High performance conductive adhesives

Isotropic conductive adhesives (ICAs) have been developed as an alternative for traditional tin/lead (Sn/Pb) solders for electronic applications. Compared to mature soldering technology, conductive adhesive technology is still in its infant stage, therefore, there are some limitations for current commercial ICAs. Two critical limitations are poor impact performance and unstable contact resistance with nonnoble metal finished components. These limitations seriously hindered the wide applications of ICA's. No current commercial ICAs show both desirable impact performance and stable contact resistance. In this paper, novel conductive adhesives were formulated using mixtures of an epoxide-modified polyurethane resin and a bisphenol-F type epoxy resin and a corrosion inhibitor. Cure profiles, rheology, and dynamic mechanical properties of the conductive adhesives were studied using a differential scanning calorimeter (DSC), a rheometer, and a dynamic mechanical analyzer (DMA), respectively. Impact strength and contact resistance with several nonnoble metals (Sn/Pb, Sn, and copper) of these conductive adhesives were tested and compared to those of a commercial conductive adhesive. It was found that these in-house conductive adhesives showed superior impact performance and substantially stable contact resistance with nonnoble metal finished components during elevated temperature and humidity aging     

Determining the impact resistance of electrically conductive adhesives using a falling wedge test

This study was conducted to determine the impact resistance of electrically conductive adhesives (ECAs). A novel falling wedge test that was used to quantitatively characterize the impact resistance of conductive adhesives at a material level has been described, and some important findings obtained from the falling wedge test are presented. This unique impact resistance testing method is not only a substitution of the conventional drop test which has several severe drawbacks, but also provides some useful information for screening adhesives at the materials level and helping formulate new conductive adhesives with improved impact performance. Three model conductive adhesives were studied in this work and the impact fracture energies of the adhesive materials were measured utilizing the falling wedge test. The effect of test temperature on the fracture behavior of ECAs was examined and the correlation between the impact resistance and damping property of the conductive adhesive was also investigated. This study suggests that 1) the falling wedge test is able to discriminate between adhesives and this technique is capable of screening adhesives for bonding purposes; 2) the viscoelastic energy has played an important role in the fracture behavior of the conductive adhesives. As a measure of the internal friction, the loss factor tan/spl delta/ is found to be a good indicator of a conductive adhesive's ability to withstand impact loading.     

Comparative studies on solder joint reliability of CTBGA assemblies with various adhesives using the array-based package shear test

This paper discusses the influences of various adhesives on board-level shear strength of ChipArray® Thin Core Ball Grid Array (CTBGA) assemblies through an innovative reliability evaluation approach, i.e. array-based package (ABP) shear test. It is found that the adhesives do enhance the shear strength for all the test categories as compared with the assemblies without adhesives (w/o A), but the degree of improvements between different strategies vary quiet a lot. The specific shear strength is affected by a number of factors, in which dispending patterns and material properties of the adhesives used influences it obviously. In general, the adhesives with high storage modulus and large dispensing volume are preferred, for example, stiff full or partial capillary flow underfills. In order to further understand the failure mechanism of the CTBGA during the ABP shear test, failure analysis on tested devices are also conducted using side view optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray (EDX), the results indicate that the predominant failure mode changes from PCB pad lift/cratering to fracture at package side intermetallic compound (IMC)/solder interface with increasing dispensing volume and storage modulus, which basically improves the solder joint reliability of CTBGA assemblies.     

脱模剂(蜡)对EMC/红胶体系间密着性影响的研究

当前电子产品正趋于小型化、微型化,这对适用于小型化的表面贴装电子元器件的要求越来越高。因此,研究环氧模塑料(Epoxy Molding Compound,EMC)对改善元器件贴装的可靠性,提高最终产品的质量具有重要意义。以EMC最常用的邻甲酚型环氧树脂(OCN)和酚醛树脂(PN)为树脂系统,选用8种不同类型的蜡为单一变量设计配方合成了EMC,以所述测试方法测试了EMC与红胶的密着力,其中5种蜡符合标准,并分析得到了蜡的类型对EMC与红胶密着力的影响规律。结合离型测试,分析了不同类型的蜡在EMC生产和使用过程中的作用。结果表明,在该树脂系统中,蜡3、4、5、6为主蜡,蜡1、7、8为辅蜡。在EMC层面上解决了红胶密着力问题,同时对提高EMC生产过程中的操作性具有一定的意义。     

