FPCB as an Acoustic Matching Layer for 1D Linear Ultrasound Transducer Arrays.

An acoustic matching layer is an essential component of an ultrasound transducer to achieve maximum ultrasound transmission efficiency. Here, we develop a flexible printed circuit board (FPCB) with a composite structure consisting of multiple polyimide and copper layers and demonstrate it as a novel acoustic matching layer. With a flexible substrate and robust ACF bonding, the FPCB not only serves as an acoustic matching layer between piezoelectric elements and the surrounding medium but also as a ground for the electrical connection between the transducer array elements and the folded substrate. A 1D linear ultrasound transducer array with the FPCB matching layer exhibits larger output pressure, wider -3dB bandwidth, and higher ultrasound beam intensity compared to that of an ultrasound transducer array with the alumina/epoxy matching layer, which is one of the most commonly applied composite matching layers. The enhanced transmission performance verifies that the proposed FPCB is an excellent matching layer for 1D linear ultrasound transducer arrays.

Acoustic Matching Layer Films Using B-Stage Thermosetting Polymer Resins for Ultrasound Transducer Applications.

Acoustic matching layer films (MLFs) were fabricated using B-stage thermosetting polymer resins with various volume fractions of alumina and tungsten powders. After making certain thickness MLFs, ultrasonic matching layers were fabricated using a simple molding process. The thickness of the matching layers can be precisely adjusted from several micrometer to hundreds of micrometer, without any grinding process. Experimental values of acoustic impedances of the matching layers were in good agreement with theoretical values calculated by the Devaney model. Using the optimized acoustic matching layer by the MLFs, the maximum intensity and the fractional bandwidth of the ultrasonic transducer were increased by 10% and 37% respectively. The effectiveness of the matching layer using MLFs was successfully verified.

Ultrathin Nanofibrous Membranes Containing Insulating Microbeads for Highly Sensitive Flexible Pressure Sensors.

Highly sensitive and flexible pressure sensors were developed based on dielectric membranes composed of insulating microbeads contained within polyvinylidene fluoride (PVDF) nanofibers. The membrane is fabricated using a simple electrospinning process. The presence of the microbeads enhances porosity, which in turn enhances the sensitivity (1.12 kPa-1 for the range of 0-1 kPa) of the membrane when used as a pressure sensor. The microbeads are fixed in position and uniformly distributed throughout the nanofibers, resulting in a wide dynamic range (up to 40 kPa) without any sensitivity loss. The fluffy and nonsticky PVDF nanofiber features low hysteresis and ultrafast response times (∼10 ms). The sensor has also demonstrated reliable pressure detection over 10 000 loading cycles and 250 bending cycles at a 13 mm bending radius. These pressure sensors were successfully applied to detect heart rate and respiratory signals, and an array of sensors was fabricated and used to recognize spatial pressure distribution. The sensors described herein are ultrathin and ultralight, with a total thickness of less than 100 µm, including the electrodes. All of the materials comprising the sensors are flexible, making them suitable for on-body applications such as tactile sensors, electronic skins, and wearable healthcare devices.

IMCs Microstructure Evolution Dependence of Mechanical Properties for Ni/Sn/Ni Micro Solder-Joints.

The current miniaturization trend of microelectronic devices drives the size of solder joints to continually scale down. The miniaturized joints considerably increase intermetallic compounds (IMCs) volume fraction to trigger mechanical reliability issues. This study investigated precise relationships between varying IMC volumes and mechanical properties of Ni/Sn(20µm)/Ni micro-joints. A designed method that followed the IMC volume as the only variable was used to prepare micro-joint samples with different IMC volumes. The continuously thickened Ni3Sn4 IMCs exhibited a noticeable morphology evolution from rod-like to chunky shape. The subsequent tensile tests showed unexpected tensile strength responses as increasing Ni3Sn4 volume, which was strongly associated with the Ni3Sn4 morphological evolutions. Fractographic analysis displayed that the ductile fracture dominates the 20%-40% IMC micro-joints, whereas the brittle fracture governs the 40%-80% IMC micro-joints. For the ductile fracture-dominated joints, an abnormal reduction in strength occurred as increasing IMCs volume from 20% to 40%. This is primarily due to severe stress concentrations caused by the transformed long rod-typed morphology of the Ni3Sn4. For the brittle fracture-dominated joints, the strength appeared a monotonous increase as the Ni3Sn4 volume increased. This may be attributed to the increased crack resistance resulting from continuous coarsening of the chunky Ni3Sn4 without any voids. Moreover, the finite element analysis was provided to further understand the joint failure mechanisms.

