers as a promising alternative way to tune the mechanical and electrical properties of the ECAs and have provided a detailed analysis of the electro-mechanical properties of silver-epoxy (Ag-epoxy) and silver-polyaniline-epoxy (Ag-PANI-epoxy) system in both partially-cured/ viscoelastic and fully-cured states. Analysis of electro-mechanical properties of silver-epoxy and silver-polyaniline-epoxy also provided the insights into electrical contact resistance of ECAs under compressive force. Electro-mechanical properties of ECAs were measured ‘in-situ’ using micro-indentation technique. We also synthesized the electrically conductive and highly crystalline nanotubes of polyaniline by mini-emulsion polymerization of aniline. The motivation behind the synthesis of polyaniline was to propose a potential filler/co-filler for replacement of metallic filler in ECAs. Electrical conductivity of polyaniline nanotubes was tuned by in-situ doping using hydrochloric acid as a dopant. Increase in dopant caused the polyaniline crystallite to grow along (400) plane. Optical, structural, electrical and thermal properties of polyaniline nanotubes are reported with varying amount of dopant. We fabricated the flexible electrically conductive coating of polyaniline tubes with uniform dispersion of polyaniline. Electrical performance of as-synthesized flexible coating is also revealed.

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.

Electrical conductive adhesives (ECAs) containing silver filler and polyaniline co-filler were characterized for their electro-mechanical properties. Polyaniline is a conductive polymer and has a moderate conductivity in between those of the silver and epoxy. Incorporation of polyaniline (μm sized) in silver-epoxy facilitated the electrical conduction in ECAs and reduced the percolation threshold- a minimum volume of filler necessary to initiate the conduction. It also prevented the localization of charge carriers due to aggregation of silver filler particles. ‘Bridging effect’ was observed due to addition polyaniline in which the polyaniline enhanced the tunneling of electrons over the silver filler particles. We have investigated the polyaniline co-fillers as a promising alternative way to tune the mechanical and electrical properties of the ECAs and have provided a detailed analysis of the electro-mechanical properties of silver-epoxy (Ag-epoxy) and silver-polyaniline-epoxy (Ag-PANI-epoxy) system in both partially-cured/ viscoelastic and fully-cured states. Analysis of electro-mechanical properties of silver-epoxy and silver-polyaniline-epoxy also provided the insights into electrical contact resistance of ECAs under compressive force. Electro-mechanical properties of ECAs were measured ‘in-situ’ using micro-indentation technique. We also synthesized the electrically conductive and highly crystalline nanotubes of polyaniline by mini-emulsion polymerization of aniline. The motivation behind the synthesis of polyaniline was to propose a potential filler/co-filler for replacement of metallic filler in ECAs. Electrical conductivity of polyaniline nanotubes was tuned by in-situ doping using hydrochloric acid as a dopant. Increase in dopant caused the polyaniline crystallite to grow along (400) plane. Optical, structural, electrical and thermal properties of polyaniline nanotubes are reported with varying amount of dopant. We fabricated the flexible electrically conductive coating of polyaniline tubes with uniform dispersion of polyaniline. Electrical performance of as-synthesized flexible coating is also revealed.

Isotropically conductive adhesives (ICAs) and inks are potential candidates for low cost interconnect materials and widely used in electrical/electronic packaging applications. Silver (Ag)filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer (SAM). The coating was found to be able to limit the re-oxidation of the Cumicron particles. The treated Cu powderswerecombined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave assisted heating methods under an inert atmosphere for the curing of the Cu loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80-150°C) and time. Tracks with electrical conductivity comparable to Ag filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in curing mechanism for the two formulations.

