Ultra thin ultrafine-pitch chip-package interconnections for embedded chip last approach

Ever growing demands for portability and functionality have always governed the electronic technology innovations. IC downscaling with Moore s law and system miniaturization with System-On-Package (SOP) paradigm has resulted and will continue to result in ultraminiaturized systems with unprecedented functionality at reduced cost. The trend towards 3D silicon system integration is expected to downscale IC I/O pad pitches from 40µm to 1- 5 µm in future. Device- to- system board interconnections are typically accomplished today with either wire bonding or solders. Both of these are incremental and run into either electrical or mechanical barriers as they are extended to higher density of interconnections. Alternate interconnection approaches such as compliant interconnects typically require lengthy connections and are therefore limited in terms of electrical properties, although expected to meet the mechanical requirements. As supply currents will increase upto 220 A by 2012, the current density will exceed the maximum allowable current density of solders. The intrinsic delay and electromigration in solders are other daunting issues that become critical at nanometer size technology nodes. In addition, formation of intermetallics is also a bottleneck that poses significant mechanical issues. Recently, many research groups have investigated various techniques for copper-copper direct bonding. Typically, bonding is carried out at 400oC for 30 min followed by annealing for 30 min. High thermal budget in such process makes it less attractive for integrated systems because of the associated process incompatibilities. In the present study, copper-copper bonding at ultra fine-pitch using advanced nano-conductive and non-conductive adhesives is evaluated. The proposed copper-copper based interconnects using advanced conductive and non-conductive adhesives will be a new fundamental and comprehensive paradigm to solve all the four barriers: 1) I/O pitch 2) Electrical performance 3) Reliability and 4) Cost. This thesis investigates the mechanical integrity and reliability of copper-copper bonding using advanced adhesives through test vehicle fabrication and reliability testing. Test vehicles were fabricated using low cost electro-deposition techniques and assembled onto glass carrier. Experimental results show that proposed copper-copper bonding using advanced adhesives could potentially meet all the system performance requirements for the emerging micro/nano-systems.M.S.Committee Chair: Prof. Rao R Tummala; Committee Member: Dr. Jack Moon; Committee Member: Dr. P M Ra

Electrical termination techniques

A technical review of high reliability electrical terminations for electronic equipment was made. Seven techniques were selected from this review for further investigation, experimental work, and preliminary testing. From the preliminary test results, four techniques were selected for final testing and evaluation. These four were: (1) induction soldering, (2) wire wrap, (3) percussive arc welding, and (4) resistance welding. Of these four, induction soldering was selected as the best technique in terms of minimizing operator errors, controlling temperature and time, minimizing joint contamination, and ultimately producing a reliable, uniform, and reusable electrical termination

Isotropically conductive adhesive filled with silver metalised polymer spheres

Isotropic conductive adhesives (ICAs) have a growing range of applications in electronics packaging and have recently emerged as an important material in photo-voltaic module interconnections, particularly for thin-film and other non-silicon technologies where soldering processes are often unsuitable due to the nature of the metallisation or the limited maximum temperature the assembly can be exposed to. ICAs typically comprise of a high volume fraction of solid metallic flakes, usually silver, in an adhesive matrix because of its highly conductive oxide however, this thesis will focus on adhesives containing a large volume fraction of silver coated/metalised mono-sized polymer spheres (Ag-MPS). Incorporating silver coated mono-sized polymer spheres is anticipated to deliver specific advantages such as a significant reduction in the required silver content, improvement of the overall mechanical properties and flexibility to tune the properties of the filler according to the application compared with conventional flake filled adhesives. In this research advancements in the understanding of Ag-MPS filled ICAs, both through theory and experiments, have been made. Analytical models to predict an individual Ag-MPS resistance and Ag-MPS filled ICA resistance have been developed. The experiments based on the flat punch nanoindentation technique have been conducted to determine individual Ag-MPS resistances. The theoretical and experimental studies establish Ag-MPS diameter, coating resistivity, coating thickness, contact radius, and contact geometry as the main contributors towards the resistance of an Ag-MPS filled ICAs. These studies showed that Ag-MPS resistance decreases with increasing coating thickness and contact radius but increases with increasing coating resistivity. The experiments have also been conducted to investigate the effect of Ag-MPS volume fraction, diameter, coating thickness, curing conditions and shrinkage (affecting contact radius) on ICA conductivity and comparisons are made with flake filled and commercial ICAs. The results showed that ICA conductivity increases with increasing volume fraction and coating thickness but decreases with diameter. More importantly the results showed that conductivities similar to those of flake filled ICAs, including those commercially available, can be obtained using 70% less silver. The results show that, Ag content can be reduced further to just 7% with use of larger 30μm Ag-MPS but with a lower resulting conductivity. Thus for applications where very high conductivity is not required larger Ag-MPS may offer even greater potential cost benefits, which is something flake filled ICAs cannot offer. This is a significant achievement which can allow tuning of ICA formulations according to the demands of the application, which is not possible with the use of silver flakes as there is only a limited range of silver flake volume fractions that will yield useful levels of conductivity

