Development and reliability of a direct access sensor using flip chip on flex technology with anisotropic conductive adhesive

Technological developments in biomedical microsystems are opening up new opportunities to improve healthcare procedures. Swallowable diagnostic sensing capsules are an example of these. In none of the diagnostic sensing capsules, is the sensor’s first level packaging achieved via Flip Chip Over Hole (FCOH) method using Anisotropic Conductive Adhesive (ACA). In a capsule application with direct access sensor (DAS), ACA not only provides the electrical interconnection but simultaneously seals the interconnect area and the underlying electronics. The development showed that the ACA FCOH was a viable option for the DAS interconnection. Adequate adhesive formed a strong joint that withstood a shear stress of 120N/mm2 and a compressive stress of 6N required to secure the final sensor assembly in place before encapsulation. Electrical characterization of the ACA joint in a fluid environment showed that the ACA was saturated with moisture and that the ions in the solution actively contributed to the leakage current, characterized by the varying rate of change of conductance. Long term hygrothermal aging of the ACA joint showed that a thermal strain of 0.004 and a hygroscopic strain of 0.0052 were present and resulted in a fatigue like process. In-vitro tests showed that high temperature and acidity had a deleterious effect of the ACA and its joint. It also showed that the ACA contact joints positioned at around or over 1mm would survive the gastrointestinal (GI) fluids and would be able to provide a reliable contact during the entire 72hr of the GI transit time. A final capsule demonstrator was achieved by successfully integrating the DAS, the battery and the final foldable circuitry into a glycerine capsule. Final capsule soak tests suggested that the silicone encapsulated system could survive the 72hr gut transition.

Study of Anisotropic Conductive Adhesive Joint Behaviour under 3-Point Bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.

The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications

The work presented in this paper focuses on the effect of reflow process on the contact resistance and reliability of anisotropic conductive film (ACF) interconnection. The contact resistance of ACF interconnection increases after reflow process due to the decrease in contact area of the conducting particles between the mating I/O pads. However, the relationship between the contact resistance and bonding parameters of the ACF interconnection with reflow treatment follows the similar trend to that of the as-bonded (i.e. without reflow) ACF interconnection. The contact resistance increases as the peak temperature of reflow profile increases. Nearly 40% of the joints were found to be open after reflow with 260 °C peak temperature. During the reflow process, the entrapped (between the chip and substrate) adhesive matrix tries to expand much more than the tiny conductive particles because of the higher coefficient of thermal expansion, the induced thermal stress will try to lift the bump from the pad and decrease the contact area of the conductive path and eventually, leading to a complete loss of electrical contact. In addition, the environmental effect on contact resistance such as high temperature/humidity aging test was also investigated. Compared with the ACF interconnections with Ni/Au bump, higher thermal stress in the Z-direction is accumulated in the ACF interconnections with Au bump during the reflow process owing to the higher bump height, thus greater loss of contact area between the particles and I/O pads leads to an increase of contact resistance and poorer reliability after reflow.

Moisture Effects on the Reliability of Anisotropic Conductive Film Interconnection for Flip Chip on Flex Applications

Anisotropic conductive film (ACF) which consists of an adhesive epoxy matrix and randomly distributed conductive particles are widely used as the connection material for electronic devices with high I/O counts. However, for the semiconductor industry the reliability of the ACF is still a major concern due to a lack of experimental reliability data. This paper reports the investigations into the moisture-induced failures in Flip-Chip-on-Flex interconnections with Anisotropic Conductive Films (ACFs). Both experimental and modeling methods were applied. In the experiments, the contact resistance was used as a quality indicator and was measured continuously during the accelerated tests (autoclave tests). The temperature, relative humidity and the pressure were set at 121°C, 100%RH, and 2atm respectively. The contact resistance of the ACF joints increased during the tests and nearly 25% of the joints were found to be open after 168 hours’ testing time. Visible conduction gaps between the adhesive and substrate pads were observed. Cracks at the adhesive/flex interface were also found. For a better understanding of the experimental results, 3-D Finite Element (FE) models were built and a macro-micro modeling method was used to determine the moisture diffusion and moisture-induced stresses inside the ACF joints. Modeling results are consistent with the findings in the experimental work.