Ultrafast UV-Curable Adhesives for Optical Pick-Ups

This paper describes novel ultraviolet (UV)-curable adhesives with an ultrafast curing rate which are fully cured within 8 s for optical pick-up (OPU) applications. Two kinds of oligomers (novolac epoxy acrylate and urethane acrylate), additives, and inorganic fillers were prepared for the formulation of the adhesives. In addition, three kinds of photo-initiator [2,2-dimethoxy-2-phenylacetophenone and 2-hydroxy-2-methylpropiophenone for surface curing and (2,4,6-trimethylbenzoyl) diphenyl phosphine oxide (TMDPO) for deep curing] were mixed to increase the curing rate. Photo-differential scanning calorimetry (photo-DSC) analyses showed that the newly formulated UV adhesives had faster curing rate than conventional UV adhesives. The UV adhesives were applied to OPUs for DVD/CD-RW, and five kinds of reliability tests, i.e., thermal shock, low-temperature storage, high-temperature storage, high temperature/high humidity, and nonoperation shock tests, were conducted to evaluate the adhesive reliability. According to the results of reliability tests and thermal stress simulations, the UV adhesives with lower storage modulus (E′) showed better thermal shock reliability due to lower thermal stresses. In addition, OPUs assembled using the UV adhesives passed all reliability tests. Consequently, the UV adhesives were successfully applied to OPUs in OPU production lines, contributing to mass production.     

Superior Low-Temperature Reversible Adhesion Based on Bio-Inspired Microfibrillar Adhesives Fabricated by Phenyl Containing Polydimethylsiloxane Elastomers

Gecko-inspired microfibrillar adhesives have achieved great progress in microstructure design and adhesion improvement over the past two decades. Space applications nowadays show great interest in this material for the characteristics of reversible adhesion and universal van der Waals interactions. However, the impact of harsh environment of space on the performance of microfibrillar adhesives, especially the extreme low temperature, is rarely addressed. Herein, microfibrillar adhesives fabricated by phenyl containing polydimethylsiloxane (p-PDMS) elastomers with superior low-temperature reversible adhesion is proposed. p-PDMS elastomers are synthesized through one-pot anionic ring-opening copolymerization, and the resulting elastomers become non-crystallizable with excellent low-temperature elasticity. Low-temperature adhesion tests demonstrate that the adhesion strength of microfibrillar adhesives fabricated by p-PDMS elastomers can be well maintained to as low as −120 °C. In contrast, the adhesion strength of pure PDMS microfibrillar adhesive reduces more than 50% below its crystallization temperature. The low-temperature cyclic adhesion tests further demonstrate that p-PDMS microfibrillar adhesives exhibit superior reversible adhesion compared to that of PDMS microfibrillar adhesives, owing to the sustainable conformal contact and even distribution of loads over repeated cycles. This study provides a new fabrication strategy for microfibrillar adhesives, and is beneficial for the practical application of microfibrillar adhesives.     

Rugged, low-cost pigtailing approach for LiNbO3integrated optic devices

A new approach for attaching optical fibers to LiNbO₃ integrated optic devices is described. The all-solder approach, which uses no organic adhesives, is fast, robust, and low cost. A three-port fiberoptic gyro chip was pigtailed, and temperature cycled from 20 to 100°C with only +/-0.5-dB change in optical insertion loss     

A study of electrically conductive adhesives as a manufacturing solder alternative

Soldering processes using tin/lead solder are standard interconnection technologies for electronic manufacturing. These processes are currently under threat from the Waste Electrical and Electronic Equipment and the Restriction of Hazardous Substances (RoHS) environmental directives, issued by the European Union in 2000. These directives explain that solder is to be free from lead by 1986, as lead has been recognized by the European Union as an environmentally harmful material. One solder alternative that has been investigated by the electronics industry is the area of electrically conductive adhesives (ECAs). This paper outlines the electrical and mechanical analysis of two isotropic conductive adhesives where the main properties of joint resistance and adhesive strength were examined before and after different environmental treatments. Joint resistance was measured with a four-probe tester and adhesive strength was examined with the use of shear testing. Cross-sectional and scanning electron microscopy analyses were used to determine problems such as oxidation and moisture absorption that may have an affect on the adhesive properties of the connection. An experimental design was carried out to examine the adhesives on a standard production line. Taguchi analysis was used to determine the build parameters required to produce an optimal adhesive joint. The results show that although ECAs can pass a functionality test, the electrical properties of ECAs are inferior to that of solder with ECAs having a high joint resistance. They also exhibit poor mechanical strength when shear or drop tested and illustrate that current visual inspection techniques used to examine solder joints are not applicable to conductive adhesives.     

