Flexible Chip-on-Flex (COF) and embedded Chip-in-Flex (CIF) packages by applying wafer level package (WLP) technology using anisotropic conductive films (ACFs)

Due to increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in electronic devices, extensive studies on flexible electronic packages have been carried out. However, there has been little research on flexible packages by wafer level package (WLP) technology using anisotropic conductive films (ACFs) and flex substrates, an innovative packaging technology that requires fewer process steps and lower process temperature, and also provides flexible packages. This study demonstrated and evaluated the reliability of flexible packages that consisted of a flexible Chip-on-Flex (COF) assembly and embedded Chip-in-Flex (CIF) packages by applying a WLP process.The WLP process was successfully performed for the cases of void-free ACF lamination on a 50 μm thin wafer, wafer dicing without ACF delamination, and a flip-chip assembly which showed stable bump contact resistances. The fabricated COF assembly was more flexible than the conventional COF whose chip thickness is about 700 μm. To evaluate the flexibility of the COF assembly, a static bending test was performed under different bending radiuses: 35 mm, 30 mm, 25 mm, and 20 mm. Adopting optimized bonding processes of COF assembly and Flex-on-Flex (FOF) assembly, CIF packages were then successfully fabricated. The reliability of the CIF packages was evaluated via a high temperature/humidity test (85 °C/85% RH) and high temperature storage test (HTST). From the reliability test results, the CIF packages showed excellent 85 °C/85% RH reliability. Furthermore, guideline of ACF material property was suggested by Finite Element Analysis (FEA) for better HTST reliability.     

Failure modes and effects analysis for high-power GaN-based light-emitting diodes package technology

In this study, nondestructive test is developed to analyze the structure failure of LED package. The relationship between thermal resistance analysis and LED package failure structure is build with T3Ster thermal transient tester and scanning electron microscope (SEM). The failure LED device with defect in the attaching layer and gap between LED chip and copper are designed advisedly. The failure factors of LED package have been measured with thermal resistance analysis and SEM cross-section images. The thermal dissipation performance of LED with defect in the attaching layer is indicated by thermal resistance analysis combined with SEM cross-section images. The blister in attaching layer results in 4.4 K/W additional thermal resistance. The gap between LED chip and copper also makes high additional thermal resistance with 8.6 K/W. Different failures of LED packages are indicated obviously using thermal transfer analysis. The LED package failure structure such as interface defect between solder and cup-shaped copper is able to forecast without destructive measurement.     

Radiation induced loss properties and hardness enhancement technique for ErYb doped fibers for avionic applications

We present radiation reliability properties and their enhancement of ErYb doped optical fibers in terms of induced loss and lifetime prediction via master curve analysis method. In this study, we are primarily concerned with the effects of ionizing radiation on the performance of double cladded ErYb doped optical fibers in an accelerated low dose γ-radiation environment (i.e. <120 rad/h rate) for high power optical amplifiers to be used in satellite communication systems. We demonstrate a novel method that utilizes pre-radiation exposure and thermal annealing, for enhancing radiation hardness of the fibers with respect to induced optical loss and lifetime prediction. Based on this method, we are able to modify radiation induced loss-rate properties of the fiber with an initial loss penalty, realizing overall loss-budget improvement for relatively long-term deployment (i.e. >5 years). In a direct comparison to non-hardened ErYb doped fibers, we demonstrate approximately 0.16 dB/m of radiation induced loss improvement including an initial loss penalty of 0.14 dB for radiation-hardened fibers over a 10-year duration in a natural low dose (i.e. <0.3 rad/h) radiation environment, i.e. low earth orbit.     

