Ming-Chuen Yip

Time and temperature-dependent mechanical behavior of underfill materials in electronic packaging application

Abstract The thermo-mechanical testing of HYSOL FP4549 polymer-filled underfill materials was conducted under different strain rate and temperature environment. A new specimen preparation procedure and further test methodology are developed to characterize the time–temperature mechanical behaviors of underfill materials. The stress–strain behavior of materials is simulated with constitutive framework, and the dependence of Young’s modulus on temperature and strain rate was evaluated. In addition, the specimens were tested with microforce testing system to evaluate the creep curve of underfill materials as a function of temperature and stress level. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with unloading–reloading technique to verify the test results and investigate its cyclic mechanical behaviors. On the other hand, the adhesion strength of underfill materials are tested between different adhesion surface by different deformation rate after some isothermal and hygro-thermal environments attack, which is to simulate the environment that the electronic components may be encountered. The results reveal that the rise of the temperature and moisture cause the apparent reduction of the surface adhesion strength, due to the microstructure transition of materials and the diffusion and concentration of moisture. For all conditions of the experiment after environmental preconditioning, the specimen fracture surfaces occur between solder mask and FR4 substrates, which means the measured strength is the adhesion strength between solder mask and FR4. Comparing different adhesion surface, the adhesion strength of underfill/FR4 is higher than solder mask/FR4. The interface of solder mask/FR4 is more sensitive to the temperature and moisture. In all of the cases, increasing the moisture level has a varying but significant effect on both fracture strength and absorption energy Ψ. The failure mode transfer and the strength degradation are attributed to the moisture uptake between the FR4/solder mask and solder mask/underfill interface.

Implementation of a gap-closing differential capacitive sensing Z-axis accelerometer on an SOI wafer

This study presents a novel capacitive-type Z-axis (out-of-plane) accelerometer implemented on an SOI wafer. This accelerometer contains special designed gap-closing differential sensing electrodes. The present Z-axis accelerometer has four merits: (1) mass of the proof mass is increased by combining both device and handle silicon layers of the SOI wafer, (2) the sensitivity is improved by the gap-closing differential electrodes design, (3) the electrical interconnection between the device and handle silicon layers of the SOI wafer is available by means of the metal-vias, and (4) the sensing gap thickness is precisely defined by the buried-oxide layer of the SOI wafer. In application, the Z-axis accelerometer is fabricated and characterized. Typical measurement results demonstrate that the presented Z-axis accelerometer has a sensitivity of 196.3 mV G−1 (42.5 fF G−1) and a maximum nonlinearity of 2% over the range of 0.1–1 G.

Biaxial fatigue crack initiation life prediction of solid cylindrical specimens with transverse circular holes

Multiaxial fatigue crack initiation lives of solid cylindrical specimens with transverse circular holes were investigated. Neubers rule and the finite element approach were both used to obtain the local stresses and strains of the hole. Neubers rule applied for the hole in this study failed to estimate the local stress state under combined push-pull and torsional fatigue loading. Several prediction models for multiaxial fatigue lives were used to examine the experimental results with the assistance of the local strain approach through the finite element analysis in this study. It was found that the maximum stress on the hole edge was uniaxial, and all models gave good predicted results by using only push-pull fatigue data of smooth specimens.

Polymer dispensing and embossing technology for the lens type LED packaging

This study presents a ring-type micro-structure design on the substrate and its corresponding micro fabrication processes for a lens-type light-emitting diode (LED) package. The dome-type or crater-type silicone lenses are achieved by a dispensing and embossing process rather than a molding process. Silicone with a high viscosity and thixotropy index is used as the encapsulant material. The ring-type micro structure is adopted to confine the dispensed silicone encapsulant so as to form the packaged lens. With the architecture and process described, this LED package technology herein has three merits: (1) the flexibility of lens-type LED package designs is enhanced; (2) a dome-type package design is used to enhance the intensity; (3) a crater-type package design is used to enhance the view angle. Measurement results show the ratio between the lens height and lens radius can vary from 0.4 to 1 by changing the volume of dispensed silicone. The view angles of dome-type and crater-type packages can reach 155? ? 5? and 175? ? 5?, respectively. As compared with the commercial plastic leaded chip carrier-type package, the luminous flux of a monochromatic blue light LED is improved by 15% by the dome-type package (improved by 7% by the crater-type package) and the luminous flux of a white light LED is improved by 25% by the dome-type package (improved by 13% by the crater-type package). The luminous flux of monochromatic blue light LED and white?light LED are respectively improved by 8% and 12% by the dome-type package as compare with the crater-type package.

Implementation of the CMOS MEMS Condenser Microphone with Corrugated Metal Diaphragm and Silicon Back-Plate

This study reports a CMOS-MEMS condenser microphone implemented using the standard thin film stacking of 0.35 μm UMC CMOS 3.3/5.0 V logic process, and followed by post-CMOS micromachining steps without introducing any special materials. The corrugated diaphragm for the microphone is designed and implemented using the metal layer to reduce the influence of thin film residual stresses. Moreover, a silicon substrate is employed to increase the stiffness of the back-plate. Measurements show the sensitivity of microphone is −42 ± 3 dBV/Pa at 1 kHz (the reference sound-level is 94 dB) under 6 V pumping voltage, the frequency response is 100 Hz–10 kHz, and the S/N ratio >55 dB. It also has low power consumption of less than 200 μA, and low distortion of less than 1% (referred to 100 dB).

