Chi-Hui Chien

Influences of the moisture absorption on PBGA package’s warpage during IR reflow process

Abstract The coefficient of temperature expansion mismatch between materials in a thin plastic ball grid array package is primarily attributed to the warpage when mounting the package to a printed circuit board, especially when the moisture concentration reaches a specified value. The influence of variations in storage conditions on warpage was investigated in this paper. The effects of the reflow parameters on warpage were also examined. Taguchi’s method was implemented for experimental design and data analyses. A critical relative moisture absorption between 0.25% and 0.30% was found for a considerable warpage response.

Optimal FSW process parameters for aluminum alloys AA5083

Friction stir welding (FSW) can produce superior mechanical properties in the weld zone. The purpose of this article is to find the optimum FSW process operating conditions for two plates of aluminum alloy AA5083 welded in butt joint. In addition to optimizing the performance characteristics, the cost is considered in the optimal FSW process. In the FSW procedure, there are four major controllable four-level factors, i.e. the tool rotation speed, transverse speed (feed rate), tool tilt angle with respect to the workpiece surface, and pin tool length. The uncontrollable factors are the ultimate tensile strength and elongation rate which can be converted to signal-to-noise ratios the larger the better. In order to achieve the aim of robustness in the multiple response process, the gray-based Taguchi method is proposed. A gray relational grade obtained from gray relational analysis is used as the multiple performance characteristic. The resulting optimum process parameters are rotation speed at 1800 rpm, transverse speed at 180 mm/min, tool tilt angle at 1°, and pin tool length at 2.9 mm for the best multiple performance characteristics with minimum cost.

Technique of microball lens formation for efficient optical coupling

A batch-fabricated microball lens array not only provides accurate coupling distances but also replaces traditional components such as aspheric lenses and expensive graded-index lenses without sacrificing performance and reduces assembly cost. The results of extensive experiments show a critical aspect ratio of approximately 0.3. That is, when the aspect ratio is larger than 0.3, the final shape of a reflowed lens changes to that of a microball rather than of a mushroom. Using a laser with a 633-nm wavelength yields an optimum coupling distance of approximately 8 microm with an insertion loss below -1.3 dB (coupling efficiency, approximately 73%).

Experimental and statistical study in adhesion features of bonded interfaces of IC packages

Abstract The existence of defects in the corresponding interfaces can gradually degrade the interfacial adhesion when the flip chip package is exposed to the high temperature and humidity. This study used the fuzzy controller to stabilize both the moisture weight gain and adhesion strength together for the button shear test specimen. Parameters design, although based on the Taguchi method, can optimize the performance characteristic through the setting of process parameters and can reduce the sensitivity of the system performance to sources of variation. However, most published Taguchi applications to date have been concerned with the optimization of a single performance characteristic only. For example, it is only concerned a single performance characteristic with the optimization of the adhesion strength or the moisture gain weight individually. In this study, the control rules of the fuzzy controller were formed using the authors’ experience and knowledge of the IC packaging process. The control parameters to be tuned were the membership functions. Therefore, the controller’s performance depended on the membership functions. The fuzzy controller combined Taguchi parameter design, which makes the control performance insensitive to the operating condition changes and noise was used to determine the membership functions.

Thermomechanical behavior of underfill/solder mask/substrate interface under thermal cycling

With the increasing application of flip-chip technology in the microelectronics industry, the adhesion strength of interfaces in flip-chip microelectronic structures has become an important issue for manufacturing and operation. In this paper we present an experimental investigation of the adhesive strength of underfill material to solder mask coated FR-4 substrate under thermal cycling. The effects of the number of thermal cycles on interfacial strength were investigated by using the button shear test. The relationship between interfacial strength and the thickness of solder mask was also examined. Furthermore, the morphologies of fracture surfaces of the test specimens were analyzed by scanning electron microscopy. The results of this study show that the interfacial strength of the underfill/solder mask/substrate joint was significantly reduced by thermal fatigue. Finally, the degradation behavior and possible mechanisms were then determined on the basis of these observations.