Effects of silicone coating degradation on GaN MQW LEDs performances using physical and chemical analyses

This work presents a physics of failure (POF) methodology coupling failure signatures with physico-chemical analyses. The aim is to work out electro-optical failure signatures located in packaged InGaN/GaN Multiple Quantum Wells Light Emitting Diodes (MQW LEDs). Electrical and optical characteristics performed after accelerated ageing tests (30 mA/85 °C/1500 h), confirm a 65% drop of optical power and an increase of one decade of leakage current spreading at the silicone oil/chip interfaces. Through measurements of silicone coating fluorescence emission spectra, we demonstrate that the polymer enlarges the LED emission spectrum and shifts central wavelength. This shift is related to silicone oil spectral instability and the central wavelength of packaged LED appears to be temperature insensitive. In this paper, we discriminate the degradation of bulk silicone oil responsible for optical losses from the polymer/chip interface inducing larger leakage current.     

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35 μm, 25 μm, and 20 μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5 mΩ, good adhesion strength of more than 600 gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85 °C/85% RH, 1000 h) and thermal cycling test (T/C test, ?40 °C to + 125 °C, 1000 cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20 μm pitch. Therefore, for less than 20 μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.     

A polymer light-emitting diode as an optical communication light source

We demonstrate improvements in the electroluminescence switching rate of polymer light-emitting diodes by reducing their active area. We explore the frequency response of devices having a range of sizes and show that LED’s having an active area of 0.02 mm² can be modulated at 16 MHz. The emission from our devices peaks at around 540 nm and thus falls within a transmission window of plastic optical fibers, suggesting possible applications of polymer LED’s as low cost optical communication sources.     

Thermal investigation of LED lighting module

This paper reports on the thermal analysis and improvement of the light emitting diode (LED) module as a lighting source. The analysis was made by transient thermal measurement and thermal simulation using the Finite Volume Method. Two basic thermal schemes were applied for the decrease of the junction temperature of the LED module. Thermal resistance was analytically defined for the LED module with multi LED packages and was confirmed by the experimental data obtained from the thermal transient method. It was found that the thermal improvement of the LED module led to the enhancement of the light output power and radiant intensity. The thermally designed LED module exhibited about 20% decrease in junction temperature compared with a basic structure before thermal design. The temperature calibrating factor, 0.046 nm/°C, was calculated from the peak wavelengths of the LED modules.     

Strength determination of high-power LED die using point-load and line-load tests

The strength of high-power light emitting diode (LED) dies, cut from wafers with a laser, has to be determined for the need of design and quality control in order to assure the good reliability of packages in manufacturing and service. The objective of this study is to determine the strength of high-power LED die with a size of 1 × 1 × 0.1 mm³ by point-load test (PLT) and line-load test (LLT) associated with a plate-on-elastic-foundation configuration. ANSYS (one of commercial finite element codes) analysis is used to calculate the stress distributions of the die under both PLT and LLT. The ANSYS models of the PLT and LLT are validated by comparing with experimental force–displacement curves, and the results are further used to convert the die failure force from the tests into the die strength. The mechanism of tensile-stress dominated die strength has been discussed and validated in detail via these test results and analyses. The results of the PLT and LLT also indicate that for the die failure on chip surface, the average die strengths are about 1.44 GPa and 1.52 GPa from the PLTs with two different-radius pins, and about 1.2 GPa from the LLT. On the other hand, for failures on sapphire surface, the average die strengths are reasonably about 1.49 GPa and 1.26 GPa from the two PLTs, but the average one from the LLT is about 0.64 GPa (with less than 50% of the values from the PLT). The inconsistent data between two PLT and LLT for failure on sapphire surfaces were found to result from the edge chipping of the die specimen observed by scanning electron microscopy. It was also observed that the thin-layer GaN material has to be taken into account in the ANSYS analyses with a bi-material model of the LED die for precisely determining the die strength for failure on the chip surface. Otherwise, these strength data would be overestimated by a few tens of percent with a uni-material model of the LED die. All in all, this study has successfully demonstrated that the LED die strength can be determined by these feasible, easy-to-use and reliable test methods.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Fabrication of nano-sized resist patterns on flexible plastic film using thermal curing nano-imprint lithography

