A reliable and environmentally friendly packagingtechnology-flip-chip joining using anisotropically conductive adhesive

There is an increasing demand to move the radio base station closer to the antenna for future mobile telecommunication systems. This requires a significant reduction in weight and volume and increased environmental compatibility. This work provides an evaluation of environmental impact and reliability when using anisotropically conductive adhesives (ACA) for flip-chip joining in radio base station applications. Conventional FR-4 substrate has been used to assemble a digital ASIC chip using an anisotropically conductive adhesive and flip-chip technology. The chip has a minimum pitch of 128 μm with 7.8 mm in chip 8 and has in total 144 bumps with a bump size of 114×126 μm². Bumping was made using electroless nickel/gold technology. Bonding quality has been characterized by optical and scanning electron microscopy and substrate planarity measurement. The main parameters affecting quality are misalignment and softening of the FR-4 substrate during assembly, leading to high joint resistance. Reliability testing was conducted in the form of a temperature cycling test between -40 and ±125°C for 1000 cycles, a 125°C aging test for 100 h and a 85/85 humidity test for 500 h. The results show that relatively small resistance changes were observed after the reliability test. The environmental impact evaluation was done in the form of a material content declaration and a life cycle assessment (LCA). By using flip-chip ACA joining technology, the content of environmentally risky materials has been reduced more than ten times, and the use of precious metals has been reduced more than 30 times compared to conventional surface mount technology     

Degradation mechanism of ag-epoxy conductive adhesive/Sn-Pb plating interface by heat exposure

The compatibility between Ag fillers and Sn-Pb plating at elevated temperature was examined by mechanical test, electrical test, and microstructure observation. The degradation at 150°C is caused by the preferential diffusion of Sn from the plating layer into Ag in the conductive adhesive. By this diffusion, Ag-Sn intermetallic compounds formed in the Ag fillers adjacent to the plating layer, and many large Kirkendall voids are formed in the Sn-Pb plating layer. Furthermore, an interfacial debonding occurred between the conductive adhesive, and the Sn-Pb plating layer is also observed near the free surface after heat exposure.     

Improvement in high-temperature degradation by isotropic conductive adhesives including Ag–Sn alloy fillers

The reliability evaluation of Cu and Sn/Ni joined with isotropic conductive adhesives (ICAs) including Ag–Sn alloy fillers with or without Ag plating instead of Ag fillers was examined using tensile tests, electrical resistivity tests and microstructural observations. For an ICA, including Ag–Sn alloy fillers added to Sn–58wt%Bi fillers, the tensile strength was found to improve, but the electrical resistivity worsened with 150 °C heat exposure. An ICA, including, Ag–Sn alloy fillers with Ag plating, was able to maintain electrical resistivity after being subjected to 150 °C heat exposure. The Ag plating on the Ag–Sn fillers reacted with the Sn in the Ag–Sn fillers, leading to the joining of the fillers with each other though metallurgical connections, and the transformation of Ag into Ag₃Sn within a 1-h curing time at 150 °C, since the Ag plating was microscopic and active. After heat exposure, the Sn distributed itself along the substrate/ICA interface by the diffusion of Sn though the connected fillers, and Cu₃Sn formed at the Cu/ICA interface, in contrast with the Ag–Sn alloy fillers without Ag plating.     

Conductivity enhancement of nano silver-filled conductive adhesives by particle surface functionalization

Five different types of surfactants were employed for nanoparticle functionalization and the effects of the surfactants on electrical properties of nano silver (Ag)-filled conductive adhesives were investigated. The Ag nanoparticles pretreated with the surfactants were incorporated into isotropic conductive adhesives (ICA) formulations as conductive fillers. By using the surfactants (S3, S4, and S5), the reduced resistivity of the nano Ag-filled adhesives could be achieved with 2×10⁻⁴ Θ-cm. The morphology studies showed that the low resistivity resulted from the sintering of nanoparticles.     

Reduced Cu interface diffusion by CoWP surface coating

Electromigration in Cu interconnections with a 10-nm thick selective electroless CoWP coating on the top surface of Cu dual damascene lines has been investigated. The grain structures of the lines embedded in SiLK semiconductor dielectric ranged from bamboo-like to polycrystalline. CoWP coated structures exhibited a greatly improved Cu electromigration lifetime which was attributed to a reduction in Cu interface diffusion.     

