各向异性导电胶膜新型填充粒子的研究

用分散聚合法合成了直径为3μm左右的聚苯乙烯核,在核表面化学镀形成带小突起的镍合金导电层,制成ACF新型填充粒子,对聚合物核做了粒径分布、比表面积测试,对核以及导电微球做了环境扫描电镜(ESEM)测试。结果表明:制备的导电微球密度小,呈单分散,镀层结构致密,镀层表面形成了近球形、锥形及其它不定形突起。这些突起可以刺穿电极的氧化膜。实践证明通过控制工艺参数实现控制镀层结构是一条可行方法。     

Study of the Formation of Bubbles in Rigid Substrate-Flexible Substrate Bonding Using Anisotropic Conductive Films and the Bubble Effects on Anisotropic Conductive Film Joint Reliability

The formation of process-related bubbles that become entrapped inside the anisotropic conductive film (ACF) layer during bonding processes remains an issue. The formation of these bubbles is strongly influenced by the process variables, such as bonding pressure and bonding temperature. Therefore, bonding process variables of bonding temperature, bonding pressure, and type of flexible substrate (FS) were changed in order to investigate the effects of the changes as they concern the formation of bubbles. According to the results, the tendency toward bubble formation was closely related to these three factors. The bubble area increased as the bonding temperature increased. Moreover, the shape and tendency of bubbles coincided with temperature distribution in␣the ACF layer. Two different types of FS, each with different surface roughnesses and energies, were used. The bubbles formed only on the FS with the larger roughness and lower surface energy. According to the results from a surface energy measurement of FS types using goniometry, a FS with a higher surface energy is favorable for a bubble-free assembly, as the higher surface energy provides better wettability. In addition, in order to investigate the effect of bubbles on the reliability of ACF joints, the pressure cooker test (PCT) was performed, and all samples with bubbles electrically failed after 72 h of a PCT, as the process-related bubbles provided a moisture penetration path and entrapment site for moisture. However, all type 1 test vehicles (TVs) survived even after 120 h of a PCT. Therefore, Ar and O₂ plasma treatments were performed on the FS with the lower surface energy in order to improve the surface energies and wettability. Following this, the bubbles were successfully removed at rigid substrate (RS)–FS bonding joints using ACFs.     

使用各向异性导电胶实现封装叠加的方法

文章阐述了一种新型的基底结构,通过使用各向异性导电胶来连接两层基底,从而实现封装叠加。这种基底结构来源于一种传统的折叠基底结构,不同的是,它采用各向异性导电胶代替折叠基底结构中的弯曲区域来实现两层基底的电路和物理连接。折叠基底结构中的弯曲区域是一个弱点,其本身存在很高的断裂几率,需要大量冗余线路支撑来提高物理强度。新的基底结构不但降低了断裂几率,消除了冗余线路,而且拓广了适用性并降低成本。     

各向异性导电胶粘接可靠性研究进展

介绍各向异性导电胶导电机理和粘接工艺,以及影响它的粘接可靠性因素和最佳参数的研究,如粘接温度、固化时间、粘接压力、粒子含量等。对各向异性导电胶粘接可靠性中的开路、短路、接触电阻与粘接压力和温度循环的关系进行了讨论,并介绍了各向异性导电胶可靠性的理论计算模型。     

Mechanical Properties of Anisotropic Conductive Adhesive Film Under Hygrothermal Aging and Thermal Cycling

Mechanical properties of anisotropic conductive adhesive film (ACF) were investigated experimentally under various environmental conditions. The temperature sweep test was conducted to investigate the effects of temperature on dynamical mechanical properties of the ACF. The ACF exhibited transitions to the glass state, viscoelastic state, and rubber state with increasing temperature, and its glass-transition temperature (T _g) was determined to be 149°C. The creep-recovery behaviors of the ACF were investigated, and it was found that the initial strains, instantaneous strains, and creep or recovery rates increased with increasing temperature. No obvious creep phenomenon was observed at low temperatures (≤0°C). The creep strain and creep rates at any time decreased with increasing hygrothermal aging time. The uniaxial tensile behaviors of the ACF were also investigated under hygrothermal aging and thermal cycling. The results show that the Young’s modulus and tensile strength of the ACF decrease with increasing hygrothermal aging time; however, they increase at first and then decrease with increasing thermal cycling time. T _g decreases slightly for the ACF after hygrothermal aging; however, it increases after thermal cycling.     