Wet‐Responsive,Reconfigurable, and Biocompatible Hydrogel Adhesive Films for Transfer Printing of Nanomembranes

This study presents a wet‐responsive and biocompatible smart hydrogel adhesive that exhibits switchable and controllable adhesions on demand for the simple and efficient transfer printing of nanomembranes. The prepared hydrogel adhesives show adhesion strength as high as ≈191 kPa with the aid of nano‐ or microstructure arrays on the surface in the dry state. When in contact with water, the nano/microscopic and macroscopic shape reconfigurations of the hydrogel adhesive occur, which turns off the adhesion (≈0.30 kPa) with an extremely high adhesion switching ratio (>640). The superior adhesion behaviors of the hydrogels are maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repeatedly. The adhesives are made of a biocompatible hydrogel and their adhesion on/off can be controlled with water, making the adhesives compatible with various materials and surfaces, including biological substrates. Based on these smart adhesion capabilities, diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues in a reproducible and robust fashion. Transfer printing of a nanoscale crack sensor onto a bovine eye further demonstrates the potential of the reconfigurable hydrogel adhesive for use as a stimuli‐responsive, smart, and versatile functional adhesive for nanotransfer printing.     

Electro-conductive adhesives for high density package and flip-chip interconnections

Dispensable isotropic conductive adhesives (ICA) and snap-curing anisotropic conductive adhesives (ACA) are developed through the EC funded Brite EuRam project DACTEL #BE95-1503. They show very promising capabilities for high-density applications when compared to benchmark electro-conductive adhesives.As first high-density application, assemblies of ceramic and plastic ball grid array/land grid array (LGA) on FR4 with DAC3-102/14 ICA are realized. Mixed assemblies solder/ICA show poor results, especially during aging. Full polymer LGA assemblies are built successfully. Daisy chains with hundreds of transitions component/substrate present resistances as low as 4 Ω. After comparison with benchmark products, CLGAs show themselves to be particularly reliable under moisture conditioning.Secondly, flip-chip assemblies on board, of medium sized chips bumped with electroless NiAu and using DAC2-143/02 ACA, are performed. Contact resistances as low as 10 mΩ are produced. For this application, reliability results are succinct.Finally, flip-chip assemblies on glass of slim chips with NiAu bump pitch down to 80 μm, by means of the newly developed DAC2-143/02 ACA, are demonstrated. The material shows better performances than a benchmark anisotropic conductive film, where measurements reveal contact resistances lower than the sheet resistance of the transparent indium tin oxide metallization used in display applications. Thermal cycling and temperature storage reveal good behavior of the ACA paste.     

微电子封装用导电胶的研究进展

介绍了导电胶的组成、分类及与传统锡-铅焊料相比导电胶的优点。阐述了导电胶的导电机理、失效机理以及国内外的研究现状。     

Novel Nonconductive Adhesives/Films With Carbon Nanotubes for High-Performance Interconnects

Novel nonconductive adhesives/films (NCAs/NCFs) with carbon nanotubes (CNTs) for high-performance interconnects were developed. A small amount of CNTs inside NCAs/NCFs could increase the thermal conductivities and at the same time decrease the coefficient of thermal expansion (CTE) for high thermomechanical reliability of the NCAs/NCFs' interconnect joints. Thermal mechanical analyzer measurements showed that the CTE value of the 0.03 wt% CNT filled NCAs/NCFs was significantly decreased. Current–voltage characterizations showed that the current carrying capabilities of the CNT (0.03 wt%) filled NCAs/NCFs were increased by 14% compared to the unfilled NCAs/NCFs due to the more efficient thermal dissipation and higher electrical conductivity by CNTs.      

Injectable Self-Healing Natural Biopolymer-Based Hydrogel Adhesive with Thermoresponsive Reversible Adhesion for Minimally Invasive Surgery

Biocompatible hydrogel adhesives with multifunctional properties, including injectability, fast self-healing, and suitable on-demand detachment, are highly desired for minimally invasive procedures, but such materials are still lacking. Herein, an injectable self-healing biocompatible hydrogel adhesive with thermoresponsive reversible adhesion based on two extracellular matrix-derived biopolymers, gelatin and chondroitin sulfate, is developed to be used as a surgical adhesive for sealing or reconnecting ruptured tissues. The resulting hydrogels present good self-healing and can be conveniently injected through needles. The strong tissue adhesion at physiological temperatures originates from the Schiff base and hydrogen bonding interactions between the hydrogel and tissue that can be weakened at low temperatures, thereby easily detaching the hydrogel from the tissue in the gelation state. In vivo and ex vivo rat model show that the adhesives can effectively seal bleeding wounds and fluid leakages in the absence of sutures or staples. Specifically, a proof of concept experiment in a damaged rat liver model demonstrates the ability of the adhesives to act as a suitable laparoscopic sealant for laparoscopic surgery. Overall, the adhesive has several advantages, including low cost and ease of production and application that make it an exceptional multifunctional tissue adhesive/sealant, effective in minimally invasive surgical applications.     

