Mark D. Poliks

Study of the interlayer expansion mechanism and thermal-mechanical properties of surface-initiated epoxy nanocomposites

Abstract The exfoliation mechanism and thermal–mechanical properties of surface-initiated epoxy nanocomposites were studied. Time-resolved high-temperature X-ray diffraction, DSC, and isothermal rheological analyses revealed that the interlayer expansion mechanism might be separated into three stages. These stages relate to the initial interlayer expansion, the steady-state interlayer expansion, and the cessation of interlayer expansion. It was found that differences in the activation energies of interlayer expansion and of curing influence the final nanostructures of the materials. The thermal–mechanical properties of the nanocomposites were studied using dynamic mechanical thermal analysis. Variations in ultimate properties were attributed to the formation of an interphase layer, where the interphase is hypothesized to be the epoxy matrix plasticized by surfactant chains.

Flexible chemiresistor sensors: thin film assemblies of nanoparticles on a polyethylene terephthalate substrate

The thin film assembly of metal nanoparticles on flexible chemiresistor (CR) arrays represents an intriguing way to address the versatility of chemical sensor design. In this work, thin film assemblies of gold nanoparticles in size range of 2–8 nm diameters with high monodispersity (unlinked or linked by molecular mediators) were assembled on a CR array with a polyethylene terephthalate (PET) substrate to demonstrate the flexible chemiresistor characteristics of the nanostructured materials. The correlation between the relative change in electrical conductivity and the change in dielectric medium constant in response to flexible wrapping of the device demonstrated the viability of manipulating the electrical responses in terms of wrapping direction. The responses of the devices in response to volatile organic compounds (VOCs) were analyzed in terms of particle size, interparticle properties, and substrate–film interactions. For molecularly linked films with small particle size and large interparticle spacing, which is characterized by a high percentage of organics and linker molecules, the relatively low electrical conductivity renders the change in interparticle spacing able to play a dominant role in the sensor response to VOCs with small dielectric constants. The combination of a high percentage of linker molecules in the thin film assembly and a high dielectric constant for the VOCs was found to produce a negative response characteristic. In contrast, the response characteristic for the unlinked film via weak interparticle interactions was dominated by the change in interparticle spacing regardless of the percentage of organics in the nanostructure. The delineation between these factors and the sensing characteristics is useful in enabling a rationale design of the nanostructures on flexible chemiresistors.

A Roll-to-Roll Photolithography Process for Establishing Accurate Multilayer Registration on Large Area Flexible Films

Roll-to-roll (R2R) flexible electronics manufacturing techniques may eventually provide a solution for continuous production of high quality flexible display devices at a significant cost reduction. The new display applications that can be enabled by using R2R technologies include inexpensive display, large area display, and etc. In this work, the R2R photolithography system and dependent materials and processes are used to establish baseline data for fabrication, registration and overlay of micron sized patterns on unsupported plastic in pieces and carried by a web. Enabling the use of unsupported plastic film is the first step in understanding the R2R overlay registration process for fabrication of high quality flexible displays. Test verniers with up to 0.1 micron measurement precision were used to read the overlay offsets. Micro-sized features with one micron overlay accuracy have been achieved on photoresist coated free standing 5 mil thick Dupont Melinex® ST507 polyethylene terephthalate (PET) substrate. Rohm & Haas LC-100 photoresist was used in this work. Soda lime glass substrates (Telic) were also used to establish the tool and process fundamental limits.

Nanoparticle-Structured Highly Sensitive and Anisotropic Gauge Sensors.

The ability to tune gauge factors in terms of magnitude and orientation is important for wearable and conformal electronics. Herein, a sensor device is described which is fabricated by assembling and printing molecularly linked thin films of gold nanoparticles on flexible microelectrodes with unusually high and anisotropic gauge factors. A sharp difference in gauge factors up to two to three orders of magnitude between bending perpendicular (B(⊥)) and parallel (B(||)) to the current flow directions is observed. The origin of the unusual high and anisotropic gauge factors is analyzed in terms of nanoparticle size, interparticle spacing, interparticle structure, and other parameters, and by considering the theoretical aspects of electron conduction mechanism and percolation pathway. A critical range of resistivity where a very small change in strain and the strain orientation is identified to impact the percolation pathway in a significant way, leading to the high and anisotropic gauge factors. The gauge anisotropy stems from molecular and nanoscale fine tuning of interparticle properties of molecularly linked nanoparticle assembly on flexible microelectrodes, which has important implication for the design of gauge sensors for highly sensitive detection of deformation in complex sensing environment or on complex curved surfaces such as wearable electronics and skin sensors.

Flexibility characteristics of a polyethylene terephthalate chemiresistor coated with a nanoparticle thin film assembly

Polyethylene terephthalate (PET) functions as a flexible substrate for the fabrication of functional devices by low-cost and scalable roll-to-roll manufacturing. Exploration of this attribute for chemical sensors requires understanding of the flexibility characteristics in correlation with the sensing properties in terms of a combination of device strain and molecular interactions in different chemical environments. This report describes new findings of an investigation of the response characteristics of PET chemiresistor sensors coated with a thin film assembly of gold nanoparticles in response to different device strains and adsorption of volatile organic compounds. The work demonstrates that the sensor response characteristics can be tuned by a combination of flexible device strain parameters. A significant finding is that the contribution to the changes in the sensing signals and sensitivities depends on not only the molecular nature of species being detected, but also a combination of the interparticle spatial, dielectric medium, and device strain properties. This combination is also associated with the orientation of the microelectrode patterns with respect to the device strain direction. These findings have an important implication for the design of nanoparticle-coupled flexible chemical sensors for effective detection of chemical or biological species in different sensing environments.

