Flexible Printed Circuit Substrate Research Articles

The Effect of Anisotropic Conductive Films Adhesion on the Bending Reliability of Chip-in-Flex Packages for Wearable Electronics Applications

In this paper, the effects of adhesion properties of anisotropic conductive films (ACFs) interconnection on the chip-in-flex (CIF) bending reliability were investigated. Oxygen plasma treatment was conducted to increase the adhesion strength between ACFs and Si chip or ACFs and flexible printed circuit (FPC) substrates. In order to characterize the enhanced adhesion properties of the CIF packages, surface energy, surface roughness, elemental composition, and peel strength were measured. A digital image correlation method was used with cross-sectional scanning electron microscopy images to visualize the stress development at the ACFs interconnection. It was found that the interface of ACFs resin and FPC substrate showed the weakest adhesion, where the delamination was initiated. As a result of the improved adhesion at the ACFs resin and FPC substrate, the location of stress concentration was changed to the interface of Si chip and ACFs resin, leading to better dynamic bending reliability. When the oxygen plasma was treated both on the Si chip and FPC substrate, the stress concentration was observed not at the ACFs interfaces, but inside of the ACFs resin, resulting in further improved dynamic bending reliability. With the optimized plasma treatment condition and the ACFs materials, the dynamic bending reliability of the CIF packages was successfully demonstrated up to 160 000 bending cycles at a 7.5-mm bending radius without any electrical failures.

Ultra-Thin Piezoelectric Strain Sensor Array Integrated on a Flexible Printed Circuit Involving Transfer Printing Methods

In this paper, we present a novel sensor array manufacturing process that involves transfer printing methods using a chip mounter with a vacuum collet. Using these methods, one can mount not only ultra-thin microsensors but also microcontroller and amplifier chips required for sensor device fabrication. We successfully transfer-printed a very fragile microelectromechanical systems-based 5-mm-long, 1-mm-wide, 5- $\mu \text{m}$ -thick high-aspect-ratio ultra-thin Pb(Zr, Ti)O3 (PZT) (1.9 $\mu \text{m}$ )/Si (3 $\mu \text{m}$ ) strain sensor onto a flexible printed-circuit (FPC) substrate with etched Cu wiring. Then, we connected the sensor to the wiring by printing a conductive paste using a screen printer. Large ferroelectric polarization–voltage hysteresis curves were obtained even after the transfer printing process. Since an output voltage corresponding to the magnitude of the strain from the developed sensor was generated, it was confirmed that ultra-thin sensors could be integrated to the FPC substrate by these transfer and screen printing techniques without damage. The PZT thin films have shown 0.18 mV/ $\mu \varepsilon $ of dynamic strain sensitivity according to the resonant vibration of the stainless plate.

Manufacturing process for piezoelectric strain sensor sheet involving transfer printing methods

In this paper, we present a novel sensor sheet manufacturing process that involves transfer printing methods using adhesive rubber stamps. By using these methods, not only ultrathin microsensors but also microcontroller and amplifier chips required for the fabrication of sensor devices can be mounted. We successfully transfer-printed a very fragile 5-mm-long, 1-mm-wide, 5-µm-thick high-aspect-ratio ultrathin strain sensor onto a flexible printed-circuit substrate. Then, we connected the sensor to the wiring by printing a conductive paste using a screen printer. A large ferroelectric polarization-voltage hysteresis curve was obtained even after the transfer printing process. Since an output voltage corresponding to the magnitude of the strain from the developed sensor was generated, it was confirmed that ultrathin sensors could be transfer-printed to the flexible substrate by this transfer technique without damage.

Implantable probe with split anchors via residual stress and induced cell growth with gelatin nanofibres

A bipolar electrode probe used for implantable nerve stimulation treatments in minimally invasive surgeries is presented. The probe is composed of a flexible printed circuit substrate and a patterned SU-8 film. This probe features a three-dimensional (3D) tweezer-like mechanism opened by residual stress from the SU-8 film, designed to fix the probe in the tissue surrounding a target nerve. Stripes on the SU-8 film direct the net residual stress in a single direction to form a curve. The holding strengths of the probes with different deformations are defined and measured by a tensile test. Results show that the fixing ability of a 3D probe is better than a plane probe. The probes with curvature heights between 13 and 14 mm have a maximum average breaking force of 0.258 N, which is 16.3 and 13.1% higher than the probes with curvature heights between 9 and 10 mm and between 10 and 11 mm, respectively. In addition, a film of gelatin fibrous membrane, produced by electrospinning, covers the fixed ends of the probe's anchors and acts as cell scaffolds to induce cell growth, which help to ensure long-term fixation in the body. 3T3 fibroblast cells are grown to verify the scaffold effect of the fibrous membrane.

Synthesis, property and application of PZT/P(VDF-TrFE) composite film fabricated on polyimide substrate

In this paper, a flexible pyroelectric sensor using PbZr0·3Ti0·7O3 PZT/P(VDF-TrFE) materials on the polyimide (PI) flexible printed circuit substrate was reported. The fabrication processes, material electric properties and infrared photoelectric response property of the designed sensor were studied. First, the PZT ceramic powder was annealed at temperatures ranging from 300 to 900°C, and then 0–3 PZT/PVDF-TrFE composites films were produced by casting PZT/PVDF-TrFE suspension onto the flexible PI substrates. The highest pyroelectric coefficient obtained using 700°C annealed PZT particulates was 96 µC m−2 K−1, which was 20% higher than that of unannealed powders. The PI thermal insulation layer and thermal insulation tanks fabricated by laser microetching have been used to improve the photoelectric response ability of the infrared sensor. The specific detectivity of the sensors was calculated from the voltage responsivity and noise. Detectivity D* of the PZT/P(VDF-TrFE) infrared sensor was 1·17×108 cm Hz1/2 W−1 at 138·3 Hz modulation frequency. These results demonstrate that the pyroelectric infrared sensor possesses potential applications in flexible electronics.