Effect of different types of silver and epoxy systems on the properties of silver/epoxy conductive adhesives

In this study, the effect of amine adduct powder (AAP) on the electrical conductivity, thermal expansion, and flexural modulus of one-part system conductive adhesives are investigated. One-part system conductive adhesives are prepared using two types of silver (silver A and silver B) and different percentage of filler loadings, 10 vol.% ?40 vol.%. Silver A adhesive systems exhibit higher electrical conductivity with lower percolation thresholds, high flexural moduli, and low coefficients of thermal expansion (CTE) compared silver B adhesive systems. A comparison of the electrical conductivity and thermal expansion properties of the one part and two-part of silver A adhesives system is undertaken. The one-part silver A adhesives system shows high electrical conductivity and low CTE values compared to the two-part system. This is due to the higher cross linking density of the one-part system compared to that of the two-part system.     

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.     

高热导率Ag-AlN/聚丙烯酸酯导电胶黏剂的制备与性能

分别采用无钯化学镀法和溶液聚合法制备了Ag-AlN和聚丙烯酸酯胶黏剂,并采用超声辅助溶液共混法制备了高热导率Ag-AlN/聚丙烯酸酯导电胶黏剂。采用XRD、EDS和SEM等对Ag-AlN的结构进行分析。结果表明:经过高温和酸性清洗液清洗等处理, AlN表面的杂质被除去,并且在AlN表面形成致密的Al₂O₃层,采用无钯化学镀法制备的Ag-AlN具有优异的电导率和热导率,其电导率由AlN的10-13 S/cm提高到了7.06×102 S/cm,热导率由AlN的170 W/(m·K)提高到了230 W/(m·K)。经过计算, Ag-AlN表面的Ag镀层质量分数约为13%, Ag镀层的厚度约为80 nm。当导电胶黏剂中Ag-AlN填料的质量分数为50%时, Ag-AlN/聚丙烯酸酯导电胶黏剂的电导率为1.9 S/cm,热导率为3.1 W/(m·K)。      

Effect of silver nanostructures on the resistivity of electrically conductive adhesives composed of silver flakes

Silver flakes are the most widely applied conductive fillers in electrically conductive adhesives (ECAs) because of their high conductivity and stable chemical properties. It is expected that there are advanced ECAs with both high electrical conductance and good adhesive strength. The high filler loadings can improve the conductance of ECAs, whereas the adhesive strength is decreased. Silver nanostructures are incorporated for the purpose of electrical conductance and adhesive strength improvement of ECAs. A simple method has enabled the synthesis of silver nanostructures by reducing silver nitrate with ethylene glycol in the presence of poly(N-vinylpyrrolidone). They are added to ECAs by dispersing them in ethanol while it is used as the diluent to adjust the volatility of ECAs, preventing them from the aggregation. This proposed process offers the possibility to effectively use silver nanostructures for improving the conductivity of ECAs at the low content of conductive fillers while good adhesive strength may be obtained.     

Influence of post-curing and coupling agents on polyurethane based copper filled electrically conductive adhesives

Effects of post curing and silane coupling agents with different functional groups such as epoxy, isocyanate and ureide on the electrical and mechanical properties of copper (Cu) filled electrically conductive adhesives (ECAs) were studied. Micron-sized Cu particles were used as conductive fillers and polyurethane resin was applied as the adhesive material. Significant differences could be observed on the as cured electrical resistivity and shear strength of the Cu filled ECAs joints prepared with different silane coupling agents. Silane coupling agents functionalized with epoxy groups yielded the lowest electrical resistivity and highest shear strength among the ECAs in this study. Besides, effect of post-curing at 170 °C for 1 h on the ECAs was also investigated. Results showed that ECAs after post-curing exhibited enhanced electrical conductivity and shear strength compared to the as cured ECAs.     

导热胶粘剂研究

导热胶粘剂因良好的导热及力学性能广泛应用于微电子封装以及热界面材料,对于电子元器件散热具有重要意义.介绍了导热胶粘剂导热原理、导热模型,分析了影响导热率的因素,以及提高导热率的途径;综述了导热非绝缘及导热绝缘胶粘剂的研究进展,最后展望了其应用前景.     

