Lilei Ye

Effect of Ag particle size on electrical conductivity of isotropically conductive adhesives

The present work is to introduce nanoparticles in micro-sized metal particles to study particle distribution in polymer matrix. Previous examinations of the silver-filled particles reveal that the micro-sized particle fillers appear as full density silver flakes, while nanoparticle fillers appear as highly porous agglomerates, similar to open-cell foams. Actually little work has been carried out to study the cross-sectional area of a particle-particle-contact in isotropically conductive adhesives (ICA). In this study, transmission electron microscope is chosen as a main measure to analyze the distribution of different-sized particles. The percentage of the nanoparticles varies from 20 wt% and 50 wt% to full percentage within micro-sized particles, and the total metal content in epoxy resin is 70 wt%. So the change of contact area and contact behavior with various volume ratio of nano-sized and micro-sized particles was investigated. At the same time, the electrical resistivity was measured, which is compared with the different level of the filler loading.

Carbon-Nanotube Through-Silicon Via Interconnects for Three-Dimensional Integration

Interconnection of carbon-nanotube (CNT)-filled through-silicon vias is demonstrated through an easy-to-implement process based on mechanical fastening. Direct CNT-to-CNT and CNT-to-Au contacts are realized at the microscale, and their specific contact resistances extracted from electrical measurements are approximately 1.2 × 10(-3) Ω cm(2) and 4.5 × 10(-4) Ω cm(2) , respectively.

Through-Silicon Vias Filled With Densified and Transferred Carbon Nanotube Forests

Through-silicon vias (TSVs) filled with densified and transferred carbon nanotube (CNT) forests are experimentally demonstrated. The filling is achieved by a postgrowth low-temperature transfer process at 200 instead of direct CNT growth in the vias normally requiring high temperature. A vapor densification method is also applied to densify the as-grown CNT forests, which allows for packing more CNTs in the vias to reduce their resistances. CNT-filled TSVs fabricated based on these two key steps show CMOS compatibility and roughly one order of magnitude reduction in resistivity compared to the TSVs filled with as-grown undensified CNT forests.

Surface-Confined Synthesis of Silver Nanoparticle Composite Coating on Electrospun Polyimide Nanofibers

A methodology for fabricating hierarchical nanostructures by surface-confined synthesis of silver nanoparticles on electrospun polyimide nanofibers is reported. Through surface-confined imide cleavage at the dianhydride domain via immersion in an aqueous KOH solution, potassium polyamate coatings of accurately defined thickness are formed (at a rate of 25 nm h(-1) ). By utilizing the ion-exchange capability of the polyamate resin, silver ions are introduced through immersion in an aqueous AgNO3 solution. Subsequent reduction of the metal ion species leads to the formation of nanoparticles at the fiber surface. Two modes of reduction, chemical and thermal, are investigated in the report, each leading to distinct morphologies of the nanoparticle coatings. Via thermal reduction, a composite surface layer consisting of monodisperse silver nanoparticles (average diameter 5.2 nm) embedded in a re-imidized polyimide matrix is achieved. In the case of chemical reduction, the reduction process occurs preferentially at the surface of the fiber, leading to the formation of silver nanoparticles anchored at the surface, though not embedded, in a polyamic acid matrix. By regulating the modification depth, control of the particle density on the fiber surface is established. In both reduction approaches, the polyimide nanofiber core exhibits maintained integrity.

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Templated Growth of Covalently Bonded Three‐Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.

Synthesis and applications of two-dimensional hexagonal boron nitride in electronics manufacturing

In similarity to graphene, two-dimensional (2D) hexagonal boron nitride (hBN) has some remarkable properties, such as mechanical robustness and high thermal conductivity. In addition, hBN has superb chemical stability and it is electrically insulating. 2D hBN has been considered a promising material for many applications in electronics, including 2D hBN based substrates, gate dielectrics for graphene transistors and interconnects, and electronic packaging insulators. This paper reviews the synthesis, transfer and fabrication of 2D hBN films, hBN based composites and hBN-based van der Waals heterostructures. In particular, this review focuses on applications in manufacturing electronic devices where the insulating and thermal properties of hBN can potentially be exploited. 2D hBN and related composite systems are emerging as new and industrially important materials, which could address many challenges in future complex electronics devices and systems.

Microstructural coarsening of lead free solder joints during thermal cycling

The present work mainly focuses on the microstructure evolution of Sn-3.5 Ag, Sn-3.4 Ag-3.0 Bi and Sn-3.2 Ag-0.5 Cu solder joints in Pb-coated lead frame and Ni/Au plated Cu surface in thermal cycling test. Eutectic Sn-Pb alloy is also studied as a reference material. The solder joints were made using typical surface mount assembly process and thermal cycling was carried out between -40/spl deg/C and 100/spl deg/C for 1000 cycles. The microstructure observation was performed by scanning electron microscope (SEM). It was found that microstructure of eutectic Pb-Sn solder joints coarsened dramatically after 1000 cycles, which caused deterioration of solder joint shear strength. The coarsening is related to the relatively high solid solubilities of Pb in Sn and vice versa, especially at elevated temperatures. Whereas, the Sn-Ag based lead-free solders have limited solubility of Ag in Sn, making them more resistant to coarsen, and the formation of Ag/sub 3/Sn and other precipitates in Sn-Ag, Sn-Ag-Bi and Sn-Ag-Cu alloys also hinder the grain growth during thermal cycling. As a result, the shear strength change of these solder joints is relatively minor. However, due to the Pb-containing lead frame, Pb-rich phase is detected for every lead-free solder joint. The microstructural coarsening effect of Pb-rich phase caused by the thermal cycling is obvious.

A carbon fiber solder matrix composite for thermal management of microelectronic devices

A carbon fiber based tin–silver–copper alloy matrix composite (CF-TIM) was developed via electrospinning of a mesophase pitch with polyimide and carbonization at 1000 °C, followed by sputter coating with titanium and gold, and alloy infiltration. The carbonized fibers, in film form, showed a thermal conductivity of ∼4 W m−1 K−1 and the CF-TIM showed an anisotropic thermal conductivity of 41 ± 2 W m−1 K−1 in-plane and 20 ± 3 W m−1 K−1 through-plane. The thermal contact resistance of the CF-TIM was estimated to be below 1 K mm2 W−1. The CF-TIM showed no reduction in effective through-plane thermal conductivity after 1000 temperature cycles, which indicates the potential use of CF-TIM in thermal management applications.

The Effect of Functionalized Silver on Rheological and Electrical Properties of Conductive Adhesives

This research used low molecular surface modifiers, and observed that chemisorptions took place through the formation of a bond between silver surface and an adsorbed molecule, which improved the dispersion of silver flakes in the organic resin. Several different functionalizers, such as thioglycolic acid, silane and di-acid, were used to functionalize the silver surface. Results of shear viscosity, bulk resistivity etc. showed that by using these low molecular organic functionalizers, isotropic conductive adhesives (ICAs) with lower shear viscosity and better electrical conductivity at high silver fillers content were obtained. The adipic acid had the greatest effect on the rheological and electrical property of ICAs, so its weight percentage in silver flakes was also optimized; ICAs displayed the maximum electrical conductivity when there was 0.5 wt% of silver flakes.