A review of the influencing factors on anisotropic conductive adhesives joining technology in electrical applications

New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications. Among all the conductive-adhesive materials, the anisotropic conductive adhesives (ACA) (or anisotropic conductive adhesive films, ACF) have gained popularity as a potential replacement for solder interconnects. The interest in using ACA instead of solder comes partly from the fact that the use of ACA for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, reduced cost, and decreased equipment needs. In this review, a summary of our understanding of the electrical, physical, thermal, chemical, environmental, and cost behaviors of ACA in conjunction with various packaging applications is elaborated. First, the formulation and curing kinetics of ACA materials, as well as the conduction mechanisms of ACA joints, are introduced; second, the influencing factors, including the boding process (boding temperature, boding pressure, curing conditions, reflow and misalignment processes, etc), the environmental factors (temperature, humidity, impact load, etc), and the properties of the components (the properties of the ACA, substrates, conductive particles, the bump height, etc), on the reliability of ACA joining technology are presented. Finally, future research areas and remaining issues are pointed out. The purpose is simply to pinpoint the most important papers that have played significant role for the advancement of the ACA bonding technology.     

Interfacial structure and strength on Ti(C,N) joint using liquid phase diffusion bonding with Ti/Ni interlayer

Abstract

Partial transient liquid phase diffusion bonding (PTLP-DB) on Ti(C,N) cermet was studied in the present paper using Ti/Ni/Ti foil sandwich structure as the interlayer. The interfacial structure and element distribution at the interface were observed using SEM, electron probe microanalysis and X-ray diffraction. The joint strength was measured using four-point bending test. The results showed that metallurgical bonding between Ti(C,N) cermet was achieved using PTLP-DB. Near Ti(C,N) cermet side, a strong chemical reaction occurred to produce an interfacial multilayer containing Ti–Al and Ti–Ni intermetallics. Different bonding times during PTLP-DB were also studied, and there was an optimum time during bonding. With a shorter bonding time, voids were observed at the interface, while with a prolonged time, the bending strength on the joints also decreased due to the overgrowth on intermetallic layer and the existence of high gradient residual stress at the interface.     

Mechanical property analyses of porous low-dielectric-constant films for stability evaluation of multilevel-interconnect structures

The mechanical properties of porous low-dielectric-constant (low-k) thin films have been investigated for the stability evaluation of multilevel-interconnect structures using nanoindentation, microscratch, and four-point bending tests. Stress–strain curves of these films are proposed to predict their strengths and to explain their deformation behaviors. Real stress–strain behaviors are analyzed and confirmed by combining the experimental data obtained from nanoindentation and microscratch tests. Soft low-k films exhibit large plastic deformation, while hard and brittle films fracture early. The interfacial adhesion strengths and delamination behaviors between thin-film layers have been also studied using microscratch and four-point bending tests. The mechanical failure of interconnect structures depends on the inferiority of film strength or interfacial adhesion.     

Flexible Wire‐Shaped Supercapacitors in Parallel Double Helix Configuration with Stable Electrochemical Properties under Static/Dynamic Bending

Wire‐shaped flexible supercapacitors (SCs) have aroused much attention due to their small size, light weight, high flexibility, and deformability. However, the previously reported wire‐shaped SCs usually involve complex assembly processes, encounter potential structural instabilities, and the influence of dynamic bending on the electrochemical stability of wire‐shaped SCs is also not clear. Here, a parallel double helix wire‐shaped supercapacitor (PDWS) protocol has been developed with two symmetric titanium@MnO₂ fiber electrodes winded on a flexible nylon fiber by a simple and reliable process. The PDWSs show an operate voltage of 0.8 V, a high capacitance of 15.6 mF cm^–2 and an energy density of 1.4 µWh cm^–2. Due to rational structure design, the PDWSs demonstrate excellent mechanical and electrochemical stability under both static and dynamic deformations. Over 3500 bending cycles, 88.0% of the initial capacitance can still be retained. In terms of dynamic bending, it is found that the cyclic voltammetry curves show periodically fluctuations simultaneously with the bending frequency and the intensity of fluctuation increases with higher bending frequency, while the dynamic capacitance is almost not affected. With extraordinary mechanical flexibility and excellent electrochemical stability, the high performance PDWS is considered to be a promising power source for wearable electronics.     