Synthesis of highly conductive cobalt thin films by LCVD at atmospheric pressure

We have deposited very low resistant Co films on SiO₂-coated Si substrates using UV pulsed laser pyrolytic decomposition of Co₂(CO)₈ with 355 nm laser radiation at atmospheric pressure. Facile decomposition of the precursors and the use of Ar curtain enable the deposition of relatively pure Co (with O less than 7% and negligible C) at the power of 1.11–3.33 W, and of pure Co at 6.67 W. The resistivity decreases from 58 to 19 µΩ-cm as the power increases from 2.22 to 3.33 W, showing inverse-linear dependence on grain size. In addition, further increase of the power to 6.67 W decreases the resistivity to 9 µΩ-cm, due to both the growth of large grains with negligible contaminants, and the adverse effect of surface roughness. The effects of oxygen contaminants on the resistivity can be minimal, because of its presence in the form of oxide. These low resistant fine metal lines deposited by a direct-writing laser chemical vapor deposition technique at atmospheric pressure have been reported for the first time.     

Low-cost direct chip attach: comparison of SMD compatible FCsoldering with anisotropically conductive adhesive FC bonding

Small modules on the basis of laminate substrates are often used as a functional subunit in electronic applications. Currently most of them are made by chip and wire technique as one kind of bare chip assembly to fulfill the requirements of size reduction. Low cost flip-chip technology is one of the most promising approaches for further cost and size reduction. In this paper a special car radio submodule is chosen for exemplification. We compare a SMD compatible FC soldering process using eutectic solder bumps and underfilling with an anisotropically conductive adhesive (ACA) FC bonding process using tape or paste materials. FC Soldering: For FC soldering an electroless, maskless Ni/Au plating for under bump metallization (UBM) was chosen. The solder deposition itself is done by stencil printing whereas other cost efficient deposition techniques in the market have been observed. The FC assembly is integrated into a standard SMD line. Different underfill methods for quick underfilling are shown and failure mechanisms and lifetime predictions of assembled flip chips are demonstrated. ACA-FC Bonding: For this process electroless Ni/Au bumping is used as well. An assembly process for ACAs using a semiautomatic FC bonder is developed. In order to reduce the time for mounting the ACA has been precured. The aspects of different process flows including ACA deposition techniques, tape and paste adhesives and filler materials are discussed. The influence of high current, climate, and thermal cycling on the contact resistance and the low frequency noise spectrum is shown. In summarizing this work we describe the benefits and disadvantages of both techniques and discuss the potential for further developments and applications     

Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly II. Different bonding pressure

The effects of different bonding temperatures during flip-chip-on-flex (FCOF) assembly in relation to the performance of anisotropic conductive adhesive (ACF) interconnect were investigated. Two types of flip chips were used in this study. It was found that Ni bumps formed better interconnections than bumpless FCOF packages. Aluminium oxide was observed and was thought to be the main cause of the increased in contact resistance after the moisture-soak tests. The conductive particles were not fully compressed by the bumps and pads and gaps were observed between the conductive particles and Cu pads in bumpless packages. Conductive particles in the Ni bump FCOF packages were tightly trapped between the bumps and pads and hence gave better connections. The performance of the ACF interconnects were affected by the degree of curing of the ACF, which was determined by the bonding temperature.     

石墨烯MEMS压力传感器Au/Sn共晶键合气密性封装

针对石墨烯MEMS压力传感器气密性封装的需求,设计出一种用于石墨烯MEMS压力传感器芯片级Au/Sn共晶键合工艺方法。石墨烯压力传感器芯片键合密封环金属采用50/400 nm的Cr/Au,基板键合密封环金属采用50/400/500/3 nm的Cr/Au/Sn/Au。随后使用倒装焊机在280℃以及8 kN的压力环境下保持6 min,完成芯片与基板的Au/Sn互溶扩散键合工艺,从而实现石墨烯压力传感器芯片的气密性封装。对键合指标进行测试,平均剪切力达20.88 MPa,平均漏率为4.91×10^-4 Pa·cm³/s,满足GJB548B-2005的要求。通过比较键合前后的芯片电学特性,石墨烯敏感结电阻平均值变化了1.1%,具有较高的稳定性。此外键合界面能谱测试结果符合Au/Sn键合金属合金元素组分,为石墨烯MEMS压力传感器低成本、高效率气密性封装奠定了基础。     