Enhanced Light Harvesting in Dye-Sensitized Solar Cell Using External Lightguide

An external lightguide (EL) for enhancing the light-harvesting efficiency of dye-sensitized solar cells (DSSCs) was designed and developed. The EL attached to the exterior of a DSSC photoelectrode directed light on a dye-covered nanoporous TiO2 film (D-NTF) of the photoelectrode. Experimental tests confirmed that the EL increased the light-harvesting efficiency of a DSSC with an active area of 0.25 cm2 by 30.69%. Photocurrent density and the power conversion efficiency were also increased by 38.12% and 25.09%, respectively.

Numerical method to predict and fabricate aspherical microlens arrays using 248-nm excimer laser ablation

A method of numerical simulations is used to predict the pro- file of a 3-D aspherical microlens array. Based on the simulated results, the desired micro-optical lens profile is obtained through excimer laser ablation. The simulation method applied in the excimer laser ablation can significantly reduce the quantity of microablation experiments. The ab- lated microstructures with surface average roughness of Ra,20 nm are successfully achieved for micro-optical components. The excimer laser ablation parameters include laser fluence, shot number, workstation scanning velocity, and repetition rate. Various profiles of microlens and microprism arrays with different dimensions can utilize numerical simu- lation and form desired geometries by laser ablation.

A Light Harvesting Policy on Black Counter Electrode for Enhanced Performance of Dye-Sensitized Solar Cells

This work presents a novel light harvesting policy for a black counter electrode (BCE) to enhance the performance of dye-sensitized solar cells (DSSCs), which uses a metal-based light scattering layer (MLSL) that is formed from Al@SiO2 core-shell microflakes prepared and coated on BCE. DSSCs based on BCE with and without the MLSL are compared as well. Analysis results of electrochemical impedance spectra (EIS) indicate that, while not affecting the charge transfer resistance at BCE, MLSL exhibits a low electron transport resistance in the TiO2/electrolyte interface. Our results further demonstrate that MLSL reflects light to the TiO2 electrode, subsequently increasing photocurrent density by 68.68% (from 2.65 to 4.47 mA/cm2) and improving the power conversion efficiency by 49.64%.

Efficiency Enhancement of Dye-Sensitized Solar Cell

Thin nanoporous TiO2 film with a metal-based light reflective layer was investigated in dye sensitized solar cell (DSSC). In cost-effective DSSC module, the thickness reduction of the nanoporous TiO2 film was used to reduce the photoactive dye loading. However, the thin nanoporous TiO2 film has poor light scattering ability so that the film is generally optical transparency, which means many transmitted light would be lost in the electrolyte solution. In order to improve the light scattering issue, we proposed a metal-based light reflective layer which was used on the thin nanoporous TiO2 film to reflect the transmitted light readdressing in the film. The results show that the photovoltaic performance of DSSC was enhanced when using the metal-based light reflective layer.

Thickness Effects on the Thermal Expansion Coefficient of ITO/PET Film

Indium Tin Oxide/Polyethylene Terephthalate(ITO/PET) thin film/flexible substrate have found many applications in dye sensitized solar cells, touch screens, energy efficient windows, etc. In this study, application of the digital image correlation method (DIC) for determining the coefficient of thermal expansion (CTE) of (ITO/PET) thin film was proposed and the effects of thickness variations of ITO and PET, respectively, on the CTE of the specimens were discussed. The observation range of experimental temperature was chosen from room temperature to the glass transition temperature of PET, 70°C. A novel DIC experimental process for reducing the errors caused from the variations of the refractive index of the surrounding heated air was proposed. As a result, the experimental error of CTE measurement was reduced from 15% ~ 23% to less than 4%. The experimental results showed that the CTE of ITO/PET specimen is anisotropic. Furthermore, the CTE of an ITO/PET specimen will be increased by decreasing the thickness of PET flexible substrate, and increased by increasing the thickness of ITO film – which means decreasing the surface resistance of ITO film.