The use of silane coupling agents in the design of electrically stable interfaces of 6061 T6 aluminum alloy surfaces and epoxy-based electrically conductive adhesives

Electrically conductive interfaces between metal surfaces, including aluminum, are commonly formed by means of bonding with an electrically conductive adhesive. Humid environments induce an increase in electrical resistance between aluminum objects bonded in this manner. However, joints that are electrically stable through stress testing in environments of elevated temperature and humidity (85°C and 80% RH for up to 137.5 h) have been obtained by treating the aluminum surface with a thin layer (less than 50 Å) of an organo-silane coupling agent, that is, a material capable of bonding chemically with the aluminum oxide surface layer and potentially bonding with the polymer binder in the adhesive. The present study shows that besides their traditional use as surface modifiers for adhesion improvement, organo-silanes can act as corrosion inhibitors of aluminum surfaces to stabilize electrical performance. Whereas other treatments used for preparation of metal surfaces for bonding, using electrically-insulating structural adhesives, typically do not ensure reliable electrical performance, minimal and reproducible increase in resistance has been observed for joints prepared using the procedure described here. Application of this method does not require any significant investment in equipment and the surface pretreatment of the metal surface is quite simple.     

Bending strength of adhesive joints in microelectronic components: Comparison of edge-bonding and underfilling

The bending strength of underfilled and edge-bonded ball grid array (BGA) microelectronic packages assembled on printed circuit boards (PCBs) was compared using double cantilever beam (DCB) specimens. All specimens with fillets of the same size and shape failed at the same load, with cracks initiating and propagating within the PCB. This was consistent with measurements of the crack initiation strain energy release rate for PCB interfacial failure, which was significantly smaller than that of cohesive failure within the adhesives. Finite element analysis (FEA) indicated that the stress state in the PCB near the PCB-fillet interface in both underfilled and edge-bonded specimens was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA, and independent of the adhesive mechanical and thermal properties over the broad range of properties of the tested adhesives. This explained why decreasing the fillet curvature in edge-bonded specimens produced a significant increase in the joint strength. The crack path in the PCB of the edge-bonded specimens was found to change with the adhesive cure temperature; however, this had a negligible effect on the failure load.     

高含量银基导电胶的制备与性能研究

以90％(质量分数)银含量,10％(质量分数)树脂为基体制备出高银含量,高性能的环氧导电胶.采用0.025 mol/L KI试剂对银粉进行简单的表面处理,原位取代银粉表面的部分润滑剂并且与银粉表面的氧化银发生反应.在日光照射条件下原位生成小尺寸的银纳米粒子,这些小尺寸的银纳米粒子在固化过程中,会发生明显的烧结现象,增加...     

Conventional and microwave-assisted processing of Cu-loaded ICAs for electronic interconnect applications

Isotropically conductive adhesives (ICAs) and inks are widely used for interconnecting components and for printed circuits. Silver (Ag)-filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. In addition, there is a need to develop systems compatible with temperature-sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer. The treated Cu powder was combined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave-assisted heating methods under an inert atmosphere for the curing of the Cu-loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80–150 °C) and time. Tracks with electrical conductivity comparable to Ag-filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in mechanism for the two formulations.     

喷墨印刷工艺对导电墨水导电性能的影响

目的研究喷墨印刷工艺对导电墨水导电性能的影响。方法以EPSON 1390万能平板打印机为输出设备,选取塑料薄膜种类、打印分辨率、打印次数、电路形状为主要工艺指标,运用单因素实验法,测试不同指标作用下各印刷线路的单位电阻值。结果塑料薄膜的表面分子结构对墨水的导电性能有直接影响,表面张力高的塑料有利于获得较好的印刷效果,导电性会得到提高;较高的打印分辨率有利于提高墨层的连接精度,从而提高导电性;适当增加打印次数有利于形成清晰完整的印刷电路,可以降低平均电阻值;印刷电子线路形状不同,导电性能也不尽相同,导电性能顺序为直线型折线型矩形波型环型。结论喷墨印刷工艺对导电墨水的导电性能具有一定的影响,选择合适的印刷工艺有利于提高导电线路的导电性。     