FABRICATION, CHARACTERIZATION AND APPLICATIONS OF HIGHLY CONDUCTIVE WET-SPUN PEDOT:PSS FIBERS

Smart electronic textiles cross conventional uses to include functionalities such as light emission, health monitoring, climate control, sensing, storage and conversion of energy, etc. New fibers and yarns that are electrically conductive and mechanically robust are needed as fundamental building blocks for these next generation textiles. Conjugated polymers are promising candidates in the field of electronic textiles because they are made of earth-abundant, inexpensive elements, have good mechanical properties and flexibility, and can be processed using low-cost large-scale solution processing methods. Currently, the main method to fabricate electrically conductive fibers or yarns from conjugated polymers is the deposition of the conducting polymer onto an inert fiber support by using different techniques. However, the volume occupied by the electrically active coating is generally very small relative to the volume of insulating fiber acting as support. Therefore, when considering the total volume, the bulk electrical conductivity of these coated textiles is usually small, often lower than 10 S/cm, which limits their applications. An interesting alternate approach would be to fabricate fibers directly from the electrically conductive material avoiding the need for an inert-fiber support. Therefore, in this work, a wet-spinning process for the fabrication of PEDOT:PSS fibers with high electrical conductivity and robust mechanical properties is described. The process includes a coagulating step, a drawing step in a dimethyl sulfoxide bath and two drying steps. The effect that drawing the fibers in the DMSO bath has on the electrical, thermoelectric and mechanical properties of the fibers is studied and correlated to the changes observed in the fibers’ structure. In general, the fibers with the highest state of preferential orientation of crystal planes are also the most conductive and stiffest. In order to further improve the electrical properties of the fibers, substituting the DMSO drawing step by a sulfuric acid drawing step in the fabrication process is investigated. The sulfuric acid drawn fibers have higher electrical conductivities and better mechanical properties than the DMSO drawn fibers. In fact, electrical conductivities as high as 4039 S/cm and break stresses around 550 MPa are obtained which, to the best of our knowledge, are the highest reported for a PEDOT:PSS fiber. The mechanism by which sulfuric acid enhances the electrical and mechanical properties of the fibers is also investigated. It is found that the sulfuric acid treatment is very efficient removing PSS from the fibers while also promoting substitution of PSS by sulfates as counterions. The removal of PSS and substitution of counterions leads to a reorganization of the crystal structure of the fibers that is more favorable for charge transport. The last part of this work focuses on the application of the fibers. The mechanical properties of the fibers are compared to traditional textile fibers. Additionally, the time stability of the electrical conductivity of the fibers is also studied. Moreover, the maximum current carrying capacity or ampacity of the fibers is investigated together with some Joule heating-based applications such as thermochromic textiles. A thermoelectric textile device is also demonstrated using the fibers as the p-type legs. Finally, electrochemical applications of the fibers are discussed and demonstrated

Toward the use of temporary tattoo electrodes for impedancemetric respiration monitoring and other electrophysiological recordings on skin

The development of dry, ultra-conformable and unperceivable temporary tattoo electrodes (TTEs), based on the ink-jet printing of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on top of commercially available temporary tattoo paper, has gained increasing attention as a new and promising technology for electrophysiological recordings on skin. In this work, we present a TTEs epidermal sensor for real time monitoring of respiration through transthoracic impedance measurements, exploiting a new design, based on the application of soft screen printed Ag ink and magnetic interlink, that guarantees a repositionable, long-term stable and robust interconnection of TTEs with external “docking” devices. The efficiency of the TTE and the proposed interconnection strategy under stretching (up to 10%) and over time (up to 96 h) has been verified on a dedicated experimental setup and on humans, fulfilling the proposed specific application of transthoracic impedance measurements. The proposed approach makes this technology suitable for large-scale production and suitable not only for the specific use case presented, but also for real time monitoring of different bio-electric signals, as demonstrated through specific proof of concept demonstrators

