K.C. Yung

Modeling Young's Modulus of Polymer-layered Silicate Nanocomposites Using a Modified Halpin—Tsai Micromechanical Model

In this study nanocomposites consisting of an epoxy matrix filled with silicate clay particles are investigated. Recent and ongoing research has shown that dramatic enhancements can be achieved in stiffness and thermal properties in these nanocomposites with small amounts of particle concentration. The resulting properties of nanocomposites are intimately related to the microstructure achieved in processing these materials. However, the ideal situation of full exfoliation, dispersion, and orientation is not usually achieved. A more common case is the partial exfoliation and intercalation. The latter is a process whereby the polymer penetrates the interlayer spaces of clay particles, causing an increase in the layer spacing (d-spacing). The region consisting of a matrix with exfoliated clay nanolayers or platelets is analyzed by assuming a near uniform-dispersion and a random orientation. The properties of intercalated clusters of clay platelets are calculated by a rule of mixtures based on a parallel platelet system. The modified composite theory of Halpin—Tsai is applied to calculate the modulus of the nanocomposite as a function of the clay concentration for various parametric variations, including the exfoliation ratio, the particle/matrix stiffness ratio Ef/Em, the particle volume fraction f, and the particle aspect ratio L/t. The modified composite theory satisfactorily captures the stiffness behavior of the polymer/clay composites.

Physics-of-Failure-Based Prognostics and Health Management for High-Power White Light-Emitting Diode Lighting

Recently, high-power white light-emitting diodes (LEDs) have attracted much attention due to their versatility in applications and to the increasing market demand for them. So great attention has been focused on producing highly reliable LED lighting. How to accurately predict the reliability of LED lighting is emerging as one of the key issues in this field. Physics-of-failure-based prognostics and health management (PoF-based PHM) is an approach that utilizes knowledge of a products life cycle loading and failure mechanisms to design for and assess reliability. In this paper, after analyzing the materials and geometries for high-power white LED lighting at all levels, i.e., chips, packages and systems, failure modes, mechanisms and effects analysis (FMMEA) was used in the PoF-based PHM approach to identify and rank the potential failures emerging from the design process. The second step in this paper was to establish the appropriate PoF-based damage models for identified failure mechanisms that carry a high risk.

Enhanced redshift of the optical band gap in Sn-doped ZnO free standing films using the sol-gel method

The optical band gap in free standing transparent zinc oxide (ZnO) films using the sol?gel method was studied. The effect of Sn doping on grain size, vibrational structure and on the optical properties of ZnO films was investigated. Contrary to the common observation, the optical band gap of Sn-doped ZnO is red-shifted from 3.38 to 3.18?eV as the doping weight percentage is increased to 3%. The redshift of the optical band gap is due to the deep states in the band gap, and a change of ~0.2?eV can only be observed when a substrate is not used. This study illustrates that removal of interaction between film boundaries and substrate is essential for developing effective band gap-tunable ZnO thin films.

XPS investigation of the chemical characteristics of Kapton films ablated by a pulsed TEA CO2 laser

Laser ablation of 125-μm-thick Kapton polyimide films was carried out in air using a pulsed TEA CO2 laser at 9.3 μm. Laser-produced fibers protruding from the ablated surface results in a bad surface quality. Changes in the composition and the chemical characteristics of the ablated surfaces were identified by X-ray photoelectron spectroscopy (XPS). The C/O and C/N atomic ratios as well as the peak area of the C 1s spectra at 284.7 eV in the ablated area increase, while the peak areas of the C 1s spectra corresponding to the carbonyl groups (CO) in the imide system and the ether groups (CO) decrease. These suggest that the fiber bundles consist mainly of carbon. Moreover, the amide groups, nitrile groups (CN) and the benzene derivatives were detected after laser irradiation due to the breakage of the CN bonds in the imide ring and ether groups. Upon increasing the fluence to 10.6 J/cm2, the shake up at 537.6 eV decreases further, yet the amide groups as well as the shake up at 291 eV almost disappeared. This is because benzene derivatives decompose completely and the carbonyl groups are eliminated from the aromatic systems due to a large temperature rise. Therefore, the increase in fluence may improve the thermal decomposition during the TEA CO2 laser ablation of the Kapton films.

Thermal Management for Boron Nitride Filled Metal Core Printed Circuit Board

This study aims at investigating novel thermal material based on filled epoxy for metal-core printed circuit boards (PCBs). The next generation PCB materials are expected to possess high heat dissipation capability in addition to low coefficient of thermal expansion (CTE) as the accumulated heat from high performance electronic devices should be removed for proper operation. In this study, boron nitride (BN) filler with different size (from micro to nano size) and content were employed to prepare thermally conductive polymer composites. Various percentage of coupling agent was used for the surface treatment of fillers in order to produce homogenous composites with lower CTE. The effect of filler and coupling agent to thermal conductivity of composite are discussed. It indicated that the experimental results fit well with the Bruggeman model with variety filler size. The use of 1% of couple agent was found to be more effective in increasing thermal conductivity of the composite. Despite the maximum content of the BN allowed to be added into the epoxy is about 30% due to the comparatively high viscosity of the varnish, as low as 13% of the micron-sized BN-filled dielectric is enough to fulfil the requirement of thermal conductivity larger than 1 W/m K and balance other critical properties used for PCB application. It is feasible to be a cost-effective advanced thermal material.