Submitted by JOSIANE SANTOS DE OLIVEIRA (josianeso) on 2018-04-19T17:05:39Z No. of bitstreams: 1 Tiago di Giovani Lunardi_.pdf: 6951536 bytes, checksum: 4d60fac658887a2701421d77cb011f8d (MD5) / Made available in DSpace on 2018-04-19T17:05:39Z (GMT). No. of bitstreams: 1 Tiago di Giovani Lunardi_.pdf: 6951536 bytes, checksum: 4d60fac658887a2701421d77cb011f8d (MD5) Previous issue date: 2018-03-21 / HT Micron / Por muitos anos a liga metálica mais utilizada como solda foi a liga Pb-Sn, material maleável e de baixa temperatura de fusão. Com a diretiva europeia 2002/95/CE (conhecida como diretiva RoHS), é homologada a suspensão do uso de substâncias nocivas como o chumbo. Ocorre a necessidade de substituir a liga de solda Pb-Sn por novos materiais e as mais utilizadas deste então são as ligas da família SAC (abreviação dos elementos que compõe a liga - Sn-Ag-Cu). Essa liga, funde em temperaturas superiores a 210ºC, o que pode vir a danificar os componentes eletrônicos, cada vez menores e com mais funções. Surge então a possibilidade da utilização de adesivos eletricamente condutivos. Estes são formulados, em sua maioria, utilizando-se partículas de prata e outros metais bons condutores como ouro e níquel, dispersas em uma matriz polimérica. Dessa forma, há economia de metais, frente à utilização de pastas de solda. Este trabalho propõe três formulações para adesivos eletricamente condutivos, obtidos a partir de uma resina epóxi com butadieno e com diferentes tipos de partículas condutivas: nanotubos de carbono e prata com diferentes morfologias. Todos os materiais foram caracterizados quanto as suas características físicas, morfológicas e elétricas através das técnicas de MEV, medição da resistência elétrica pelo método 4 pontas, choque térmico e envelhecimento. A resistência elétrica de cada material e a resistência de contato foram testadas e comparadas com adesivos comerciais e a referência solda metálica SAC305. Foi identificado que os adesivos demonstraram resiliência em relação ao contato com a PCI mesmo após sucessivos ciclos de envelhecimento em 85°C com 85% UR ou ensaios de choque térmico. A resistência elétrica medida no adesivo formulado a partir das nanofolhas de prata aproxima-se muito dos valores obtidos com a referência comercial, na ordem de 10-4 cm. Foi também o único material que apresentou variação de resistência elétrica inferior a 20% após o choque térmico, comprovando seu bom desempenho frente às demais formulações deste trabalho. / For many years the most commonly used metal alloy as solder was the Pb-Sn alloy, a malleable material with a low melting temperature. With the European Directive 2002/95/CE (as known as RoHS Directive), the suspension of the use of hazardous substances such as lead is approved. It is necessary to replace the Pb-Sn solder alloy with new materials and the most used of these are the alloys of the SAC family (metal alloy composed by Sn-Ag-Cu). This alloy melts at temperatures above 210°C, which can damage the electronic components, that are becoming smaller and with more functions. The possibility of using electrically conductive adhesives arises. These are mostly formulated using silver particles and other good conducting metals such as gold and nickel, dispersed in a polymer matrix. In this way, there is economy of metals, compared to the use of solder pastes. This work proposes three formulations for electrically conductive adhesives, obtained from an epoxy resin with butadiene with different types of conductive fillers: carbon nanotubes and silver with different morphologies. All the materials were characterized as their physical, morphological and electrical characteristics through MEV, electrical resistance measurement by the 4-point probes method, thermal shock and aging. Electrical resistance of each material and the contact resistance were tested and compared with commercial adhesives and the reference SAC305 alloy solder. It was identified that the adhesives demonstrated resilience in relation to the contact with the PCI even after successive aging cycles at 85°C with 85% RH or thermal shock tests. Electrical resistance measured on the adhesive formulated using silver nanosheets is very close to the values obtained with the commercial reference, in the order of 10-4 cm. It was also the only material that presented less than 20% variation of electrical contact resistance after thermal shock, proving its good performance against the other formulations of this work.

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.