Highly Reliable Ag Nanowire Flexible Transparent Electrode with Mechanically Welded Junctions

Deformation behavior of the Ag nanowire flexible transparent electrode under bending strain is studied and results in a novel approach for highly reliable Ag nanowire network with mechanically welded junctions. Bending fatigue tests up to 500 000 cycles are used to evaluate the in situ resistance change while imposing fixed, uniform bending strain. In the initial stages of bending cycles, the thermally annealed Ag nanowire networks show a reduction in fractional resistance followed by a transient and steady‐state increase at later stages of cycling. SEM analysis reveals that the initial reduction in resistance is caused by mechanical welding as a result of applied bending strain, and the increase in resistance at later stages of cycling is determined to be due to the failure at the thermally locked‐in junctions. Based on the observations from this study, a new methodology for highly reliable Ag nanowire network is proposed: formation of Ag nanowire networks with no prior thermal annealing but localized junction formation through simple application of mechanical bending strain. The non‐annealed, mechanically welded Ag nanowire network shows significantly enhanced cyclic reliability with essentially 0% increase in resistance due to effective formation of localized wire‐to‐wire contact.     

Large‐Scale Direct‐Writing of Aligned Nanofibers for Flexible Electronics

Nanofibers/nanowires usually exhibit exceptionally low flexural rigidities and remarkable tolerance against mechanical bending, showing superior advantages in flexible electronics applications. Electrospinning is regarded as a powerful process for this 1D nanostructure; however, it can only be able to produce chaotic fibers that are incompatible with the well‐patterned microstructures in flexible electronics. Electro‐hydrodynamic (EHD) direct‐writing technology enables large‐scale deposition of highly aligned nanofibers in an additive, noncontact, real‐time adjustment, and individual control manner on rigid or flexible, planar or curved substrates, making it rather attractive in the fabrication of flexible electronics. In this Review, the ground‐breaking research progress in the field of EHD direct‐writing technology is summarized, including a brief chronology of EHD direct‐writing techniques, basic principles and alignment strategies, and applications in flexible electronics. Finally, future prospects are suggested to advance flexible electronics based on orderly arranged EHD direct‐written fibers. This technology overcomes the limitations of the resolution of fabrication and viscosity of ink of conventional inkjet printing, and represents major advances in manufacturing of flexible electronics.     

Fatigue‐Free,Electrically Reliable Copper Electrode with Nanohole Array

Design and fabrication of reliable electrodes is one of the most important challenges in flexible devices, which undergo repeated deformation. In conventional approaches, mechanical and electrical properties of continuous metal films degrade gradually because of the fatigue damage. The designed incorporation of nanoholes into Cu electrodes can enhance the reliability. In this study, the electrode shows extremely low electrical resistance change during bending fatigue because the nanoholes suppress crack initiation by preventing protrusion formation and damage propagation by crack tip blunting. This concept provides a key guideline for developing fatigue‐free flexible electrodes.     

Influence of bending rotations on three and four-point bend end notched flexure tests

It has been experimentally observed that mode II critical energy release rate (G_IIC) values determined by four-point end notched flexure test and three-point end notched flexure test are different for the same material. At the present work correction factors related to bending rotations are introduced to explain the differences between values of G_IIC obtained by three point and four point end notched flexure tests. The bending angle leads to the contact zones between specimen and supports and specimen and load rollers changing in both test configurations. The present analysis has been carried out by the classical beam theory, neglecting shear effects and assuming the hypothesis of small rotated angles. Results show that the relative differences between corrected and uncorrected values of G_IIC are greater in the case of four-point end notched flexure than in the case of three-point end notched flexure test.     

