Corrosion protection of anisotropically conductive adhesive joined flip chips

Corrosion is a common cause of failure in electronic devices. Conformal coatings may be used to prevent corrosion of electronics packages. Water and contaminants are thus not in direct contact with the package surface, which may improve resistance to corrosion. However, it is important to study the effects of corrosion on both coating materials and on the electronics itself. The protection method and the level of protection can then be selected.This study presents a salt spray test of anisotropically conductive adhesive joined flip chip components on FR-4 substrate, where half of the test samples were parylene C coated. A dramatic difference was seen between these two test lots. Parylene C proved to be an excellent barrier against salt spray. The test lots without conformal coating suffered severe corrosion, and were thus considerably less reliable. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) revealed the reasons for the failures in the non-coated test samples. The main reason was found to be severe corrosion of the nickel layer. There were no failures among the parylene C coated test samples during the 3000 h test. Moisture absorption of the FR-4 substrate material is also studied in this paper. Both plain FR-4 and parylene C coated FR-4 materials were studied. These tests were performed both at room temperature and in boiling water.     

Experimental and theoretical characterization of electrical contactin anisotropically conductive adhesive

Electrical conduction through anisotropically conductive adhesive (ACA) is caused by deformation of metal fillers under pressure and heat. In this work, the hardness of the electrical particles under various deformation degrees was determined by nano-indentor measurements and the electrical resistance of the electrical contacts was measured under various deformation degrees. Theoretical model and simulation have been developed for the microscopic mechanism of the electrical conduction through metal fillers in the anisotropically conductive adhesive. By comparing with experimental data it is concluded that the deformation of the metal filler in our ACA is plastic even at rather low external load. Further theoretical simulation reveals two important aspects of the conductance characteristics. The conductance is improved by increasing the external load but the dependence of the conductance on the spatial position of the metal filler becomes stronger. Design and optimization of the ACA with respect to the absolute value of the electric conductance and its dependence on the spatial position of the metal filler are of essential importance for the electronics packaging application of the anisotropically conductive adhesives     

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     

Reliability investigations on LIFT-printed isotropic conductive adhesive joints for system-in-foil applications

The reliability of a commercially available isotropic conductive adhesive (ICA) deposited via laser induced forward transfer (LIFT) printing is reported. ICAs are particularly important for surface mount device (SMD) integration onto low-cost, large-area system-in-foil (SiF) applications such as radio frequency identification (RFID) transponder tags. For such tags, and for SiF in general, the reliability of the printed interconnects under harsh circumstances is critical. In this study, the reliability of surface mounted resistors bonded onto screen-printed conductive circuitry on polymer foil was assessed. The prepared samples were subjected to thermal shock testing (TST), accelerated humidity testing (AHT) and flexural testing, while electrical measurements were conducted at regular intervals. Die shear testing was performed to evaluate the bond strength. The reliability characteristics of the LIFT-printed samples were benchmarked against current industry standard stencil printing process. Finally, the applicability of the LIFT–ICA process for practical applications is demonstrated using RFID transponder integration and testing.     

基于电子纸技术的无线电子标签研究

RFID电子标签技术即无线射频识别技术,是一种非接触式的自动识别技术.首先介绍无线电子标签的工作原理和应用特点,以及电子纸的驱动显示技术,针对一种常用的无线电子标签设计感应模块和数据存储处理模块,并提出一种新型应用.     

基于电子纸技术的无线电子标签研究

RFID电子标签技术即无线射频识别技术,是一种非接触式的自动识别技术。首先介绍无线电子标签的工作原理和应用特点,以及电子纸的驱动显示技术,针对一种常用的无线电子标签设计感应模块和数据存储处理模块,并提出一种新型应用。     

Inkjettable conductive adhesive for use in microelectronics and microsystems technology