Laser ablation of Upilex-S polyimide: influence of laser wavelength on chemical structure and composition in both ablated area and halo

Abstract Blind craters (diameter 200 μm) on Upilex-S polyimide films (80-μm thick) were drilled by irradiation with three different laser systems [KrF excimer: wavelength λ=248 nm (UV); acoustic optical Q-switch Nd:YAG: λ=355 nm (UV); and TEA CO 2 : λ≈9.3 μm (IR)] in air. Modifications of chemical structure and surface morphology in both the ablated area and halo were examined using X-ray photoelectron spectroscopy (XPS) and a scanning electron microscope (SEM). In the halo, nano-particles were observed with UV lasers, but submicro-particles were observed with the IR laser. The results of XPS analysis show that the C content increased, while the O content and N content decreased in the ablated area at all wavelengths, due to photo-thermal or photochemical decomposition of polyimide. These are substantiated by decreases of the carbonyl groups (CO) at 288.2 eV and an increase of CC groups at 284.8 eV. However, the N content in the ablated area with TEA CO 2 laser is higher than that with UV lasers. Also, amide groups were detected in the ablated area with TEA CO 2 laser and 355-nm Nd:YAG laser, but few with 248-nm excimer laser. Furthermore, in the halo, the O content with the 355-nm laser is higher than that of the 248-nm laser. Also, the shoulder of C 1s peak was observed at 288.5 eV in the halo due to oxidation of fragments erupted from the plume in air, but did not occur at the 248-nm wavelength. These findings indicate that the chemical structure and composition are highly dependent on laser wavelength.

High repetition rate effect on the chemical characteristics and composition of Upilex-S polyimide ablated by a UV Nd:YAG laser

Abstract X-ray photoelectron spectroscopy was applied to investigate the chemical and structural modification of Upilex-S polyimide films ablated by a 355 nm pulsed Nd:YAG laser at different high repetition rates. The changes in the chemical characteristics and composition of the ablated area were found to be markedly dependent on the repetition rate. With every increase in the repetition rate, the relative carbon content of the ablated area increased, while the nitrogen and oxygen contents were reduced. After being irradiated by the UV laser, a new component was detected at 287.4 eV, assigned to be the amide structure, as a result of a breakage of the imide ring occurring between the nitrogen and carbonyl carbon atoms. The peak area of the CC group also increased, while the peak areas of CO and the amide group decreased with each increase in the repetition rate. These results are attributed to both the cumulative heat and the increase of the input energy.

Modeling the Etching Rate and Uniformity of Plasma-Aided Manufacturing Using Statistical Experimental Design

The response characteristics of an O2/CF4-based plasma process used to desmear and etch back multi-layer rigid-flex printed circuit board were examined using a two-level fractional factorial experimental design. The effects of variation in RF power, temperature, gas proportion and gas flow (CF4 and O2) on several output variables, including etch rate, process uniformity and selectivity were investigated. The screening factorial experiment was designed to isolate the most significant input parameters. In the experiments conducted, increases in the etching rate generally corresponded to decreases in uniformity. Etch uniformity was strongly dependent on temperature and gas proportion. The relative significance of polymer deposition and ion bombardment was separated. Using this information as a platform from which to proceed, the subsequent phase of the experiment developed empirical models of etch behavior using response surface 3-D plots. The models were subsequently used to optimize the etching process.

The effects of pulse plating parameters on copper plating distribution of microvia in PCB manufacture

The introduction of microvias to printed circuit boards has revolutionized the entire printed circuit board (PCB) industry. In many instances, the plating of microvias creates a bottleneck in the manufacture of high-density circuitry. In this study, the effects of pulse plating parameters and different shaped waveforms on the quality of microvias have been investigated. The results showed that, within the scope of this study, the reverse current cycle time has little effect on throwing power. Indeed, a decrease in forward current, or an increase in reverse current could significantly improve the throwing power. The study also found that using a triangular, instead of the traditional rectangular waveform, could increase the throwing power further, with a more uniform distribution of copper plating. Finally, the advantage of the cathode vibrating during plating was demonstrated.

Investigation of laser surface alloying of copper on high nickel austenitic ductile iron

Abstract Laser surface alloying (LSA) of pure copper on high nickel austenitic ductile iron was carried out using a 2 kW continuous wave (CW) CO 2 laser with two processing conditions: laser beam spot size D =5 mm, scanning speed V =2 mm s −1 , power density E =1.02×10 4 W cm −2 (condition 1) and D =2 mm, V =9 mm s −1 , 6.37×10 4 W cm −2 (condition 2). Microstructure in the alloyed zone was found to consist mainly of primary γ-austenite dendrites and an interdendritic eutectic of γ-austenite and M 7 C 3 carbides with copper wetting the dendritic boundaries. Distribution of the copper in the alloyed zone is fairly uniform across the depth of the pool and the concentration of copper in the alloyed zone highly depends on the power density. Copper spheroidal particles can be observed at the interdendritic boundaries under condition 1, indicating that the melt could be near the metastable miscibility gap and liquid phase separation (LPS) takes place due to rapid solidification. After LSA, hardness of the alloyed zone is considerably higher than before, which is attributed to both precipitation hardening and work hardening caused by the thermal stress.