Effects of novel carboxylic acid-based reductants on the wetting characteristics of anisotropic conductive adhesive with low melting point alloy filler

A low viscosity epoxy resin with carboxylic acid-based novel reductants was introduced to improve the process ability and reliability of an anisotropic conductive adhesive (ACA) resin with a low melting point alloy (LMPA) filler system. The curing degree of the ACA resin and the melting of the LMPA filler were investigated using differential scanning calorimetry (DSC) conducted in the dynamic mode in order to control the curing conditions such as the reaction temperature and a melting temperature of solder. The temperature-dependent viscosity characteristics of the ACA resin were investigated using a rheometer. The compatibility between the viscosity of a polymer matrix and a melting temperature of solder was characterized to optimize the processing cycle. Three different types of carboxyl acid-based reductants were added to remove the oxide layer on the surfaces of the filler particles and the conductive pad. Good wetting properties were achieved between the LMPA filler and the Cu pad in the epoxy resin using these carboxylic acid-based reductants.     

Analysis of mode conversion in waveguide transition sections with surface impedance boundaries applied to VLF radio propagation

First- and second-order correction terms are derived for the reflection and transmission scattering coefficients for a step-type transition between two uniform waveguides with surface impedance boundaries. This analysis yields rigorous expressions for the differential transmission and reflection coefficients defined for an infinitesimal step transition. These coefficients are essential in the derivation of the coupled differential equations for the wave amplitudes in a transition section of gradually varying height.     

Orthogonalization by principal components applied to CPM

The orthonormal basis for the space spanned by a given signal set can be chosen in many different ways. However, when the basis is truncated to fewer dimensions, the quality of the approximated signals differs, depending on the choice of the original basis. We study two energy-related quality measures and show that the optimal lower-dimensional approximation is given by the principal components (PC) method, which is also a simple and efficient alternative to Gram-Schmidt techniques. In addition, we derive and bound the average decrease in squared Euclidean distances over one symbol interval caused by the PC method. This measure is relevant in serially concatenated continuous phase modulation, where a manifold of signal pairs contributes to the bit-error rate for low-to-medium signal-to-noise ratios. By a numerical evaluation, we find that for this measure, the decrease is lower than that of a previous method by J. Huber and W. Liu (see IEEE J. Select. Areas Commun., vol.7, p.1437-49, 1989). Finally, we compare the minimum squared Euclidean distance for error events, which is relevant for uncoded CPM systems. Here, the loss with the PC method is generally larger than with Huber and Liu's method, although examples of the opposite exist.     

Digital half-toning by error diffusion with perturbation

An error diffusion method is well known as one of the half-toning methods for displaying grey tone pictures on a bilevel display. However, the error diffusion method has some shortcomings, such as the appearance of correlated artefacts and directional hysteresis. To overcome these shortcomings, the author proposes a new error diffusion method with perturbation     

Low-light image enhancement by diffusion pyramid with residuals

With the advancement of the camera-related technology in mobile devices, the vast amount of photos have been taken and shared in our daily life. However, many users still have unsatisfactory experiences with low-visible photos, which are frequently acquired under complicated real-world environments. In this paper, a novel yet simple method for low-light image enhancement has been proposed without any learning procedure. The key idea of the proposed method is to estimate properties of the scene illumination both in global and local manner by exploiting the diffusion pyramid with residuals. Specifically, the residual of each scale level in the diffusion pyramid is combined with the corresponding input. This restored result efficiently highlights local details across different scale spaces, thus it is helpful for preserving the boundary of illuminations. By conducting max-pooling with restored results from different levels of the diffusion pyramid, which are resized to the original resolution, the illumination component is accurately inferred from a given image. Compared to recent learning-based approaches, one important advantage of the proposed method is to effectively avoid the overfitting problem to the specific training dataset. Experimental results on various benchmark datasets demonstrate the efficiency and robustness of the proposed method for low-light image enhancement in real-world scenarios.     