环境对各向异性导电胶膜性能参数的影响

各向异性导电胶膜(ACF)的玻璃转化温度T_g是它的一个重要性能参数,用差示扫描热示计(DSC)分别测定商用各向异性导电胶膜固化前和固化后的玻璃转化温度,并确定不同的固化时间对它的玻璃转化温度的影响,以及高温高湿(85℃,85%RH)环境对它的玻璃转化温度影响,得到各向异性导电胶玻璃转化温度下降是其粘接强度下降的原因之一。     

各向异性导电胶粘剂膜的研究进展

介绍了ACF的结构、原理、组成、应用及发展前景,详述了导电粒子、粘结剂、添加剂等组分在体系中的作用和对ACF性能的影响,综述了热固性树脂中离子聚合树脂型ACF和自由基聚合树脂型ACF的现状,并指出自由基聚合树脂型ACF是将来的发展趋势。     

Effects of Heating Rate on Material Properties of Anisotropic Conductive Film (ACF) and Thermal Cycling Reliability of ACF Flip Chip Assembly

In this paper, the effects of heating rate during anisotropic conductive film (ACF) curing processes on ACF material properties such as thermomechanical and rheological properties were investigated. It was found that as the heating rate increased, the coefficient of thermal expansion (CTE) of the ACF increased, and the storage modulus and glass transition temperature $(T _{g})$ of the ACF decreased. Variation of the ACF material properties are attributed to cross-linking density, which is thought to be related with the ACF density. In addition, as the heating rate increased, the minimum viscosity of the ACF decreased and the curing onset temperature increased during the curing process. The similar phenomenon was also found in in-situ contact resistance measurement. As the heating rate increased, contact resistance establishing temperature increased and the contact resistances of the ACF flip chip assemblies decreased. The decrease in contact resistance was due to larger conductive particle deformation which leads to larger electrical contact area. The effect of the heating rate of ACFs on thermal cycling (T/C) reliability of flip chip assemblies was also investigated. As the heating rate increased, the contact resistances of the ACF flip chip assembly rapidly increased during the T/C test. The T/C reliability test result was analyzed by two terms of shear strain and conductive particle deformation. Reduced gap of joints due to reduced ACF viscosity resulted in larger shear strain. Moreover, many cracks were observed at metal-coated layers of conductive particles due to larger deformation.      

Hygrothermal Effects on the Tensile Properties of Anisotropic Conductive Films

The effect of temperature and hygrothermal aging on the tensile properties of three types of anisotropic conductive films (ACFs) with different conductive particle percentages (4.69 vol.%, 6.14 vol.%, and 8.07 vol.%) are experimentally studied. Glass-transition temperatures of the three types of ACF are measured using a dynamic mechanical analyzer (DMA). The effect of the volume fraction of particles on the tensile properties is discussed. An increase in the volume fraction of the conductive particles enhances the Young’s modulus to some extent, but reduces the tensile strength. It is also investigated how hygrothermal aging impacts the tensile properties of ACFs at temperatures of 40°C, 80°C, and 120°C after aging times of 200 h and 1000 h. It is found that the mechanical properties of the ACFs basically decline with the hygrothermal aging time.     

Evaluation of Anisotropic Conductive Film for a Flex to Silicon Interconnection

A commercial anisotropic conductive film (ACF) was used to form electrical interconnections between the aluminum metallization on a silicon wafer and tin plated copper pads on a polyimide flexible circuit. Some samples were subjected to temperature/humidity aging at 85degC and 85% RH for 300h, and others were subjected to thermal cycling from 0degC to 125degC for 1500 cycles. The electrical resistance of the bonds was monitored periodically during testing by taking the samples out of the chamber and measuring the resistance as a function of the electrical current. A model incorporating the a-spots on the individual conducting particles in the ACF was used to extract the constriction resistance and film resistance associated with the bond. In the model, the constriction resistance is computed from the measured resistance change versus electrical current, and the film resistance is derived from the constriction resistance and a fit to the data evaluated at zero current. This analysis predicted an upper limit on the a-spot radius for a typical sample of 6.1nm. Typically, after an initial rapid resistance increase with aging, which was attributed to a loss of electrical contact area, the resistance became stable for the remainder of the stress testing     

