Xiuzhen Lu

The promising application of graphene oxide as coating materials in orthopedic implants: preparation, characterization and cell behavior

To investigate the potential application of graphene oxide (GO) in bone repair, this study is focused on the preparation, characterization and cell behavior of graphene oxide coatings on quartz substrata. GO coatings were prepared on the substrata using a modified dip-coating procedure. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy results demonstrated that the as-prepared coatings in this study were homogeneous and had an average thickness of ~67 nm. The rapid formation of a hydroxyapatite (HA) layer in the simulated body fluid (SBF) on GO coated substrata at day 14, as proved by SEM and x-ray diffraction (XRD), strongly indicated the bioactivity of coated substrata. In addition, MC3T3-E1 cells were cultured on the coated substrata to evaluate cellular activities. Compared with the non-coated substrata and tissue culture plates, no significant difference was observed on the coated substrata in terms of cytotoxicity, viability, proliferation and apoptosis. However, interestingly, higher levels of alkaline phosphatase (ALP) activity and osteocalcin (OC) secretion were observed on the coated substrata, indicating that GO coatings enhanced cell differentiation compared with non-coated substrata and tissue culture plates. This study suggests that GO coatings had excellent biocompatibility and more importantly promoted MC3T3-E1 cell differentiation and might be a good candidate as a coating material for orthopedic implants.

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.

New Nano-Thermal Interface Material for Heat Removal in Electronics Packaging

The need for faster, smaller, and more reliable and efficient products has resulted in increase of heat generated in microelectronic components. The removal of the heat generated is an important issue in electronic packaging. The present research work aims at developing a new class of nano-thermal interface material (nanoTIM) that has low thermal resistance, high thermal conductivity and mechanical strength using the electrospinning process. With the electrospinning process, polymer nano-fibers with nano-scale diameter are formed. Nano-particles such as nano-silver particles, nano-carbon nanotubes (CNT) and nano-silicon carbide particles were embedded into the nano-fibers to enhance the thermal conductivity and to reduce the thermal resistivity. Optical and scanning electron microscopy (SEM) analysis techniques were used to determine the morphology of the nano-composite fibers obtained. Thermal resistivity, conductivity and mechanical strength of the nano-composite materials formed were measured. In addition, the manufactured nano-materials were characterized using the thermo gravimetric analyzer (TGA) and the differential scanning calorimetric (DSC) analysis techniques to study the softening, melting as well as degradation behavior. The mechanical strength was also studied using a multi-functional mechanical tester. The results show that the nano-fiber based composite nano-TIMs have similar thermal conductivity, 3 to 9 times lower thermal resistivity, similar operation temperature range and degradation behavior, 2 to 5 times higher ultimate tensile strength, in comparison with commercially available TIMs. By adding adhesive functions into the process, a new class of nano-TIM tape has been produced

New Nano-Thermal Interface Materials (Nano-TIMs) with SiC Nano-Particles Used for Heat Removal in Electronics Packaging Applications

Heat dissipation of semiconductor packages has become one of the limiting factors in miniaturization. A new Nano Thermal Interface Material (Nano-TIM) with potential high thermal conductivity and excellent mechanical properties manufactured by electrospinning process has been published earlier. In the present work, SiC nano-particles were added into the electrospinning solution to improve the thermal conductivity of the film. The produced Nano-TIM has a nano scale structure with embedded nano-SiC distributed randomly. Scanning Electrical Microscope (SEM) was used to determine the morphology of the film. Thermal properties were measured. The results show that the Nano-TIM has good thermal properties and it can be further improved by changing the mass concentration of the SiC nano particles.

Adhesion behavior between epoxy molding compound and different leadframes in plastic packaging

Adhesion has been identified as one of the key elements in solving failure problems in electronic packaging. Understanding and improving the adhesion between epoxy molding compound (EMC) and leadframes is thought to be a key step towards improving package performance. The objective of this work was to study the effect on adhesion behavior and delamination of plastic packages using various metal coated leadframes that had been subjected to preconditions. In this work, the effects of moisture absorption testing and reflow process on the interface adhesion strength of the EMC/metal leadframes were studied. Three kinds of metal coated leadframes were used: copper, silver coated copper and nickel/palladium/gold coated copper. Adhesion strength was measured using the tab pull test. These data were correlated to some extent with the findings of package delamination observed by the C-mode Scanning Acoustic Microscope (C-SAM). Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were employed to characterize the surface of EMC and metal leadframes after separation along the vertical plane.

Reliability analysis of embedded chip technique with design of experiment methods

This paper presents reliability and failure analysis of embedded components on the liquid crystalline substrate. First, a demonstration module for embedded technology is fabricated. Then it is put into thermal cycling to find out the most obvious failure. 3D FEM analysis is employed to investigate the failure mode. A design of experiment method is conducted to obtain the optimal structure of embedded chip components.