Moisture Effects on the Reliability of Anisotropic Conductive Films

Anisotropic conductive film (ACF) which consists of an adhesive epoxy matrix and randomly distributed conductive particles are widely used as the connection material for electronic devices with high I/O counts. However, for the semiconductor industry the reliability of the ACF is still a major concern due to a lack of experimental reliability data. This paper reports an investigation into the moisture effects on the reliability of ACF interconnections in the flip-chip-on-flex (FCOF) applications. A macro-micro 3D finite element modeling technique was used in order to make the multi-length-scale modeling of the ACF flip chip possible. The purposes of this modeling work was to understand the role that moisture plays in the failure of ACF flip chips, and to look into the influence of physical properties and geometric characteristics, such as the coefficient of the moisture expansion (CME), Young's modulus of the adhesive matrix and the bump height on the reliability of the ACF interconnections in a humid environment. Simulation results suggest that moisture-induced swelling of the adhesive matrix is the major cause of the ACF joint opening. Modeling results are consistent with the findings in the experimental work.

Nano-scale morphological and electrical characterization of nc-SiOxNy thin layers for photovoltaic applications

Il presente lavoro di tesi propone uno studio approfondito di proprietà morfologiche e di trasporto di carica di film sottili di SiOxNy amorfi (a-SiOxNy) e nanocristallini (nc-SiOxNy), che trovano importanti applicazioni in celle fotovoltaiche ad eterogiunzione in silicio, ad alta efficienza. Lo studio è condotto mediante caratterizzazione elettrica e morfologica attraverso tecniche di microscopia a forza atomica (AFM). Sono stati studiati campioni di a-SiOxNy cresciuti con tecnica PECVD (Plasma Enhanced Chemical Vapor Deposition), in cui è stata variata unicamente la distanza tra gli elettrodi durante la deposizione. Sono stati inoltre studiati campioni di nc-SiOxNy, cresciuti con PECVD con una differente percentuale di N2O come gas precursore e un differente tempo di annealing. In entrambi i casi si tratta di un materiale innovativo, le cui proprietà fisiche di base, nonostante le numerose applicazioni, sono ancora poco studiate. L'analisi morfologica, condotta mediante AFM e successiva analisi statistica delle immagini, ha permesso di determinare alcune proprietà morfologiche dei campioni. L’analisi statistica delle immagini è stata validata, dimostrandosi stabile e consistente per lo studio di queste strutture. Lo studio delle proprietà di trasporto è stato condotto mediante acquisizione di mappe di corrente con tecnica conductive-AFM. In questo modo si è ottenuta una mappa di conducibilità locale nanometrica, che permette di comprendere come avviene il trasporto nel materiale. L'analisi di questo materiale mediante tecniche AFM ha permesso di evidenziare che l'annealing produce nei materiali nanocristallini sia un clustering della struttura, sia un significativo aumento della conducibilità locale del materiale. Inoltre la distanza tra gli elettrodi in fase di deposizione ha un leggero effetto sulle dimensioni dei grani. È da notare inoltre che su questi campioni si sono osservate variazioni locali della conducibilità alla nanoscala. L’analisi delle proprietà dei materiali alla nanoscala ha contribuito alla comprensione più approfondita della morfologia e dei meccanismi di trasporto elettronico.

Electrical Characterization of Airborne Vehicle Exhaust Plume

he induced current and voltage on the skin of an airborne vehicle due to the coupling of external electromagnetic field could be altered in the presence of ionized exhaust plume. So in the present work, a theoretical analysis is done to estimate the electrical parameters such as electrical conductivity and permittivity and their distribution in the axial and radial directions of the exhaust plume of an airborne vehicle. The electrical conductivity depends on the distribution of the major ionic species produced from the propellant combustion. In addition it also depends on temperature and pressure distribution of the exhaust plume as well as the generated shock wave. The chemically reactive rocket exhaust flow is modeled in two stages. The first part is simulated from the combustion chamber to the throat of the supersonic nozzle by using NASA Chemical Equilibrium with Application (CEA) package and the second part is simulated from the nozzle throat to the downstream of the plume by using a commercial Computational Fluid Dynamics (CFD) solver. The contour plots of the exhaust parameters are presented. Eight barrel shocks which influence the distribution of the vehicle exhaust parameters are obtained in this simulation. The computed peak value of the electrical conductivity of the plume is 0.123 S/m and the relative permittivity varies from 0.89 to 0.99. The attenuation of the microwave when it is passing through the conducting exhaust plume has also been presented.