Experimental and Analytical Studies on the High Cycle Fatigue of Thin Film Metal on PET Substrate for Flexible Electronics Applications

This paper addresses the behavior of thin-film metal coated flexible substrates under high cyclic bending fatigue loading. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are widely used substrates in the fabrication of microelectronic devices. Factors affecting the fatigue life of thin-film coated on a flexible PET substrates were studied, including thin-film thickness, film material, bending radius, temperature, and humidity. A series of experiments for sputter-deposited copper and aluminum coated on a PET substrate were conducted. Electrical resistance and crack growth rate were monitored during the experiments at specified time intervals. In addition, a finite element model was built to simulate the bending of thin-films on flexible substrate structure. Layered shell elements were used in the model. Stress intensity and stress distribution across the film were obtained and compared with the experiments. Initial results of copper-coated PET showed a great agreement between the model and the experimental results.

High capacitance, large area, thin film, nanocomposite based embedded capacitors

This paper discusses thin film technology based on barium titanate (BaTiO3)-epoxy polymer nanocomposites. In particular, we highlight recent developments on high capacitance, large area, thin film passives, their integration in PWB substrates and the reliability of the embedded capacitors. A variety of nanocomposite thin films ranging from 2 microns to 25 microns thick were processed on PWB substrates by liquid coating or printing processes. SEM micrographs showed uniform particle distribution in the coatings. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance and dissipation factor (loss) measurements. Nanocomposites resulted in high capacitance density (10-100 nF/inch2) and low loss (0.02-0.04) at 1 MHz. The manufacturability of these films and their reliability has been tested using large area (13 inch times 18 inch or 19.5 inch times24 inch) test vehicles. Reliability of the test vehicles was ascertained by IR-reflow, thermal cycling, PCT (pressure cooker test) and solder shock. Capacitors were stable after PCT and solder shock. Capacitance change was less than 5% after IR reflow (assembly) preconditioning (3X, 245 degC) and 1400 cycles DTC (deep thermal cycle)

Package-Interposer-Package (PIP): A breakthrough Package-on-Package (PoP) technology for high end electronics

This paper discusses a new 3D “Package-Interposer-Package” (PIP) solution suitable for combining multiple memory, ASICs, stacked die, stacked packaged die, etc., into a single package. Recent work on interposers to join multiple packages is highlighted, with particular attention paid to the processing of the electrical joints formed between the interposer and package. A variety of package-interposer-package joining approaches were considered. Photographs were used to investigate the joining, conducting mechanism and path. Traditional Package-on-Package (PoP) approaches use direct solder connections between the packages and are limited to use of single (or minimum) die in the bottom package(s) in order to avoid warpage and poor reliability performance. This is because each package may have a different warpage trend from room temperature to reflow temperature when combined with other packages. For PIP, the stability imparted by the interposer reduces warpage and increases stability, allowing assemblers of the PIP to select the top and bottom components (packages, dies, stacked die, modules) from various suppliers. PIP can accommodate multiple stacks of dies. PIP can use modules with stacked die where modules can be organic, ceramic, or silicon board, where each can be detached and replaced without affecting the rest of the package. Thus PIP will be economical for high-end electronics, where a damaged, non-functional part of the package can be selectively removed and replaced. The paper also describes interconnect construction for a PIP. The present process allows fabrication of PIP interconnect joints having diameters in the range of 55–300 microns, allowing finer pitch, higher density packaging structures. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed.

Reliability of Sputtered Aluminum Thin Film on Flexible Substrate Under High Cyclic Bending Fatigue Conditions

Aluminum thin films on flexible substrates are very popular as a back electrode in solar photovoltaic technology. However, during their manufacturing and use, the package is subject to cyclic bending, which leads to cracks in the conductive thin film and ultimately failure of the package. This paper investigates the effect of film thickness, bending diameter (BD), and number of cycles (NOC) on crack development and the percentage change in electrical resistance (PCER) of aluminum thin films under cyclic bending conditions. PCER-NOC diagrams are constructed at all considered factor-level combinations. These curves are used in comparisons between high and low levels of BD and film thickness. The Design of Experiment tool is used to investigate the effect and significance of film thickness, BD, NOC, and the interactions between them on the PCER. In this regard, all factors are found to be significant. Furthermore, thickness-NOC and BD-NOC interactions are significant, while thickness-BD interaction is not significant. Moreover, a finite element model is built to investigate the area of the highest stress on the aluminum thin film, in other words, the area with the most fatigue potential.

Development of rigid-flex and multilayer flex for electronic packaging

Recent development work on flex joining using different pre-pregs is highlighted, particularly with respect to their integration in laminate chip carrier substrates, and the reliability of the joints formed between the rigid and flex surfaces. A variety of rigid-flex structures were fabricated, with 1 to 3 flex layers laminated into printed wiring board substrates. Photographs and optical microscopy were used to investigate the joining, bending, and failure mechanism. Flexibility decreased with increasing number of metal layers. The flexibility of the various flexes was characterized by roll diameter and bend angle. Flex substrates exhibited roll diameter with polyimide dielectric as low as 180 mils for 2 metal layers, and as high as 1300 mils for 6 metal layers. Similarly, bending for 12 metal layers flex with thin and thick dielectric were <1 inch and >1 inch, respectively. Reliability of the rigid-flex was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. There was no delamination for Resin coated copper (rigid)-polyimide (flex) samples after IR-reflow, PCT, and solder shock. The paper also describes a novel approach for the fabrication of flexible electronics on PDMS substrates. It was found that with increasing thickness, the flexibility of the polydimethylsiloxane (PDMS) based substrate decreased less due to stretching property of PDMS. The present process evaluates the fabrication of PDMS substrates using different circuit lines and spaces.