Direct Visualization of Cross-Sectional Strain Distribution in Flexible Devices.

For flexible devices that inevitably undergo repetitive deformations, it is important to evaluate and control the mechanical strain imposed on the flexible systems for enhancing the reliability. In this paper, a novel experimental method to directly visualize cross-sectional strain distribution in the thin flexible devices is proposed. Digital image correlation (DIC) is effectively adapted by using microscopic images of the cross section for accurate analysis of the microscale deformations. To conduct the DIC strain analysis, speckle patterning is accomplished by using microparticles from diamond-abrasive suspensions with optimized fabrication conditions. First, the cross-sectional micro-DIC analysis is performed successfully for 100 µm-thick substrates. Full-field strain quantification and easy inspection of a neutral plane are demonstrated and compared with results of finite element analysis simulation. Using the presented method, generation of multiple neutral planes is clearly visualized for a trilayer structure with a very soft adhesive midlayer, where strain decoupling occurs by severe shear deformation of the soft adhesive layer. Furthermore, bending strain distribution in a flexible fabric-reinforced polymer (FRP) substrate is also investigated to analyze and predict fatigue fracture in the complex inner structure under repetitive bending loading.

Effects of nanofiber on the electrical properties of anisotropic conductive adhesives (ACAs).

The effects of nanofiber on the electrical properties of anisotropic conductive films (ACFs) were investigated from the perspectives of the joint and insulation resistances. To obtain stable electrical properties for fine-pitch chip-on-film (COF) packages, two kinds of nanofiber ACFs were demonstrated: (1) polystyrene (PS) and polyacrylonitrile (PAN) nanofiber ACFs, which were formed by laminating ACFs on the top and bottom sides of PS and PAN nanofibers, and (2) PAN nanofiber coupled with conductive particle (PAN/Cp nanofiber) ACF, which was made by laminating non conductive films (NCFs) on both sides of a PAN/Cp nanofiber. The effects of the nanofiber thickness, melting, and structure on the electrical properties of the nanofiber ACFs were analyzed. Among the two different nanofiber ACFs, the PAN/Cp nanofiber ACF showed the most stable joint resistance (below 4 mOmega) and insulation resistance (above 10(8) Omega (between 7 microm bump space) due to the thin insulation layer.

Enhanced thermal conductivity of epoxy-graphene composites by using non-oxidized graphene flakes with non-covalent functionalization.

Homogeneous distribution of graphene flakes in a polymer matrix, still preserving intrinsic material properties, is key to successful composite applications. A novel approach is presented to disperse non-oxidized graphene flakes with non-covalent functionalization of 1-pyrenebutyric acid and to fabricate nanocomposites with outstanding thermal conductivity (∼1.53 W/mK) and mechanical properties (∼1.03 GPa).

Enhanced electrical conductivity of nanocomposites containing hybrid fillers of carbon nanotubes and carbon black.

Nanocomposites reinforced with hybrid fillers of carbon nanotubes (CNTs) and carbon black (CB) are developed, aiming at enhancing the electrical conductivity of composites with balanced mechanical properties while lowering the cost of the final product. Epoxy-based nanocomposites were prepared with varying combinations of CNTs and CB as conducting fillers, and their electrical and mechanical properties were evaluated. It was shown that the addition of CNTs in CB composites enhanced the electrical conductivity of composites: a low percolation threshold was achieved with 0.2 wt % CNTs and 0.2 wt % CB particles. The CB particles also enhanced the ductility and fracture toughness of nanocomposites, confirming the synergistic effect of CB as a multifunctional filler. The novelty of this work lies in the synergy arising from the combination of two conducting fillers with unique geometric shapes and aspect ratios as well as different dispersion characteristics, which have not been specifically considered previously.