Electrical characterization of surface defects in GaSb created by hydrogen plasma

Using steady state and transient capacitance measurements, the electrical characteristics of a defect layer on the surface of bulk GaSb created during the hydrogen plasma treatment is presented. The trap density, activation energies, and the thickness of the defect layer have been calculated. The trap densities are comparable in magnitude to the carrier concentration. The defects introduce multiple energy levels in the band gap. Typical defect layer thicknesses range from a few angstroms to a fraction of a micron. © 1995 American Institute of Physics.

Electrical characterization of Ba(Zr0.1Ti0.9)O-3 thin films grown by pulsed laser ablation technique

In situ annealed thin films of ferroelectric Ba(Zr0.1Ti0.9)O-3 were deposited on platinum substrates by pulsed laser ablation technique. The as grown films were polycrystalline in nature without the evidence of any secondary phases. The polarization hysteresis loop confirmed the ferroelectricity, which was also cross-checked with the capacitance-voltage characteristics. The remnant polarization was about 5.9 muC cm(-2) at room temperature and the coercive field was 45 kV. There was a slight asymmetry in the hysteresis for different polarities, which was thought to be due to the work function differences of different electrodes. The dielectric constant was about 452 and was found to exhibit low frequency dispersion that increased with frequency, This was related to the space-charge polarization. The complex impedance was plotted and this exhibited a semicircular trace, and indicated an equivalent parallel R - C circuit within the sample. This was attributed to the grain response. The DC leakage current-voltage plot was consistent with the space-charge limited conduction theory, but showed some deviation, which was explained by assuming a Poole-Frenkel type conduction to be superimposed on to the usual space-charge controlled current. (C) 2002 Elsevier Science B.V. All rights reserved.

Structural, thermal and electrical characterization of NdLiMo2O8 electroceramics, using impedance spectroscopy

We report electrical property of a polycrystalline NdLiMo2O8 ceramics using complex impedance analysis. The material shows temperature dependent electrical relaxation phenomena. The d.c. conductivity shows typical Arrhenius behavior, when observed as a function of temperature. The a.c. conductivity is found to obey Jonscher's universal power law. The material was prepared in powder form by a standard solid-state reaction technique. Material formation and crystallinity have been confirmed by X-ray diffraction studies. Impedance measurements have been performed over a range of temperatures and frequencies. The results have been analyzed in the complex plane formalism and suitable equivalent circuits have been proposed in different regions. The role of bulk and grain boundary effect in the overall electrical conduction process is discussed with proper justification. (C) 2011 Elsevier Ltd. All rights reserved.

Effect of film thickness and annealing on optical properties of TiO2 thin films and electrical characterization of MOS capacitors

Titanium dioxide (TiO2) thin films were deposited on glass and silicon (100) substrates by the sol-gel method. The influence of film thickness and annealing temperature on optical transmittance/reflectance of TiO2 films was studied. TiO2 films were used to fabricate metal-oxide-semiconductor capacitors. The capacitance-voltage (C-V), dissipation-voltage (D-V) and current-voltage (I-V) characteristics were studied at different annealing temperatures and the dielectric constant, current density and resistivity were estimated. The loss tangent (dissipation) increased with increase of annealing temperature.

Parasitic feedthrough cancellation techniques for enhanced electrical characterization of electrostatic microresonators

Capacitive parasitic feedthrough is an impediment that is inherent to all electrically interfaced micron scale resonant devices, resulting in increased challenges to their integration in more complex circuits, particularly as devices are scaled to operate at higher frequencies for RF applications. In this paper, a technique to cancel the undesirable effects of capacitive feedthrough that was previously proposed is here developed for an on-chip implementation. The method reported in this paper benefits from the simplicity of its implementation, and its effectiveness is demonstrated in this paper. This technique is demonstrated for two disk-plate resonators that have been excited in the wine glass mode at 5.4 MHz, though applicable to almost any electrically interfaced resonator. Measurements of the electrical transmission from these resonators show that the magnitude of the frequency response of the system is enhanced by up to 19 dB, while the phase is found to shift through a full 180° about the resonant frequency. This method is proposed as a useful addition to other techniques for enhancing the measured response of electrostatic micromechanical resonators. © 2009 Elsevier B.V. All rights reserved.