导电胶的研究进展

目的 综述电子封装中用于代替锡铅焊料的导电胶的研究进展,对导电胶未来研究方向进行展望,为导电胶的应用提供参考。方法 从导电胶的组成、导电机理、类型入手,重点介绍导电胶应用时的关键性能要求与测试方法,并总结近几年在提高导电性、稳定性及降低固化温度、成本等方面的研究进展。结果 对导电胶中基体树脂进行改性并选择合适的导电填料(形状、组成),可改善导电胶的固化条件,并提高导电胶的导电性能、黏结性能、耐久性,满足苛刻应用环境下对器件连接高可靠性的要求。结论 相比传统铅锡焊料焊接的方式,导电胶具有绿色环保、连接温度低、分辨率高等特点。因此导电胶适用于电子封装与智能包装领域。目前导电胶的研究方向主要为提高导电性、黏结强度以及黏结稳定性。但是在面对固化时间长、耐湿热性弱、成本较高等缺点时,仍需不断优化组成,以满足实际应用要求。     

紫外光固化导电胶基体固化动力学分析

紫外光固化导电胶作为新型电子连接材料,具有环保、节能等优点。导电胶中基体胶的固化性能是影响导电胶使用可靠性的重要因素。利用实时红外分析研究环氧树脂基体胶的固化过程和固化过程的动力学特征。研究表明基体胶的固化是通过体系中-C=C-双键的互联来实现,随固化反应的进行,双键的相对浓度降低。光引发剂的添加量影响体系的固化速度和固化程度,光引发剂的添加量存在阈值Cmax。     

A review of interconnect materials used in emerging memory device packaging: first- and second-level interconnect materials

The main motivation of this review is to study the evolution of first and second level of interconnect materials used in memory device semiconductor packaging. Evolutions of bonding wires from gold (Au) to silver (Ag) or copper (Cu) have been reported and studied in previous literatures for low-cost solution, but Au wire still gives highest rating in terms of the performance of temperature humidity test, high temperature storage, and bond-ability, etc. However, a new bonding wire material, Au-coated Ag, is recently developed to be an alternative solution which gives comparable performance, but lower cost compared to Au wire. In the first section of the article, the influence of a variety of factors were reviewed, which includes reliability performance and interfacial reaction that determines the performance of Au-coated Ag to reach for developing high reliability of bonded devices. With respect to second-level interconnects, SAC305 and SAC302 solder alloys give a balance performance between temperature cycling testing and drop testing, which are widely used in many field applications, such as mobile, consumer and computer. SAC405 and LF35 are developed for specific requirements such as SAC405 owns better temperature cycling performance, whereas LF35 gives excellent drop performance compared to SAC305 or SAC302. However, with market demands on automotive electronics get strong in recent years, solder joint reliability is being reviewed and discussed, especially in temperature cycling performance. Typical solder alloys on Ni/Au surface finish were not designed for automotive application to fulfill the requirement of board level reliability. Hence, newly developed solder alloys with Sn/Ag/Cu/Bi/Ni elements and Cu-OSP substrate surface finishes will be reviewed in the second section of the article.

     

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.     

Magnetic nanoparticle-based solder composites for electronic packaging applications

Sn–Ag–Cu (SAC) alloys are regarded as the most promising alternative for traditional Pb–Sn solders used in electronic packaging applications. However, the higher reflow temperature requirement, possible intermetallic formation, and reliability issues of SAC alloys generate several key challenges for successful adoption of Pb-free solder for next generation electronic packaging needs. Localized heating in interconnects can alleviate thermal stresses by preventing subjection of entire package to the higher reflow temperatures associated with the SAC solders. It had been demonstrated that SAC solder–FeCo magnetic nanoparticles (MNPs) composite paste can be reflowed locally with AC magnetic fields, enabling interconnect formation in area array packages while minimizing eddy current heating in the printed circuit board.Solder/magnetic nanocomposite pastes with varying MNP concentration were reflowed using AC magnetic fields. Differential scanning calorimetry results show a reduced undercooling of the composite pastes with the addition of MNPs. TEM results show that the FeCo MNPs are distributed in Sn matrix of the reflowed solder composites. Optical and SEM micrographs show a decrease in Sn dendrite regions as well as smaller and more homogeneous dispersed Ag₃Sn with the addition of MNPs. The MNPs promote Sn solidification by providing more heterogeneous nucleation sites at relatively low undercoolings. The mechanical properties were measured by nanoindentation. The modulus, hardness, and creep resistance, increase with the MNP concentration. The enhanced mechanical properties are attributed to grain boundary and dispersion strengthening.The reflow of solder composites have been modeled based on eddy current power loss in the substrate and magnetic power losses in the solder bumps. Induction reflow of pure solder bumps (<300 μm) in an area array package using 500 Oe magnetic field at 300 kHz requires excessive eddy current power loss in the substrate, resulting in extreme temperatures that lead to blistering and delamination of the substrate. Solder–MNP composites with modest MNP loading showed temperature increases sufficient to achieve solder reflow when subjected to the same AC magnetic fields. Thermomechanical behavior of a solder joint was also modeled under cyclic temperature variations. The stress and strain are highly localized at the interface between solder and substrate. Plastic work accumulated per cycle can be used for lifetime prediction.In this article we review lead-containing and lead-free solder systems, and the electronic packaging technologies pertinent to soldering process. Recent research on the effects of MNPs on localized heating, microstructure evolution, mechanical properties, and thermomechanical reliability are summarized.     