An energy-based damage parameter for the life prediction of AISI 304 stainless steel subjected to mean strain

Abstract This study extends the plastic strain energy approach to predict the fatigue life of AISI 304 stainless steel. A modified energy parameter based on the stable plastic strain energy density under tension conditions is proposed to account for the mean strain and stress effects in a low cycle fatigue regime. The fatigue life curve based on the proposed energy parameter can be obtained directly by modifying the parameters in the fatigue life curve based on the stable plastic strain energy pertaining to fully reversed cyclic loading. Hence, the proposed damage parameter provides a convenient means of evaluating fatigue life on the mean strain or stress effect. The modified energy parameter can also be used to explain the combined effect of alternating and mean strain/stress on the fatigue life. In this study, the mean strain effects on the fatigue life of AISI 304 stainless steel are examined by performing fatigue tests at different mean strain levels. The experimental results indicate that the combination of an alternating strain and a mean strain strongly influences the fatigue life. Meanwhile, it is found that the change in fatigue life is sensitive to changes in the proposed damage parameter under the condition of a constant strain amplitude at various mean strain levels. A good agreement is observed between the experimental fatigue life and the fatigue life predicted by the proposed damage parameter. The damage parameter proposed by Smith et al. (1970) is also employed to quantify the mean strain effect. The results indicate that this parameter also provides a reasonable estimate of the fatigue life of AISI 304 stainless steel. However, a simple statistical analysis confirms that the proposed damage parameter provides a better prediction of the fatigue life of AISI 304 stainless steel than the SWT parameter.

?Flip glass substrate? package technology for LED yield and performance enhancement

This study presents the ?flip glass substrate? approach for the package of an LED chip. The transparent glass substrate covered with phosphor is employed as the carrier. This LED package scheme and its related processes have three merits: (1) light is allowed to emit in all directions for the LED packaged in a flipped glass substrate, and thus the view angle is increased, (2) the chromaticity coordinate distribution of the glass substrate coated with phosphor could be evaluated before LED chip is packaged. Thus glass substrate with wrong chromaticity coordinates could be reworked in early stage, and the color bin yield is enhanced and (3) the uniformity of angular CCT (correlated color temperature) for the LED packaged in the flip glass coated with phosphor is much better than the commercial LED packaged with random distributed phosphor. In application, the glass substrate for packaging is implemented and tested. Measurements show that the packaging technology provides a wide and symmetry view angle (140? ? 1.5?) for the LED chip. In comparison, 95% of the existing commercial single-chip LED packages have a view angle of less than 125?. The maximum deviation in angular CCT is 1300?K, whereas the CCT is 2500?K for random-distributed phosphor type LED.

Preparation, thermal stability and flame-retardant properties of halogen-free polypropylene composites

A flame retardant containing phosphorus and nitrogen was prepared. This halogen-free flame retardant was blended with polypropylene (PP) by hot melting to improve the flame-retardant capability and thermal stability of the composites. Fourier transform infrared spectroscopy, energy dispersive X-ray measurements, thermogravimetric analysis, limiting oxygen index (LOI) measurements, and UL-94 measurements were applied to characterize the structure and thermal and flame-retardant properties of the composites. When the flame-retardant concentration was 40 wt%, the LOI value of the composite was 40, passing the V-0 rating of the UL-94 test. The LOI and UL-94 data showed the composites have an excellent flame-retardant property. For a kinetic study of thermal degradation, Ozawa’s method was applied to calculate the activation energies of pure PP and the composites. Analytical results indicate that the composites had higher values, meaning they have better thermal stability.

A study on mechanical properties of CNT-reinforced carbon/carbon composites

Carbon/carbon composites (C/C composites) possess superior characteristics of low density, high strength, extremely low coefficient of thermal expansion, and high fatigue resistance. In carbonization process, the high-temperature pyrolysis made of carbon, hydrogen, oxygen, and other elements results in a lot of voids and cavities generated in the interior of C/C composites. Therefore, the C/C composites are densified to fill the voids by using repeated impregnation. But densification is a time-wasting and complex process, which increases production costs in the manufacturing process. In this study, themultiwall carbon nanotubes (MWNTs) were adopted as a reinforcement material for C/C composites to reduce the existence of voids or cavities and enhance the mechanical properties of C/C composites. According to the experimental results, the CNT-added C/C composite containing 1.2wt% CNT possesses the greatest flexure strength, flexure modulus, and interlaminar shearing strength. Plus, the abovementioned strength and modulus are increased by 23%, 19.2%, and 30%, respectively.

Development of a “Flip Glass Substrate” LED package technology for color bin yield and view angle enhancement

This study presents a “Flip Glass Substrate” LED package design. The transparent glass is employed as the substrate. This LED package architecture and process has two merits, (1) Large view angle: Light is allowed to emit in all directions for the LED packaged in a flip glass-substrate. (2) Color bin yield enhancement: CIE (The International Commission on Illumination) chromaticity coordinate distribution (CCx, CCy) of glass-substrate coated with phosphor could be evaluated before LED chip package. Thus glass-substrate with wrong chromaticity coordinates could be reworked in early stage (like the concept of known-good-die). Moreover, the advantage of remote phosphor is also achieved for this package architecture. In application, the glass substrate for packaging is implemented and tested. Measurements show the chromaticity relation between unpackaged-LED and packaged-LED is determined as 0.017 in CCx. The view angle of packaged device can reach 142°.