This thesis is devoted to the research and development of low processing temperature lead-free interconnect materials for microelectronic packaging applications with an emphasis on fundamental studies of nanoparticles synthesis, dispersion and oxidation prevention, and nanocomposites fabrication. Oxide-free tin (Sn), tin/silver (96.5Sn3.5Ag) and tin/silver/copper (96.5Sn3.0Ag0.5Cu) alloy nanoparticles with different sizes were synthesized by a low temperature chemical reduction method. Both size dependent melting point and latent heat of fusion of the synthesized nanoparticles were obtained. The nano lead-free solder pastes/composites created by dispersing the SnAg or SnAgCu alloy nanoparticles into an acidic type flux spread and wet on the cleaned copper surface at 220 to 230 ¡æ. This study demonstrated the feasibility of nano sized SnAg or SnAgCu alloy particle pastes for low processing temperature lead-free interconnect applications in microelectronic packaging. Surface functionalized silver nanoparticles and silver fakes were used as fillers for electrically conductive adhesives (ECAs) applications. During the curing of epoxy resin (150 ¡æ), the surfactants were debonded from the particles and at the same time the oxide layers on the particle surfaces were removed which facilitated the sintering of Ag nanoparticles. The contact interfaces between fillers were significantly reduced and an ultra highly conductive ECA with a resistivity of 5 ¡Á 10-6 ohm.cm was obtained. To enhance the adhesion of carbon nanotube (CNT) films to substrates, an ultra highly conductive ECA were used as a media to transfer the CNT films to copper substrates. The polymer wetted along the CNTs during curing process by the capillary force. An ohmic contact was formed between the copper substrates and the transferred CNTs. This process could overcome the serious obstacles of integration of CNTs into integrated circuits and microelectronic device packages by offering low processing temperatures and improved adhesion of CNTs to substrates. The transferred CNTs can be used to simultaneously form electrical and mechanical connections between chips and substrates.

The objective of this study was to evaluate and gain a better understanding of the short-term impact performance and the long-term durability of electrically conductive adhesives for electronic interconnection applications. Three model conductive adhesives, designated as ECA1, ECA2 and ECA3, supplied by Emerson & Cuming, were investigated, in conjunction with printed circuit board (PCB) substrates with metallizations of Au/Ni/Cu and Cu, manufactured by Triad Circuit Inc. Effects of environmental aging on the durability of conductive adhesives and their joints were evaluated. All the samples for both mechanical tests and thermal tests were aged at 85%, 100%RH for periods of up to 50 days. Studies of bulk conductive adhesives suggested that both plasticization, which is reversible and further crosslinking and thermal degradation, which are irreversible, might have occurred upon exposure of ECAs to the hot/wet environment. The durability of electrically conductive adhesive joints was then investigated utilizing the double cantilever beam (DCB) test. It was observed that the conductive adhesive joint was significantly weakened following hydrothermal aging, and there was a transition from cohesive failure to interfacial failure as aging continued. A comparative study of the durability of different conductive adhesive and substrate metallization combinations suggested that the resistance of the adhesive joints to moisture attack is related to the adhesive properties, as well as the substrate metallizations. It was noted that the gold/adhesive interface had better resistance to moisture attack than the copper/adhesive interface. A reasonable explanation of this phenomenon was given based upon the concept of surface free energy and interfacial free energy. XPS analysis was performed on the fractured surfaces of DCB samples. For adhesive joints with copper metallization, copper oxide was detected on the failed surfaces upon exposure of the conductive adhesive joints following aging. XPS analysis on the fractured surfaces of adhesive joints with Au metallization suggested that diffusion of Cu to the Au surface might have happened on the Au/Ni/Cu plated PCB substrates during aging. The impact performance of conductive adhesives was quantitatively determined using a falling wedge test. This unique impact resistance testing method could serve as a useful tool to screen conductive adhesives at the materials level for bonding purpose. Moreover, this test could also provide some useful information for conductive adhesive development. This study revealed that the viscoelastic energy, which is a result of the internal friction created by chain motions within the adhesive material, played an important role in the impact fracture behavior of the conductive adhesives. This study also demonstrated that the loss factor, evaluated at the impact environment conditions, is a good indicator of a conductive adhesive's ability to withstand impact loading. / Ph. D.