A simple, low-cost approach to prepare flexible highly conductive polymer composites by in situ reduction of silver carboxylate for flexible electronic applications

In recent years, efforts to prepare flexible highly conductive polymer composites at low temperatures for flexible electronic applications have increased significantly. Here, we describe a novel approach for the preparation of flexible highly conductive polymer composites (resistivity: 2.5 × 10⁻⁵ Ω cm) at a low temperature (150 °C), enabling the wide use of low cost, flexible substrates such as paper and polyethylene terephthalate (PET). The approach involves (i) in situ reduction of silver carboxylate on the surface of silver flakes by a flexible epoxy (diglycidyl ether of polypropylene glycol) to form highly surface reactive nano/submicron-sized particles; (ii) the in situ formed nano/submicron-sized particles facilitate the sintering between silver flakes during curing. Morphology and Raman studies indicated that the improved electrical conductivity was the result of sintering and direct metal-metal contacts between silver flakes. This approach developed for the preparation of flexible highly conductive polymer composites offers significant advantages, including simple low temperature processing, low cost, low viscosity, suitability for low-cost jet dispensing technologies, flexibility while maintaining high conductivity, and tunable mechanical properties. The developed flexible highly conductive materials with these advantages are attractive for current and emerging flexible electronic applications.     

Effect of filler content on the dielectric properties of anisotropic conductive adhesives materials for high-frequency flip–chip interconnection

The dielectric property of anisotropic conductive film (ACF) as an interconnect materials in the flip–chip joints is becoming important concern for device packaging solution at high-frequency due to low parasitic effect on the signal transfer. The effects of non-conductive, dielectric filler content on dielectric properties of ACA materials, like dielectric constant, loss factor and loss tangent, and conductivity at high-frequency were investigated. Frequency is dominating factor in determining dielectric constant, loss factor, and conductivity. However, the filler content is dominant only on dielectric constant, not on the loss factor, and conductivity at low-frequency range. The effect of low dielectric constant (low-k) filler addition on high-frequency behavior of ACF interconnection in flip–chip assembly was also investigated. Impedance parameters of low-k ACF with Ni filler and low-k SiO₂ filler extracted from measurement were compared with that of conventional ACF with only Ni filler. The resonant frequency of conventional ACF flip–chip interconnect was 13 GHz, while the resonant frequency of low-k ACF including low-k SiO₂ filler was found at 15 GHz. This difference is originated from capacitance decrease of polymer matrix between bump and substrate pad due to change in dielectric constant of polymer matrix, which was verified by measurement-based modeling. The high-frequency property of the conductive adhesive flip–chip joint, such as resonant frequency can be enhanced by low-k polymer matrix.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.     