3D Graphene‐Infused Polyimide with Enhanced Electrothermal Performance for Long‐Term Flexible Space Applications

Polyimides (PIs) have been praised for their high thermal stability, high modulus of elasticity and tensile strength, ease of fabrication, and moldability. They are currently the standard choice for both substrates for flexible electronics and space shielding, as they render high temperature and UV stability and toughness. However, their poor thermal conductivity and completely electrically insulating characteristics have caused other limitations, such as thermal management challenges for flexible high‐power electronics and spacecraft electrostatic charging. In order to target these issues, a hybrid of PI with 3D‐graphene (3D‐C), 3D‐C/PI, is developed here. This composite renders extraordinary enhancements of thermal conductivity (one order of magnitude) and electrical conductivity (10 orders of magnitude). It withstands and keeps a stable performance throughout various bending and thermal cycles, as well as the oxidative and aggressive environment of ground‐based, simulated space environments. This makes this new hybrid film a suitable material for flexible space applications.     

Thermo-electrical characterization of Sn–Zn alloys

The thermo-electrical properties of Sn–Zn alloys have been investigated for four different compositions. The SEM micrographs and EDX analysis of the samples have been obtained. The electrical resistivity measurements were performed by using four-point probe technique in the temperature range 300 K–575 K. The resistivity of samples increases linearly with temperature and the electrical conductivity is inversely proportional to temperature. The electrical current preferentially flows through the pure Sn phase having lower resistivity in the matrix. Also, thermal conductivity of samples has been measured by using the radial heat flow method. It has been found that the thermal conductivity decreases with the increasing temperature and composition of Sn. The results were consistent with available experimental results of other studies. Finally, the temperature coefficient of electrical resistivity and the temperature coefficient of thermal conductivity have been determined, which were independent from the composition of Sn and Zn.     

玻璃纤维增强树脂基复合材料夹芯板-钢板连接接头的弯曲性能

以PVC泡沫或Balsa轻木为芯材的玻璃纤维增强树脂基复合材料（GRP）夹芯板目前广泛应用于船舶与海洋工程结构中。论文设计不同参数的GRP夹芯板-钢板混合接头模型,进行四点弯曲加载下的静力及疲劳试验研究,同时运用ABAQUS软件结合MSC.fatigue软件对接头的静态及疲劳弯曲失效进行数值模拟,分析了接头的弯曲强度、刚度和失效模式,并研究了接头填充区材料及长度、钢板嵌入填充区长度等参数对接头弯曲性能的影响。结果表明：弯曲载荷作用下接头破坏发生在连接结合部,失效模式则因填充区的不同设计而不同;对提高接头的弯曲性能较为明显的设计参数包括将钢板延伸到接头填充区或者选择Balsa轻木替代PVC泡沫芯材;对于受到疲劳弯曲载荷的接头模型,在较大疲劳载荷水平下,所有试件在未达到10⁶次循环时均发生了疲劳破坏;而在相对较小的疲劳载荷水平下,经过10⁶次循环后所有试件全部完好,并且接头的剩余强度与疲劳试验前的静强度相近,表明小载荷水平下接头的疲劳次数对其承载能力无影响。     

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.     

Four-point bending fatigue behaviour of an iron-based laser sintered material

New material processing methods such as laser sintering of metal powder necessitates new knowledge and characterization of the material to support its implementation in technical applications. Fatigue behaviour of a laser sintered FeNiCu-alloy was studied with emphasis on crack path, initiation and propagation. Fatigue crack growth was investigated by surface replication in four-point bending fatigue tests. The fatigue behaviour was controlled by the complex layered structure. Pores on or under the surface were preferable places for crack initiation. Crack linkage and deflection occurred due to crack tip interaction with microstructure and sinter layers where microcracks initiated at pores adjacent to the advancing crack tip. Crack growth rate and stress intensity factor were calculated from surface replicas and showed an oscillating behaviour.     