Ink jet is an accepted technology for dispensing small volumes of material (50–500 picolitres). Currently traditional metal-filled conductive adhesives cannot be processed by ink jetting (owing to their relatively high viscosity and the size of filler material particles). Smallest droplet size achievable by traditional dispensing techniques is in the range of 150 μm, yielding proportionally larger adhesive dots on the substrate. Electrically conductive inks are available on the market with metal particles (gold or silver) <20 nm suspended in a solvent at 30–50 wt%. After deposition, the solvent is eliminated and electrical conductivity is enabled by a high metal ratio in the residue. Some applications include a sintering step. These nano-filled inks do not offer an adhesive function. Work reported here presents materials with both functions, adhesive and conductive. This newly developed silver filled adhesive has been applied successfully by piezo-ink jet and opens a new dimension in electrically conductive adhesives technology.The present work demonstrates feasibility of an inkjettable, isotropically conductive adhesive in the form of a silver loaded resin with a two-step curing mechanism: In the first-step, the adhesive is dispensed (jetted) and precured leaving a ‘dry’ surface. The second step consists of assembly (wetting of the 2nd part) and final curing.     

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     

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.     

Research on the contact resistance,reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications

Although there have been many years of development, the degradation of the electrical performance of anisotropically conductive adhesive or film (ACA or ACF) interconnection for flip-chip assembly is still a critical drawback despite wide application. In-depth study about the reliability and degradation mechanism of ACF interconnection is necessary. In this paper, the initial contact resistance, electrical performance after reliability tests, and degradation mechanisms of ACF interconnection for flip-chip-on-flex (FCOF) assembly were studied using very-low-height Ni and Au-coated Ni-bumped chips. The combination of ACF and very-low-height bumped chips was considered because it has potential for very low cost and ultrafine pitch interconnection. Contact resistance changes were monitored during reliability tests, such as high humidity and temperature and thermal cycling. The high, initial contact resistance resulted from a thin oxide layer on the surface of the bumps. The reliability results showed that the degradation of electrical performance was mainly related to the oxide formation on the surface of deformed particles with non-noble metal coating, the severe metal oxidation on the conductive surface of bumps, and coefficient of thermal expansion (CTE) mismatch between the ACF adhesive and the contact conductive-surface metallization. Some methods for reducing initial contact resistance and improving ACF interconnection reliability were suggested. The suggestions include the removal of the oxide layer and an increase of the Au-coating film to improve conductive-surface quality, appropriate choice of conductive particle, and further development of better polymeric adhesives with low CTE and high electrical performance.     

智能家居网络控制器退耦技术应用

针对物联网技术广泛运用,智能家居以及智慧社区的开展,网络控制器广泛应用于室内家用电器和室内外传感器的控制.由于应用领域的特殊性,设计产品必须具有高可靠性和高安全性,需要在具体的试验环境中进行相关的产品测试.通过在具体原理图中退耦技术应用和PCB布局设计,并在具体的电气和环境试验(电脉冲群干扰试验和静电放电试验)中进行分析和总结,对产品进行持续改进,试验后手动和移动控制效果提升明显,设计的产品符合国家标准和企业标准的具体性能要求.本课题退耦技术应用适用于智能家居控制系统中市电电压输入控制的各类触摸控制器.     

A general Weibull model for reliability analysis under different failure Criteria-application on anisotropic conductive adhesive joining technology

In this paper, a generic four-parameter model has been developed and applied to the anisotropic conductive adhesive (ACA) flip-chip joining technology for electronics packaging applications. The model can also be used to predict any minimum failure cycles if the maximum acceptable failure criterion (in this case, a preset electrical resistance value) is set. The original reliability testing from which the test data was obtained was carried out on flip-chip anisotropically conductive adhesive joints on an FR-4 substrate. In the study, nine types of ACA and one nonconductive film (NCF) were used. In total, nearly 1000 single joints were subjected to reliability tests in terms of temperature cycling between -40/spl deg/C and 125/spl deg/C with a dwell time of 15 min and a ramp rate of 110/spl deg/C/min. The reliability was characterized by single contact resistance measured using the four-probe method during temperature cycling testing up to 3000 cycles. A single Weibull model is used for two failure definitions defined as larger than 50 m/spl Omega/ and larger than 100 m/spl Omega/ respectively using the in situ electrical resistance measurement technique. The failure criteria are incorporated into this Weibull model. This paper shows the flexibility and usefulness of Weibull distribution in this type of applications.     