电子封装用环氧树脂固化温度与应变的三维有限元模拟

环氧树脂因具有许多优异的性能而被广泛用作电子封装材料,然而环氧树脂在固化过程中产生的内应力会对封装产品的性能产生严重影响。针对一种用于电子封装的环氧树脂,通过实验分析了其固化动力学、密度、导热系数、玻璃化转变温度、弹性模量、化学收缩应变和热应变等性能参数,建立了固化过程中的数学模型。通过ABAQUS建立三维有限元模型,采用顺序耦合分析方式,分步进行传热分析和应力应变分析,模拟环氧树脂固化过程中的温度场、固化度场和应力应变场。最后采用光纤布拉格光栅（FBG）监测环氧树脂在固化过程中内部的温度和应变变化,并与模拟进行对比,结果表明本文所建立的有限元模型具有较高的可靠性。      

Lead-free Solders in Microelectronics

Practically all microelectronic assemblies in use today utilize Pb–Sn solders for interconnection. With the advent of chip scale packaging technologies, the usage of solder connections has increased. The most widely used Pb–Sn solder has the eutectic composition. Emerging environmental regulations worldwide, most notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable “Pb-free” solders an important issue for microelectronics assembly. Approximately 70 Pb-free solder alloy compositions have been proposed thus far. There is a general lack of engineering information, and there is also significant disparity in the information available on these alloys. The issues involved can be divided into two broad categories: manufacturing and reliability/performance. A major factor affecting alloy selection is the melting point of the alloy, since this will have a major impact on the other polymeric materials used in microelectronic assembly and encapsulation. Other important manufacturing issues are cost, availability, and wetting characteristics. Reliability related properties include mechanical strength, fatigue resistance, coefficient of thermal expansion and intermetallic compound formation. The data available in the open literature have been reviewed and are summarized in this paper. Where data were not available, such as for corrosion and oxidation resistance, chemical thermodynamics was used to develop this information. While a formal alloy selection decision analysis methodology has not been developed, less formal approaches indicate that Sn-rich alloys will be the Pb-free solder alloys of choice, with three to four alloys being identified for each of the different applications. Research on this topic continues at the present time at a vigorous pace, in view of the imminence of the issue.     

A review of soft errors and the low α-solder bumping process in 3-D packaging technology

This study reviews soft errors in modern electronic assemblies, through silicon via (TSV), and low α-solder bumping techniques for 3-D microelectronic packaging. The TSV fabrication involves deep reactive ion-etching process of Si wafers to form vertical holes, which are further filled with copper and joined to solder bumps. The solder bumps in close proximity to Si die thus impose a serious threat of soft errors. These soft errors responsible for the malfunction of electronic systems have become a critical issue in miniaturized and high-density packaging, like 3-D packaging. Various low α-solder bumping techniques have been reported to minimize these errors in modern microelectronic devices. A low α-solder is one that has low levels of α-particle emission, as compared to the conventional solder. In addition, it improves the performance and reliability of the solder joints, prompting the need to adopt low α-solder for bumping in TSV packaging. Thus, this paper discusses the various aspects of TSV fabrication, functional layer deposition, Cu filling into TSV, and low α-solder bumping on TSV by solder ball reflow methods.     

Novel high performance no flow and reworkable underfills for flip-chip applications

 Flip-chip interconnect is the emerging technology for the high performance, high I/O (Inputs/Outputs) IC devices. Due to the thermal mismatch between the silicon IC (CTE=2.5 ppm/⁰ C) and the low cost organic substrate such as FR-4 printed wiring board (CTE=18–22 ppm/°C), the flip chip solder joints experience high shear stress during temperature cycling testing. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder joint interconnects. Novel no-flow underfill encapsulant is an attractive flip-chip encapsulant due to the simplification of the no-flow underfilling process. To develop the no-flow underfill material suitable for the no-flow underfilling process of flip-chip solder joint interconnects, we have studied and developed a series of metal chelate latent catalysts for the no-flow underfill formulation. The latent catalyst has minimal reaction with the epoxy resin (cycloaliphatic type epoxy) and the crosslinker (or hardener) at the low temperature (<180° C) prior to the solder reflow and then rapid reaction takes place to form the low-cost high performance underfills. The effects of the concentration of the hardener and the catalyst on the curing profile and physical properties of the cured formulations were studied. The kinetics and exothermic heat of the curing reactions of these formulations were investigated by differential scanning calorimetry (DSC). Glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of these cured resins were investigated by using thermo-mechanical analyzer (TMA). Storage moduli (G′) and crosslinking density of the cured formulations were measured by dynamic-mechanical analyzer (DMA). Weight loss of these formulations during curing was investigated by using thermo-gravimetric analyzer (TGA). Additionally, some comparison results of our successful novel generic underfills with the current commercial experimental no-flow underfills are reported. Additionally, approaches have been taken to develop the thermally reworkable underfill materials in order to address the non-reworkability problem of the commercial underfill encapsulants. These include introducing the termally cleavable blocks to thermoset resins, and adding additives to thermoset resins. For the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with the hardener and the catalyst. Then the properties of these mixtures including Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy: ERL-4221D. These mixtures all decompose at lower temperature than the standard formulation. Moreover, one mixture – Epoxy5 – showed acceptable Tg, low viscosity, and fairly good adhesion. For the latter approach, two additives were shown that after added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties of cured epoxy system, yet provide die removal capability to the epoxy. Furthermore, the combination of the two approaches showed positive results. Received: 28 September 1998 / Reviewed and accepted: 1 October 1998     