填充不同材料条件下管材三维自由弯曲变形行为研究

目的 研究三维自由弯曲过程中,不同填充材料对管材成形性能的影响,比较不同填充材料的影响效果。方法 建立相关力学模型,分析比较固体填充物以及柔性填充物对成形质量不同的影响机理,并基于ABAQUS有限元仿真软件,对成形过程进行数值模拟分析。结果 与填充PU棒时成形结果相比,当填充SS304钢珠时,管材具有更好的成形性以及更小的厚度变化率,同时,当内部填充材料时,管材椭圆率均会降低,且当填充材料为SS304钢珠时,椭圆度最低可达2.6%。结论 将三维自由弯曲技术与填充成形技术相结合,可以有效提高管材成形性能,并且固体填充物要好于柔性填充物,有限元模拟及实验验证了模型的正确性。     

厚钢板激光弯曲成形机理的模拟研究

对厚钢板的激光弯曲成形过程进行数值模拟.建立了钢板激光弯曲成形的三维非线性瞬态热力耦合有限元模型,模型中考虑了材料热物性参数和力学性能参数与温度的相关性.计算了钢板激光弯曲成形过程中的温度和应力,并预测了钢板的弯曲角度.模拟结果表明,厚度方向的温度梯度是导致钢板弯曲变形的主要原因.对数值模拟的结果进行了相应的实验,模拟结果与实验结果符合较好.     

A Finite-Element Approach to Predict Permanent Deformation Behaviour of Hot Mix Asphalt Based on Fundamental Material Tests and Advanced Rheological Models

Abstract:  An ongoing research project, undertaken at the Christian Doppler Laboratory for 'performance-based optimisation of flexible road pavements', focuses on the evaluation of advanced rheological models (i.e. Power Law, Huet and Huet–Sayegh) to describe permanent deformation behaviour of hot mix asphalt (HMA) and their implementation in a finite-element (FE) code. To accomplish this, an appropriate algorithm was developed for the summation of the different contributions of the stress history to determine the resulting creep strain tensor to simulate real-life stress/strain situations in flexible road pavements. Furthermore, the mathematical background for parameter identification from dynamic stiffness tests (four-point bending beam and dynamic tension compression tests) has been developed and a straightforward programme for data fitting is presented. For the validation of the implemented constitutive equations derived for the selected rheological models, a FE simulation of triaxial tests on cylindrical HMA specimens was carried out.     

Investigation of Hot-Staking Process Parameters and Evolution of Improvement in Joint Strength

The capability of hot-staking in providing mechanical as well as electrical joints along with its versatility and cost effectiveness makes it a process of preference in electrical and electronics appliances. However, the effectiveness of the process depends only on the mechanical contact and there is no welding or fusion between the joining surfaces. This necessitates proper selection and control of process parameters. This study investigates using Taguchi experiments the importance of process parameters for hot-staked joints in armatures of DC motors. Results show that the electrode material, geometry, and its position are significantly important. Based on the findings, a hypothesis is reasoned about process-product relationship to enable evolving in joint strength.     

Self-healing interconnects for flexible electronics applications

In this paper, we present the results of a study of self-healing flexible interconnect structures which utilize silver metallization on thin solid electrolyte films. Silver/electrolyte (Ag-Ge-Se) bilayers were fabricated on polyimide substrates and subsequently damaged by small radius bending. A small bias was used to stimulate Ag electrodeposition in the stress-induced cracks, thereby repairing the electrical discontinuity and returning the interconnect to a resistance close to its initial (unbroken) value. The healed structures were shown to be stable for small signal DC and AC conditions but were unable to withstand long periods of increased DC voltage stress due to interconnect erosion by ion migration.