Engineering Micropatterned Dry Adhesives: From Contact Theory to Handling Applications

Reversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long‐term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications.     

Effect of Gold Stud Bump Topology on Reliability of Flip Chip on Flex Interconnects

The flip chip technique using conductive adhesives have emerged as a good alternative to solder flip chip methods. Different approaches of the interconnection mechanism using conductive adhesives have been developed. In this paper, test chips with gold stud bumps are flip-chipped with conductive adhesives onto a flexible substrate. An experimental study to characterize the bonding process parameters is reported. Initial results from the environmental studies show that thermal shock test causes negligible failure. On the other hand, high humidity test causes considerable failure in flip chip on flex assemblies. Improvements in the reliability of the assembly are achieved by modifying the shape of the gold stud bumps.     

TAPE: A Medical Adhesive Inspired by a Ubiquitous Compound in Plants

Adhesives play an important role in industrial fields such as electronics, architectures, energy plantation, and others. However, adhesives used for medical purpose are rather under‐developed compared with those used in industry and consumer products. One key property required for medical adhesives is to maintain their adhesiveness in the presence of body fluid. Here, an entirely new class of medical adhesives called TAPE is reported; this is produced by intermolecular hydrogen bonding between a well‐known polyphenol compound, tannic acid, and poly(ethylene glycol). The preparation method of TAPE is extremely easy, forming a few liters at once by just the simple mixing of the two compounds without any further chemical synthetic procedures. TAPE shows a 250% increase in adhesion strength compared with fibrin glue, and the adhesion is well maintained in aqueous environments. It is demonstrated that TAPE is an effective hemostatic material and a biodegradable patch for detecting gastroesophageal reflux disease in vivo. Widespread use of TAPE is anticipated in various medical and pharmaceutical applications such as muco‐adhesives, drug depots, and others, because of its scalability, adhesion, and facile preparation.     

金属粉末-聚合物复合导电胶研究进展

随着电子封装工艺的发展,越来越多的人们致力于研发更高性能、环境友好的新连接材料,以聚合物为基体的复合导电胶在电子封装领域的应用也由此越来越广泛。文章介绍了导电胶的分类和组成,以及改善导电胶性能的方法与技术,从宏观和微观角度对导电机理进行了探讨并对研究前景做了展望。     

Novel Isotropical Conductive Adhesives for Electronic Packaging Application

Isotropical conductive adhesives (ICAs) have recently received a lot of focus and attention from the researchers in electronic industry as a potential substitute to lead-bearing solders. In this paper, a novel kind of isotropical conductive adhesives was made by using silver (Ag) nanowires and nanosized silver particles as conductive fillers. These Ag nanowires with dismeters in the range of 30–50 nm and lengths of up to ${sim 50}~mu {rm m}$ could be synthesized by a solution-phase method, which was proceed by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). Electrical property including bulk resistivity and mechanical property including shear strength were investigated and compared with that of conventional ICA filled with micrometer-sized Ag particles or nanometer-sized Ag particles. Our results indicated that ICA filled Ag nanowires exhibited higher conductivity, higher shear strength and low percolation threshold value than traditional ICA. Possible conductive mechanism was discussed based on theory calculation.      

粘结材料对CTBGA封装组件热循环可靠性的影响

对使用了8种粘结材料的12 mm SAC305薄型球形栅格阵列(CTBGA)封装组件进行了热循环测试。结果显示,所有的粘结材料都降低了组件的热循环可靠性。具有较低线(热)膨胀系数,较高玻璃转化温度和储能模量的粘结材料可使组件热疲劳寿命相对更长,如使用了材料E(a11=48×10-6/℃;a12=184×10-6/℃;θ...     

导电胶粘剂的研究

文章综述了当前电子组装业中导电胶粘剂的研究情况,主要介绍国外正在重点研究的几大类导电胶粘剂及其组成,与传统锡铅焊料的对比,以及它们的电性能、机械性能、热性能等。同时简要介绍其发展趋势。