Process-dependent Thermal-Mechanical Behaviors of an Advanced Thin-Flip-Chip-on-Flex Interconnect Technology with Anisotropic Conductive Film Joints

User experiences for electronic devices with high portability and flexibility, good intuitive human interfaces and low cost have driven the development of semiconductor technology toward flexible electronics and display. In this study proposes, an advanced flexible interconnect technology is proposed for flexible electronics, in which an ultra-thin IC chip having a great number of micro-bumps is bonded onto a very thin flex substrate using an epoxy-based anisotropic conductive film (ACF) to form fine-pitch and reliable interconnects or joints (herein termed ACF-typed thin-flip-chip-on-flex (TFCOF) technology). The electrical and thermal -mechanical performances of the micro-joints are the key to the feasibility and effectiveness of the technology. Thus, the main goal of the study is to assess the process-induced thermal-mechanical behaviors of the interconnect technology during the bonding process. To undertake the process modeling, a process-dependent simulation methodology that integrates both thermal and nonlinear thermal-mechanical finite element (FE) analyses together with ANSYS^® birth-death modeling technique is proposed. The validity of the process modeling is confirmed through various temperature and warpage measurements. Subsequently, the contact behaviors of the ACF joints under four-point bending and static bending tests are characterized through FE modeling. The simulated contact stresses are further correlated with the measured electrical resistance data using four-point probe method, by which the minimum threshold contact stress for achieving a reliable contact electrical performance is determined.     

延长自组织InAs／GaAs量子点发光波长的研究进展

在近几年的InAs／GaAs自组织量子点的研究中，如何荻得1．3～1．55μm。长波长量子点材料是一个很热门的课题。本文综述了各种延长自组织InAs／GaAs量子点发光波长的方法，并提出了实用化的最佳途径。     

Testing of a solder replacement technology

Environmental issues have an ever-increasing influence on the selection of material and processes in electronic manufacturing. This paper discusses the use of conductive adhesives as a replacement for solder on SMT printed circuit boards. As a result of a world-wide market survey, a number of conductive adhesives has been selected. One of the two key issues of this paper has been to uncover the market for adhesive types and their composition. The other key issue has been the technical investigation of the influence of component termination and printed circuit surface types on adhesive bonding stability. Four different types of adhesives on two different metal surfaces are compared with conventional solder technology. Each adhesive has been applied to the PCBs by either screen printing or dispensing according to the manufacturer's recommendation, followed by curing. All PCBs went through thermal and humidity cycling followed by electrical measurements of resistance. Finally all variants have been adhesion tested. All adhesive variants have been microsectioned for metallurgical and microstructure examination. Energy dispersive analysis of X-ray (EDAX) of the metal particles in the adhesive has been carried out and documented. Rework of conductive joints is briefly commented on. Finally, aspects of occupational health are discussed concerning work with adhesive types. Work with epoxy based adhesives has been brought into special focus.     

本征导热高分子材料研究进展

导热高分子复合材料因轻质、设计自由度大及易加工等优势获得了广泛的工业应用,但也面临着热导率k与介电强度E_b之间无法协同提高的严峻问题,严重影响和限制了其在高压电力绝缘设备领域的工业应用。而基于提高无序结构聚合物的结构有序性而获得本征导热高分子(ITCP)的策略,不但同步提高E_b及k,还保留了其自身卓越的综合性能。本文讨论了本征聚合物的导热机制,系统分析了本征k的影响因素,综述了两类不同结构ITCP的研究进展,探讨了聚合物微结构、取向、氢键作用、液晶基元及固化剂、加工方式等因素对本征k的影响机制,阐述了提高聚合物有序结构及本征k的途径。最后总结了当前ITCP研究中存在的问题及未来研究方向,综合性能优异的ITCP在高密度封装微电子、高电压及大功率电力设备等领域具有重要用途,代表了导热高分子的未来发展方向。      