Piezo-actuators modeling for smart applications

This paper deals with piezoelectric actuators modeling. The objective is to contribute in piezo-models library construction. Compared to existing models in commercial packages, the proposal model integrate nonlinear effects, especially hysteresis, while not requiring high computing efforts. This will help in the generalization of piezo-actuator usage for smart applications. Our methodology and approaches are clearly justified in the paper.     

Development of conductive adhesive materials for via fillapplications

As mobile computing and telecommunication electronics are spreading fast, a demand for microelectronics packaging schemes for high density, fine pitch, high performance and low cost becomes even more severe. Specifically, the conventional printed circuit board (PCB) with plated-through-hole (PTH) vias is difficult to justify its manufacturing cost as well as the packaging density requirement. To meet this challenge, several new manufacturing schemes of high density and high performance PCB have been recently introduced such as surface laminar circuit (SLC), any layer inner via hole (ALIVH) and others. This new PCB fabrication process is categorized as sequential build-up (SBU) or build-up multilayer (BUM) using laminate-based substrates, where via holes are filled with a conductive paste material to make reliable vertical or Z-interconnects. In this paper, a new electrically conducting paste material to be used for via filling is introduced. The new conducting material consists of a conducting filler powder coated with a low melting point metal or alloy, a mixture of several thermoset resins, and other minor organic additives. By varying the filler content and resin chemistry, several formulations have been produced to fill via holes with a high aspect ratio. Via fill experiments have been performed to demonstrate void-free microstructure with good electrical continuity. Various bulk properties such as thermal, electrical and mechanical have also been characterized to understand the material behavior during via filling as well as the field service     

A versatile half-micron complementary BiCMOS technology formicroprocessor-based smart power applications

A modular, high density 0.5 μm Complementary BiCMOS technology with integrated high-voltage Lateral Diffused MOS (LDMOS) and conductivity modulated Lateral Insulated Gate Bipolar Transistor (LIGBT) structures designed for high performance, multi-functional integrated circuit applications is described. The advantages of VLSI processing and 0.5 μm compatible layout rules have been applied to the design and fabrication of the tight-pitch high-voltage devices without sacrificing the performance of 0.5 μm dual-poly (N+/P+) gate CMOS and complementary vertical bipolar transistors. Single chip integration of VLSI microprocessors with high-voltage and/or high-current input and output functions for “Smart Power” applications can be achieved using this technology     

导电银胶触变性的研究

以环氧树脂为基体,银粉为填料,制备出IC封装用导电银胶。研究了环氧树脂与银粉的比例、银粉的形貌和粒径以及触变剂SiO2的用量对导电银胶触变性的影响。结果表明,银胶触变性随树脂/银粉质量比的减小而增大,当其为0.2时,触变指数达到5.68。在一定粒径范围内,银胶的触变性与银粉粒径呈反比关系;银胶触变性随SiO2的用量增大而增大。     

光标记技术研究

光标记技术是光标记交换的核心技术。本文对最新提出的多种光标记技术进行了描述与讨论,并比较研究了各自的优缺点及实用化前景。     

Prediction of electrical contact resistance for anisotropic conductive adhesive assemblies

Anisotropic conductive adhesive (ACA) assembly is emerging as one of the most flexible and cost effective packaging interconnect methods in the microelectronics industry. One of the major impediments to the full realization of the fine pitch (<200 /spl mu/m) capabilities of this assembly method is accurate prediction and control of electrical contact resistance. This paper presents a detailed review and direct comparisons of the different models that have been used to predict electrical contact resistance in ACA interconnects. It is found that large discrepancies exist among these models and between contact resistance values experimentally measured and what these models predict. The governing equations and assumptions underlying the models and their implications are examined, and possible rationales for the observed discrepancies and future directions for developing improved models are identified. The study shows that important issues generally not considered in current models are tunneling resistance, multimaterial layers, edge effect, rough surfaces, elastic recovery and residual forces, interaction between nearby particles and variation on the radii of multiple particles.