A new 2-D FDTD method applied to scattering by infinite objects with oblique incidence

A new two-dimensional (2-D) finite-difference time domain (FDTD) method applied to scattering by infinite objects with oblique incidence is proposed. 2-D Maxwell's equations, differential equations, and perfectly matched layer (PML) absorbing boundary conditions (ABC) are derived. The incident wave, computed by the 1-D FDTD method, is set on the connecting boundary. The accuracy and the efficiency of the proposed method have been verified by comparing the results of the split-field periodic FDTD method, the sine-cosine method, and the transmission line theory method with the proposed method.     

Theory of diffusion constant-, lifetime- and surface recombination velocity-measurements with the scanning electron microscope

The theory given in this paper shows how to determine separately lifetime, surface recombination velocity and diffusion constant by using harmonic beam intensity modulation and measuring the amplitude and phase of the induced photocurrents as a function of the distance of the beam from the contact. Our analysis which is based on expansion of the minority carrier density into orthogonal functions is more general than previous ones, since we can take into account a slicelike sample geometry. Moreover, the collecting junction may lie parallel to the surface. This is an important experimental arrangement because easily prepared Schottky contacts may be used.     

Effect of pressure on surface passivation of silicon solar cell by forming gas annealing

Forming gas annealing (FGA) of single crystalline commercial silicon solar cell was carried out at various conditions of temperature, pressure, time, and gas ratio for different samples. We report here the role of working pressure during annealing to improve the performance of the solar cells. The improvement basically comes from the surface passivation. It was found that the annealing of solar cells at high temperature in forming gas ambient could bring improvement in conversion efficiency only if the process pressure is sufficient to suppress the out diffusion of hydrogen from the silicon nitride layer that acts as anti-reflection coating. The outcome of this experiment can be instrumental in providing guidelines for improving the low grade commercial cells by introducing an easy and low cost step of forming gas annealing in appropriate condition.     

如何克服OXC级连造成的性能退化

在全光网络中，多个波长交换节点（WSN）级连的损耗和噪声有可能累积起来，造成光信号劣化和光信噪比降低。通过使用合适的滤波器、波长锁定的激光器、低噪声的光放大器 和低损耗的光纤等方式，可以增加级连的节点数目。     

Grey relational clustering associated with CAPRI applied to FPGA placement

Grey relational clustering is used to minimise wire length during field programmable gate arrays (FPGA) placement and routing. The proposed Grey Relational Clustering Apply to Placement (GRAP) algorithm combines grey relational clustering and convex assigned placement for regular ICs method to construct a placement netlist, which was successfully used to solve the problem of minimising wire length in an FPGA placement. Upon calculating the grey relational grade, GRAP can rank the sequence and analyse the minimal distance in configuration logic blocks based on the grey relational sequence and combined connection-based approaches. The experimental results demonstrate that the GRAP effectively compares the Hibert, Z and Snake with bounding box (BB) cost function in the space-filling curve. The GRAP improved BB cost by 0.753%, 0.324% and 0.096% for the Hilbert, Z and Snake, respectively. This study also compares the critical path with the space-filling curve. The GRAP approach improved the critical path for Snake by 1.3% in the space-filling curve; however, the GRAP increased critical path wire by 1.38% and 0.03% over that of the Hilbert and Z of space-filling curve, respectively.     

Altering the Mechanical Properties of Sn Films by Alloying with Bi: Mimicking the Effect of Pb to Suppress Whiskers

Stress is believed to be the main driving force for whisker formation in Sn coatings on Cu. This suggests that whiskering can be suppressed by enhancing stress relaxation in the Sn layer, which is believed to be the reason why Sn-Pb alloys do not form whiskers. However, Pb is no longer acceptable for use in electronics manufacturing. As an alternative, we used pulsed plating to create Sn-Bi coatings with an equiaxed microstructure similar to that of Sn-Pb alloys. An optical wafer curvature technique was used to measure stress relaxation kinetics in Sn, Sn-Pb and Sn-Bi alloy thin films during thermal cycles. The results show that Sn-Bi films have significantly enhanced stress relaxation relative to pure Sn films. Comparison between Sn-Bi samples with equiaxed and columnar microstructures shows that both microstructure and alloy composition play a role in enhancing the stress relaxation.