Due to polymer’s excellent flexibility, transparency, reliability and light weight, it is a good candidate material for substrate of devices including organic electronic devices, biomedical devices, and flexible displays (LCD and OLED). In order to build such devices on polymer, nano- to micron-sized patterning must be accomplished. Since polymer materials reacts with organic solvents or developer solutions which are inevitably used in photolithography and cannot bear high temperature (∼140 °C) process for photoresist baking, conventional photolithography cannot be used to polymer substrate. In this research, monomer based thermal curing imprinting lithography was used to make as small as 100 nm dense line and space patterns on flexible PET (polyethylene-terephthalate) film. Compared to hot embossing lithography, monomer based thermal curing imprint lithography uses monomer based imprint resin which consists of base monomer and thermal initiator. Since it is liquid phase at room temperature and polymerization can be initiated at 85 °C, which is much lower than glass temperature of polymer resin, the pattern transfer can be done at much lower temperature and pressure. Hence, patterns as small as 100 nm were successfully fabricated on flexible PET film substrate by monomer based thermal curing imprinting lithography at 85 °C and 5 atm without any noticeable degradation of PET substrate.     

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Impact toughness,hardness and shear strength of Fe and Bi added Sn-1Ag-0.5Cu lead-free solders

In this study, the new Fe/Bi-bearing Sn-1Ag-0.5Cu (SAC105) solder alloys were studied for their mechanical properties, including impact toughness, hardness and shear strength. Charpy impact tester with impact speed of 5.4 m/s was used to determine the impact absorbed energy during impact tests. With the 0.05 wt.% Fe and 1 wt.% Bi addition to the SAC105 alloy, the impact absorbed energy increased from 8.1 J to 9.7 J by about 20% and literally no further improvement was observed by increasing the Bi content in the alloy. Vickers hardness tests were performed with a load of 245.2 mN and load dwell time of 10 s. The addition of Fe/Bi to SAC105 increased the hardness of the alloy from 10.5 HV to 22.6 HV showing an increase of more than two fold. Shear tests were performed with a shear speed of 0.25 mm/min. Shear strength almost doubled for the Fe/Bi added SAC105, as compared to the base alloy, increasing from 17.8 MPa to 34.3 MPa. The microstructure study shows that Bi is dissolved in the solder bulk and strengthens the solder alloys by its solid solution strengthening mechanism. The β-Sn grain size, as revealed by cross-polarized optical microscopy, significantly reduced from 60–100 μm to 20–40 μm with Fe/Bi addition to SAC105. The micrographs of field emission scanning electron microscopy (FESEM) with backscattered electron detector and their further analysis via ImageJ software indicated that Fe/Bi addition to SAC105 significantly reduced the Ag₃Sn and Cu₆Sn₅ IMCs size and refined the microstructure. These changes in the microstructure of Fe/Bi added SAC105 expectedly resulted in such improvement in their mechanical properties.     

Effect of 805 nm on reliability of 735/805/850-nm LED involved near-infrared spectroscopy biomedical device

Near-infrared spectroscopy (NIRS) has been widely used in biomedicine due to its capability of noninvasively detecting hemodynamic variations in relative deep tissue. Most NIRS devices utilized multiple-wavelengths integrated LED as the sources, of which the 735/805/850-nm LED was mostly employed. As we known, the 735/850-nm combination is enough for quantifying the changes of oxy-hemoglobin (∆ HbO₂) and deoxy-hemoglobin (∆ Hb). Then how is the effect of the wavelength 805 nm of 735/805/850-nm LED on the measurement reliability? Here we performed blood model experiments with 57 human blood samples and recorded optical density variations at above 3 wavelengths. Both of the least squares method and multi-variable linear regression analysis were used to quantify ∆ HbO₂ and ∆ Hb with three-wavelength combination (735/805/850-nm) and two-wavelength combination (735/850-nm) respectively. By comparing the quantified values with the real values, we found that the results obtained from 735/850-nm combination are more close to reality than the 735/805/850-nm combination. This study reported, for the first time, that 805 nm actually took a negative effect on measurement reliability of NIRS. It indicates to get rid of 805 nm from such LED design to reduce the LED cost and get higher reliability for NIRS instrumentation.     