High-Ampacity Power Cables of Tightly-Packed and Aligned Carbon Nanotubes

We characterize the current-carrying capacity (CCC), or ampacity, of highly-conductive, light, and strong carbon nanotube (CNT) fibers by measuring their failure current density (FCD) and continuous current rating (CCR) values. We show, both experimentally and theoretically, that the CCC of these fibers is determined by the balance between current-induced Joule heating and heat exchange with the surroundings. The measured FCD values of the fibers range from 10$^7$ to 10$^9$ A/m$^2$ and are generally higher than the previously reported values for aligned buckypapers, carbon fibers, and CNT fibers. To our knowledge, this is the first time the CCR for a CNT fiber has been reported. We demonstrate that the specific CCC (i.e., normalized by the linear mass density) of our CNT fibers are higher than those of copper.Comment: 14 pages, 8 figure

Stretchability : the metric for stretchable electrical interconnects

Stretchable circuit technology, as the name implies, allows an electronic circuit to adapt to its surroundings by elongating when an external force is applied. Based on this, early authors proposed a straightforward metric: stretchability—the percentage length increase the circuit can survive while remaining functional. However, when comparing technologies, this metric is often unreliable as it is heavily design dependent. This paper aims to demonstrate this shortcoming and proposes a series of alternate methods to evaluate the performance of a stretchable interconnect. These methods consider circuit volume, material usage, and the reliability of the technology. This analysis is then expanded to the direct current (DC) resistance measurement performed on these stretchable interconnects. A simple dead reckoning approach is demonstrated to estimate the magnitude of these measurement errors on the final measurement

Venyvän elektroniikan vaihtoehtoiset valmistus- ja tutkimusmenetelmät

Stretchable electronics are used in wearable applications to implement intelligent features. The main characteristic of stretchable electronics is stretchability enabling deformation required in wearable objects such as bandages and clothes. In this thesis, the stretchable electronics consist of elastic substrates, printed stretchable interconnections, adhesives and rigid modules with traditional electronic components. The modules on the elastic substrate form rigid islands that allow the substrate to stretch. Stretchable electronics can endure only a specific amount of elongation before their electrical interconnections fail. Adhesion and deformation mechanisms in the joint and in the joint area of the module and the substrate affect elongation. The durability of stretchable electronics can be improved by improving adhesion and controlling the deformations via optimizing the structure of the joint and the joint area. In this thesis, the stretchable electronics were studied on several levels. A thermoplastic polyurethane (TPU) film was used as the elastic substrate. Wettability and effectiveness of pre-treatments on wettability were examined. The substrate was investigated by measuring contact angles of droplets with a drop shape analyzer. Adhesion and peel behavior of non-conductive adhesives between the TPU-film and the rigid substrates were studied with a floating roller peel test setup. Finally, tensile testing was used to investigate deformations and elongation of the fabricated stretchable electronics samples. In the tensile test samples, width of the interconnection, the amount of the conductive adhesive and the use of a supportive frame structure were varied. The tests presented new results that can be adopted alone or as whole. The wettability of the TPU-film improved most with a plasma pre-treatment that decreased the contact angles up to 63 percent. The peel tests showed that the sample with one cyanoacrylate adhesive with a primer had the highest momentary bond strength (0,5 N/mm). The high bond strength made the TPU-film elongate during the peeling test. Unlike the tested structural adhesives, an elastic transfer tape adhesive had the most even peeling force during the tests (between 0,2 – 0,3 N/mm) and was the easiest adhesive to process. According to the stress peaking concept, in the tensile testing, when the samples elongated, stress concentrated close to the attached module and broke the samples. The strongest interconnection elongated 91,7 % before failure. The referred sample type had the supportive frame and conductive adhesive only under the contacts. Similarly, according to the concept, the stress exerted on this sample was more uniform compared to the other tensile test samples, which explains the good results