Investigation of accelerated surface oxidation of Sn-3.5Ag-0.5Cu solder particles by TEM and STEM

The composition and thickness of surface oxide of solder particles has a direct effect on adhesion and electrical resistance of soldering joint and resultant the quality of interconnect and the reliability of packaged system. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to examine the oxide layer on solder powders in the present paper. However, for the surface oxide layer of a lead-free solder particle, the TEM sample for the oxide layer has never been done for studying its thickness or appearance before. And it is the first time in this work to use Focus Ion Beam (FIB) technology to prepare TEM specimen for solder particles and show TEM pictures of their surface oxide layer. High angle annular dark field (HAADF) pattern was applied to distinguish between the oxide layer and the solder matrix by the contrast of average atomic number. The solder powders were exposed in air (70% relative humidity) at 150°C for 0, 120 and 240 h to simulate the accelerated growth of oxide. The surface oxide thickness was 6 nm and 50 nm measured by TEM for 0 h and 120 h samples respectively. Confirming by AES measurement, the thickness of 5 nm and 50 nm were gotten using intersection analysis method for AES depth profiles. It is found that the increase of surface oxide thickness of solder particles is proportional to the rooting of time. The elemental distribution along the oxide was quantified by line scanning using STEM and the atomic ratio of Sn to O in the oxide layer nearer to the outer, the middle, and the inner (adjacent to the solder matrix) were found to be 1:2, 2:3 and 1:1, respectively. The result was validated using XPS which gave Sn to O ratio of 1:2 at 5 nm depth of surface oxide.

A novel isotropic conductive adhesive with Ag flakes, BN and SiC nanoparticles

Isotropic conductive adhesives (ICAs) with lower bonding temperature, higher resolution and environmental friendly have been used extensively in packaging process. In order to improve the electrical and thermal conductive properties of ICAs, two kinds of bimodal high temperature stable ICAs with matrix SHT6 and fillers with composition of macro silver flakes and boron nitride nanoparticles or macro silver flakes and silicon carbide nanoparticles were studied. In these two kinds of adhesives, the silver flakes were 75wt%, and the contents of nanoparticles were 0wt%, 0.5wt%, 1.5wt%, 2.5wt%, 3wt%, 5wt% in weight. All the samples were cured at 150°C for 1 hour. SEM images and EDS results show the nanoparticles disperse randomly in the ICA. The electrical resistivity of these ICAs depends on the contents of silver flakes and is hardly affected by BN nanoparticles and SiC nanoparticles. The thermal conductivity of these ICAs increases firstly with the weight increase of the BN nanoparticles and SiC nanoparticles. And then it decreases when the content of the nanoparticles beyond a certain point.

Effects of BN and SiC nanoparticles on properties of conductive adhesive

Isotropic conductive adhesives (ICAs) are a type of interconnect material used more and more widely in computer, robot, mobile phone, LED and so on. Compared with traditional solders, isotropic conductive adhesives have better working plasticity, creep resistance and heat resistance. In particular, isotropic conductive adhesives are more responsive in high density packaging than solder, which determines the dominance of ICAs in high density packaging in the future. In order to improve the thermal conductivity of ICA with acceptable electrical conductivity, Ag flakers, BN and SiC nanoparticles were added into the matrix. The content of silver flakes was 75wt%, and the content of nanoparticles (BN or SiC) in the isotropic conductive adhesives were 0wt%, 0.5wt%, 1.5wt%, 2.5wt%, 3wt%, 5wt% in weight. The conductive adhesives were coated on the PCBs with stencil printing and fifty SR1206 chip components were mounted on a PCB using conductive adhesive. All samples were cured at 150°C for 1h. Further research into the reliability of the above isotropic conductive adhesives after temperature & humidity and thermal-cycling was carried out to analyze the effects of BN and SiC nanoparticals on the properties of ICA. The condition of the temperature & humidity test was 85 °C/85%RH, 500h. The thermal-cycling test was -40°C~125°C, 500 cycles and the soaking time and ramping rate were 19min and ±15°C/min. Changes to electrical resistance were used to estimate the reliability of the isotropic conductive adhesives in this study. The microstructure of the failure samples was observed using a Scanning Electron Microscope (SEM). The water absorption of all ICAs is the same after 94h temperature and humidity aging and the rate of water absorption is also the same during the aging. The ICA with 3% boron nitride nanoparticles and 75% micron silver flakes shows the best temperature and humidity reliability, with the fewest cracks on the interface between ICA and component. After 500h thermal cycling aging, the resistance reduces in the first 100h and maintains in a certain value after 332h cycled thermal. The samples have no wide cracks but a few small ones on the interface.

Studies on microstructure of epoxy molding compound (EMC)-Leadframe interface after environmental aging

The Leadframe-Epoxy molding compound (EMC) interface is known to be one of the weakest interfaces in an electronic packaging exhibiting delamination during reliability test. Interfaces of EMC and leadframes with different metal coatings exhibit different failure mode behavior after environmental aging because of the different adhesion strengths. In this paper, the interface microstructure of EMC-leadframes with different metal coatings was studied using Scanning Electron Microscopy (SEM). The leadframes used in this study were copper, copper with Ag coating, Ni coating and Ni/Pd/Au coating. The results of tab pull testing showed the adhesion strength of the EMC/copper leadframe interface was strongest while that of the EMC/leadframe with Ni/Pd/Au coating interface was weakest. Little delamination appeared on the EMC/Cu sample, including the samples after the moisture and reflow treatment. Delamination appeared on the interface of EMC/leadframe with Ag coating after treatment, and serious delamination was found on the interface of EMC/leadframe with Ni coating. The SEM micrographs showed that there were some microcracks between the filler of EMC and the leadframe.