Thin dielectric-layer-enabled low-voltage operation of fully printed flexible carbon nanotube thin-film transistors.

The quality of printable dielectric layer has become one of the major obstacles to achieving high-performance fully printed transistors. A thick dielectric layer will require high gate voltage to switch the transistors on and off, which will cause high power dissipation in printed devices. In response to this challenge, fully printed carbon nanotube (CNT)-based thin-film transistors (TFTs) have been fabricated on flexible membranes such as polyimide and liquid crystal polymer using aerosol jet printing. These devices can be operated at bias voltages below ±10 V (drain/gate voltages around ±6 V). This is much smaller than the previously reported values for fully printed CNT-TFTs because of using xdi-dcs (mixture of poly(vinylphenol)/poly (methylsilsesquioxane)) as the dielectric and using a single printing method. The lower voltage is a consequence of a thin dielectric layer (∼300 nm) and good uniformity in the printed CNT network. The printed CNT-TFTs show on/off ratio >105, and mobility >5 cm2V-1s-1. Layer-by-layer deposition of CNT allows highly uniform and dense network formation, and the optimization of the xdi-dcs concentration using natural butyl alcohol provides high-yield printing of a thin dielectric layer. Collectively, this work shows the potential of using fully printed CNT-TFTs in various flexible electronic applications such as wearable sensors, actuators, artificial skin, displays and wireless tags and antennas.

Additively manufactured flexible on-package phased array antennas for 5G/mmWave wearable and conformal digital twin and massive MIMO applications.

This paper thoroughly investigates material characterization, reliability evaluation, fabrication, and assembly processes of additively manufactured flexible packaging and reconfigurable on-package antenna arrays for next-generation 5G/mmWave wearable and conformal applications. The objective is to bridge the technology gap in current Flexible Hybrid Electronics (FHE) designs at mmWave frequencies and address the challenges of establishing future design standards for additively manufactured flexible packages and System-on-Package (SoP) integrated modules. Multiple 3D printed flexible materials have been characterized for their electrical and mechanical properties over the 5G/mmW frequency band (26-40 GHz), and the inkjet printed interconnects on 3D printed Polypropylene (PP) substrates demonstrated excellent electrical and mechanical performance during a 10,000-time cyclic bending test over typical wearable flexible radii down to 1 inch. A proof-of-concept flexible on-package phased array with an integrated microfluidic cooling channel on 3D printed substrates was fabricated and measured, demonstrating [Formula: see text] beam steering capability with efficient cooling. The proposed reconfigurable design and low-temperature fabrication approach using additive manufacturing can be widely applied to next-generation highly-complex on-demand FHE, flexible multi-chip-module integration, and on-package phased-array modules for 5G/mmWave wearable and conformal smart skin, digital twin and massive MIMO applications.

Ag3Sn Compounds Coarsening Behaviors in Micro-Joints.

As solder joints are being scaled down, intermetallic compounds (IMCs) are playing an increasingly critical role in the reliability of solder joints, and thereby an in-depth understanding of IMCs microstructure evolutions in micro-joints is of great significance. This study focused on coarsening behaviors of Ag3Sn compounds in Sn-3.0Ag-0.5Cu (SAC305) micro-joints of flip chip assemblies using thermal shock (TS) tests. The results showed that the Ag3Sn compounds grew and rapidly coarsened into larger ones as TS cycles increased. Compared with such coarsening behaviors during thermal aging, TS exhibited a significantly accelerating influence. This predominant contribution is quantitatively determined to be induced by strain-enhanced aging. Moreover, based on observations for Ag3Sn microstructure evolutions during TS cycling, one particular finding showed that there are two types of coarsening modes (i.e., Ostwald ripening and Necking coalescence) co-existing in the Ag3Sn coarsening process. The corresponding evolutions mechanism was elucidated in a combination of simulative analysis and experimental validation. Furthermore, a kinetic model of the Ag3Sn coarsening was established incorporating static aging and strain-enhanced aging constant, the growth exponent (n) was calculated to be 1.70, and the predominant coarsening mode was confirmed to be the necking coalescence.

A bio-enabled maximally mild layer-by-layer Kapton surface modification approach for the fabrication of all-inkjet-printed flexible electronic devices.

A bio-enabled, environmentally-friendly, and maximally mild layer-by-layer approach has been developed to surface modify inherently hydrophobic Kapton HN substrates to allow for great printability of both water- and organic solvent-based inks thus facilitating the full-inkjet-printing of flexible electronic devices. Different from the traditional Kapton surface modification approaches which are structure-compromising and use harsh conditions to target, and oxidize and/or remove part of, the surface polyimide of Kapton, the present Kapton surface modification approach targeted the surface electric charges borne by its additive particles, and was not only the first to utilize environmentally-friendly clinical biomolecules to build up a thin film of protamine-heparin complex on Kapton, but also the first to be conducted under minimally destructive and maximally mild conditions. Besides, for electrically charged ink particles, the present surface modification method can enhance the uniformity of the inkjet-printed films by reducing the "coffee ring effect". As a proof-of-concept demonstration, reduced graphene oxide-based gas sensors, which were flexible, ultra-lightweight, and miniature-sized, were fully-inkjet-printed on surface modified Kapton HN films and tested for their sensitivity to dimethyl methylphosphonate (a nerve agent simulant). Such fabricated sensors survived a Scotch-tape peel test and were found insensitive to repeated bending to a small 0.5 cm radius.

Magnetic alignment of hexagonal boron nitride platelets in polymer matrix: toward high performance anisotropic polymer composites for electronic encapsulation.

We report magnetic alignment of hexagonal boron nitride (hBN) platelets and the outstanding material properties of its polymer composite. The magnetically responsive hBN is produced by surface modification of iron oxide, and their orientations can be controlled by applying an external magnetic field during polymer curing. Owing to the anisotropic properties of hBN, the epoxy composite with aligned hBN platelets shows interesting properties along the alignment direction, including significantly reduced coefficient of thermal expansion, reaching ∼28.7 ppm/°C, and enhanced thermal conductivity, 104% higher than that of unaligned counterpart, both of which are observed at a low filler loading of 20 wt %. Our modeling suggests the filler alignment is the major reason for these intriguing material properties. Finite element analysis reveals promising applications for the magnetically aligned hBN-based composites in modern microelectronic packaging.