FINITE ELEMENT ANALYSIS OF ANISOTROPIC CONDUCTIVE FILM FOR CHIP ON GLASS PROCESS

Anisotropically conductive film (ACF) is a smart electronic packaging material that consumes minimal space for connecting integrated circuit (IC) chips to a liquid crystal display (LCD) panel or printed circuit board. It consists of an adhesive resin and fine conductive fillers such as metallic particles or metal-coated polymer balls. The fillers are compressed and maintain a certain elastic capability while conducted between electrodes. The size of the contact area and the shape of the fillers are important factors in determining conductivity. The process of applying ACF is modeled into three consecutive steps, and the stress and deformation states are studied by finite element analysis in each. In step 1 of the manufacturing process, external load is applied to compress the conductive particles at a temperature of 190°C so that the matrix resin is in a fluid condition. In step 2, an external load is maintained at this temperature to allow the matrix resin to solidify; then, the load is released and the particles spring back to create tensile stresses in the bonding resin matrix. The last step considers the bonded and conducting ACF cooling from 190°C to room temperature. The state of stress and deformation will be readjusted due to different contraction properties between the filler and matrix resin. The results presented in this paper show that changing the thickness of a coated nickel layer does not clearly affect the conductivity of ACF, and the deformation should be greater than 40% for the stability of conductivity.     

Effects of hybrid fillers based on micro- and nano-sized silver particles on the electrical performance of epoxy composites

In the present study, electrically conductive adhesives produced from hybrid fillers based on micro- and nano-sized silver (Ag) was developed. The influence of the hybrid filler composition on the electrical properties of the hybrid system was studied. The electrical conductivity of the epoxy composites filled with micro- and nano-silver was correlated with their morphologies. A positive effect was observed in the electrical conductivity result when the composition of micro- and nano-sized Ag particles reached a 50:50 weight ratio. The nano-sized Ag particles became interconnecting particles in the interstitial spaces between micro-sized particles. Micrograph scanning shows that the particles were well distributed and dispersed, the separation between lumps of Ag filler by the insulating matrix was significantly reduced, leading to the formation of continuous linkages. The increased electrical conductivity resulted in a charge around the particle distribution, which led to the high capacity. Hence, these particles increased the conductivity of the system.     

Direct flip-chip bonding of bare dies to polypropylene-coated paper substrates without adhesives or solders

Paper-based electronics is an emerging concept with the prospect of developing recyclable, low cost, flexible, and green products such as paper displays, smart labels, RFID tags, smart packages, electronic magazines, biological and medical devices. Compared to conventional printed circuit board (PCB) materials, utilizing paper as an electronics substrate has both physical and chemical challenges. Nowadays, the integration of components on papers are mainly conducted using adhesives [such as anisotropic conductive paste (ACP), isotropic conductive paste (ICP), and non-conductive pastes (NCP)] or low-temperature solders. The application of adhesives and solders in a roll-to-roll fabrication line of papers requires an additional dispensing or printing unit, which has its own drawbacks. Therefore, alternative approaches such as pre-applied adhesive films either on bare dies or papers can gain significant attention. In this study, by exploiting the unique properties of a paper coating material (i.e., polypropylene) as a non-conductive adhesive, it was shown that direct flip-chip bonding of the bare dies and devices could be successfully performed on paper without using any additional adhesives or solders. The electrical and mechanical performance of the flip chip-bonded dies on the polypropylene-coated paper substrate were assessed utilizing daisy-chain contact resistance measurement and die-shear analysis, respectively. Moreover, for an RFID tag application, RFID chips were flip chip bonded to the coated papers and functional tests via NFC communication were also successfully exerted. It was concluded that the polypropylene film on the paper can be considered as an intrinsic NCP layer for flip-chip integration of bare dies.