Fatigue and fracture assessment for reliability in electronics packaging

Modern electronics products relentlessly become more complex, higher in density and speed, and thinner and lighter for greater portability. The package of these products is therefore critical. The reliability of the interconnection of electronics packaging has become a critical issue. In this study, the novel testing methods for electronic packaging are introduced and failure mechanisms of electronic packaging are explained. Electronics packaging is subjected to mechanical vibration and thermal cyclic loads which lead to fatigue crack initiation, propagation and the ultimate fracture of the packaging. A small-sized electromagnetic-type bending cycling tester, a micro-mechanical testing machine, and thermal fatigue testing apparatus were specially developed for the reliability assessment of electronics packaging. The long-term reliability of an electronic component under cyclic bending induced high-cycle fatigue was assessed. The high-cycle bending-fatigue test was performed using an electromagnetic-type testing machine. The time to failure was determined by measuring the changes in resistance. Using the micro-mechanical tester, low cycle fatigues were performed and compared with the results of a finite element analysis to investigate the optimal shape of solder bumps in electronic packaging. Fatigue tests on various lead-free solder materials are discussed. To assess the resistance against thermal loads, pseudo-power cycling method is developed. Thermal fatigue tests of lead-containing and lead-free solder joints of electronic packaging were performed using the pseudo-power cycling tester. The results from the thermal fatigue tests are compared with the mechanical fatigue data in terms of the inelastic energy dissipation per cycle. It was found that the mechanical load has a longer fatigue life than the thermal load at the same inelastic energy dissipation per cycle.     

导热高分子材料在包装印刷领域的研究进展

目的综述导热高分子材料在包装印刷领域的应用及研究现状,拓展导热高分子材料的应用领域。方法首先介绍2类导热高分子材料的制备方法,即本征型和填充型导热高分子材料;其次全面综述用于包装印刷领域的导热膜/纸、导热胶黏剂和导热油墨;最后总结常用的各类导热机理模型。结果与本征型导热高分子相比,填充型导热高分子具有加工简单、成本低廉、应用面广等优点,是目前研究最多的导热高分子材料。导热膜/纸、导热胶黏剂和导热油墨具有广泛的研究基础,市场需求旺盛。导热预测模型虽能够有效预测复合材料的热导率,但会受到填料含量和粒子形貌的影响。结论导热高分子材料在包装印刷领域拥有巨大的应用需求,开展导热高分子的研究具有重要的现实和理论意义。     

二维原子晶体材料中的各向异性研究概述

二维原子晶体材料简称二维材料,因载流子迁移和热量扩散都被限制在二维平面内,展现出了许多奇特的性质而受到了广泛关注。二维材料的带隙可调特性在场效应管、光电器件、热电器件等领域应用广泛。另外二维材料的自旋自由度和谷自由度的可控性使得二维材料在自旋电子学和谷电子学等领域也引发了深入的研究。不同的二维材料由于晶体结构的特殊性质导致了不同的电学特性或者光学特性的各向异性,包括拉曼光谱、光致发光光谱、二阶谐波谱、光吸收谱、热导率、电导率等性质的各向异性。这些各向异性特性在偏振光电器件、偏振热电器件、仿生器件、偏振光探等领域拥有巨大的发展潜力。二维材料的各向异性还能够用于实现器件性能的最优化。文章介绍了各种二维材料的各向异性的最新研究进展。     

二维原子晶体材料中的各向异性研究概述

二维原子晶体材料简称二维材料,因载流子迁移和热量扩散都被限制在二维平面内,展现出了许多奇特的性质而受到了广泛关注.二维材料的带隙可调特性在场效应管、光电器件、热电器件等领域应用广泛.另外二维材料的自旋自由度和谷自由度的可控性使得二维材料在自旋电子学和谷电子学等领域也引发了深入的研究.不同的二维材料由于晶体结构的特殊性质导致了不同的电学特性或者光学特性的各向异性,包括拉曼光谱、光致发光光谱、二阶谐波谱、光吸收谱、热导率、电导率等性质的各向异性.这些各向异性特性在偏振光电器件、偏振热电器件、仿生器件、偏振光探等领域拥有巨大的发展潜力.二维材料的各向异性还能够用于实现器件性能的最优化.文章介绍了各种二维材料的各向异性的最新研究进展.