Process optimization of RTA on the characteristics of ITO-coated GaN-based LEDs

We present a detailed study on the optimization of rapid thermal annealing (RTA) on GaN-based light emitting diodes (LEDs). 14 mil × 28 mil GaN-based LED chips are fabricated with indium tin oxide (ITO) layer treated by RTA under various temperatures and times. Through the optical and electrical property analyses of ITO film, it is found that the transmittance and sheet resistance are improved after RTA process due to the better ITO crystallization and bigger grain size, compared with ITO treated by conventional furnace annealing. By employing electroluminescence measurement for the LED chips with RTA treatment, the forward voltage is found to be low as a result of low sheet resistance and contact resistance, and light output power (LOP) is high due to high ITO transmittance and good current density uniformity. Under RTA temperature of 550 °C and time of 3 min, the optimized LOP and forward voltage at 60 mA injection current are 71.2 mW and 2.97 V, respectively. Moreover, the reliability of the chips with RTA is better than those with furnace annealing.     

Thermal characterization of multicolor LED luminaire

The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire.     

Experimental observation of the post-annealing effect on the dark current of InGaAs waveguide photodiodes

The post-annealing effect on the dark current of the InGaAs waveguide photodiodes, which are developed for 40-Gbps optical receiver applications, is experimentally investigated. The interesting experimental phenomena were observed that the dark current is significantly decreased and the breakdown voltage is slightly increased after annealing at 250 and 300 °C whereas the dark current and the breakdown voltage are almost constant after annealing at 200 °C. Based on the experimental results, the post-annealing is more effective for the dark current improvement than the conventional curing process.     

Utilization of self-injection Fabry–Perot laser diode for long-reach WDM-PON

In this investigation we propose and demonstrate a wavelength widely tunable laser source employing a self-injected Fabry–Perot laser diode (FP-LD) for long-reach wavelength-division-multiplexed passive optical network (WDM-PON). By using a tunable bandpass filter and an optical circulator inside the gain cavity, a stable and single-longitudinal-mode (SLM) laser output is achieved. Besides, the proposed laser sources are directly modulated at 2.5 Gb/s for both downlink and uplink transmissions of 85 km single mode fiber (SMF) in PON without dispersion compensation.     

Wafer-level LED-SiP based mobile flash module and characterization

In this paper, we have developed and revealed the wafer-level LED system-in-packaging (WL-LED-SiP) design and process platform. This platform provided an LED system solution with high packaging density, high performance, high reliability and cost effectiveness. Camera phone flash module was adopted as a test vehicle for demonstrating LED-SiP platform technologies, including wafer-level (WL) process integration with micro electromechanical system (MEMS) reflector, cavity-based lithography and through silicon via (TSV) filling. Meanwhile, LED flash module performance, including correlated color temperature (CCT), color rendering index (CRI), light energy and distribution, was characterized. The delivered LED-SiP flash module showed wide-view angle design of 80° × 50° and light energy of 5.7 lx s @ 1/30 s. The volume of the module is 8 × 11 × 4 mm³.     

Low-coherence interferometric fiber sensor with improved resolution using stepper motor assisted optical ruler

Low-coherence interferometric sensing is typically used to detect phase changes without simultaneous optical ruler calibration in order to by-pass light intensity fluctuations and the periodic nature of the interferometric signal. An interferogram from a two-staged optical low-coherence Mach–Zehnder interferometer is proposed to double the sensitivity improvement for fiber strain sensing. A 1310-nm wavelength distributed feedback laser implemented in an optical ruler achieved 655-nm resolved characterization from its high-coherence interferogram, which could further be enhanced to an average of 18.9 nm using a stepper motor assisted optical ruler. A 2.7-nε high strain resolution was then demonstrated on a 3-m long fiber sensing arm in a Mach–Zehnder interferometer. The relative movement distances between the interferograms were utilized to experimentally show the strain and force sensitivity as 6.8 μm/με and 8.5 μm/mN, respectively.