     

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。     

Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

Correlation between dispersion state and electrical conductivity of MWCNTs/PP composites prepared by melt blending

Non-conductive polymers filled with conductive carbon nanotubes (CNTs) often do not show detectable conductivity due to poor dispersion of carbon nanotubes in the polymer matrix and the lack of conductive networks formed from CNTs. In this work, we attempted two ways to improve the dispersion of multi-walled carbon nanotubes (MWCNTs) in a polypropylene (PP) matrix: chemical modification of MWCNTs and addition of a master batch as a compatibilizer, followed by melt blending using a micro-compounder. The relationship between the dispersion state of MWCNTs and the electrical conductivity of the CNTs/PP composites have been investigated by controlling several factors such as CNTs modification, compatibilization by a master batch, melt mixing, and post-heat treatment. The enhanced interfacial adhesion between the CNTs and the polymer could improve the dispersion of CNTs but it could also reduce the electrical conductivity of the composites. Meanwhile, it is interestingly found that the post-heat treatment could increase the conductivity remarkably due to the connection of CNTs into networks. Thus, it is concluded that the balance between dispersion of CNTs and the formation of conductive networks plays an important role in enhancing the electrical conductivity of composites.     

Preparation of conductive transparent adhesive films from carbon nanomaterials and polar acrylate

Electrically conductive optically clear adhesives (ECOCAs) were prepared using a nanostructured carbon material (CMK-3(150)) as a conductive filler. The mesoporous carbon material, CMK-3(150), was synthesized using an ordered mesoporous silica template to produce inverse replica ordered mesoporous carbon material with an approximately 10 nm pore diameter. An adhesive solution of acrylic monomers containing polar acrylate, CMK-3(150), and thermal initiator was reacted at 80 degrees C to prepare the ECOCA composite which had appropriate viscosity for further processing. The adhesive composite was adhered to various surfaces including ITO films upon thermal processing at 60 degrees C to afford a highly transparent and adhesive film. Tensile strength of the ECOCA films was increased with the contents of conductive filler up to 4 wt%. The percolation concentration of the CMK-3(150) in the composite was approximately 7 wt%, which is much less than those of typical conductive fillers. The optimum content of CMK-3(150) to assure optical clarity, tensile strength, and high conductivity was 2-3 wt%.     

Rheology,electrical conductivity and crystallinity of a polyurethane/graphene composite: Implications for its use as a hot-melt adhesive

Homemade graphene based polyurethane composites elaborated using a facile melt mixing method were studied. The polyurethane (PUR) was a hot-melt adhesive and the research was carried out considering the perspective of electrically conductive adhesive composites. The crystallization of PUR was significantly modified by graphene; for instance the crystallization temperature was shifted to higher temperatures. Avrami equation results indicated that the composites constituted nucleated axialites, instead of espherulites. These results were used to explain the initial decrease of conductivity, followed by a recovery, observed during the crystallization process.The observed huge conductivity enhancement, together with the increase of the crystallization temperature, opens interesting perspectives in the development of electrically conductivity adhesives which offer a rapid welding.     

Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices

Stress corrosion cracking of transparent conductive layers of indium tin oxide (ITO), sputtered on polyethylene terephthalate (PET) substrates, is an issue of paramount importance in flexible optoelectronic devices. These components, when used in flexible device stacks, can be in contact with acid containing pressure-sensitive adhesives or with conductive polymers doped in acids. Acids can corrode the brittle ITO layer, stress can cause cracking and delamination, and stress-corrosion cracking can cause more rapid failure than corrosion alone.The combined effect of an externally-applied mechanical stress to bend the device and the corrosive environment provided by the acid is investigated in this work. We show that acrylic acid which is contained in many pressure-sensitive adhesives can cause corrosion of ITO coatings on PET. We also investigate and report on the combined effect of external mechanical stress and corrosion on ITO-coated PET composite films. Also, it is shown that the combination of stress and corrosion by acrylic acid can cause ITO cracking to occur at stresses less than a quarter of those needed for failure with no corrosion. In addition, the time to failure, under ~ 1% tensile strain can reduce the total time to failure by as much as a third.