各向异性导电胶倒装封装电子标签的可靠性

各向异性导电胶(ACA)广泛用于RFID电子标签芯片封装,具有芯片对位方便、热压温度低和工艺时间短的优点.但ACA互连本质上是机械接触,其互连可靠性强烈依赖于粘接界面性质、胶水粘接力及环境稳定性.本文试验表明,168 h高温高湿和D20 mm心轴弯曲对芯片粘接点的电接触性能有所影响;铜模组良品率显著高于铝天线Inlay.     

Investigation of Mechanical and Electrical Characteristics for Cracked Conductive Particle in Anisotropic Conductive Adhesive (ACA) Assembly

In an anisotropic conductive adhesive (ACA) assembly, the electrical conduction is usually achieved with the conductive particles between the bumps of integrated circuit (IC) and corresponding conductive tracks on the glass substrate. Fully understanding of the mechanical and electrical characteristics of ACA particles can help to optimize the assembly process and improve the reliability of ACA interconnection. Most conductive particles used in the ACA assembly are with cracks in the metal coating of the particles after the ACA bonding. This paper introduced the fracture analysis by applying the cohesive elements in the numerical model of the nickel-coated polymer particle and further simulating the cracks initiation and propagation in the nickel coating during the ACA bonding. The simulation results showed that the stress distribution on the nickel-coated particle with cracks was significantly different from that on the nickel-coated particle without crack, indicating that the stress analysis by taking the crack into consideration is very important for the reliability assessment of the ACA interconnection. The stress analysis of cohesive elements indicated that the cracks initiated at the central area of the nickel coating and propagated to the polar area. Furthermore, by the introduction of a new parameter of the virtual resistance, a mathematical model was established to describe the electrical characteristics of the nickel-coated particle with cracks. The particle resistance of the nickel-coated particle with cracks was found to be much higher than that of the particle without crack in the optimized bonding pressure range, indicating that it is necessary to take the crack into consideration for the particle conduction analysis as well. Therefore, the fracture analysis on the conductive particle by taking the crack into consideration could accurately evaluate the reliability of ACA interconnection and avoid serious reliability issues.     

Analysis of Failure Mechanism in Anisotropic Conductive and Non-Conductive Film Interconnections

Failure behaviors of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects were investigated by measuring the connection resistance. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit. The interconnection reliability was evaluated by multiple reflow process. The connection resistance of the ACF joints was markedly higher than that of NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mOmega at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the reflow process, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

倒装芯片各向异性导电胶互连的剪切结合强度

采用冲击试验方法研究了各向异性导电胶膜(ACF)互连的玻璃和柔性基板上倒装芯片(COG和COF)的剪切结合强度.结果表明:COF比COG的剪切强度高.ACF的固化程度达85 %时有最大的结合强度.键合温度、导电颗粒状态、缺陷等因素对ACF互连的结合强度有较大影响,而键合压力的影响不大.     

倒装芯片各向异性导电胶互连的剪切结合强度

采用冲击试验方法研究了各向异性导电胶膜(ACF)互连的玻璃和柔性基板上倒装芯片(COG和COF)的剪切结合强度.结果表明:COF比COG的剪切强度高.ACF的固化程度达85%时有最大的结合强度.键合温度、导电颗粒状态、缺陷等因素对ACF互连的结合强度有较大影响,而键合压力的影响不大.     

Effects of Anisotropic Conductive Film Viscosity on ACF Fillet Formation and Chip-On-Board Packages

In this paper, the effects of anisotropic conductive film (ACF) viscosity on ACF fillet formation and, ultimately, on the pressure cooker test (PCT) reliability of ACF flip chip assemblies were investigated. The ACF viscosity was controlled by varying the molecular weight of the epoxy materials. It was found that the ACF viscosity increased as the increase of molecular weight of the epoxy materials. However, there was little variation of the thermomechanical properties among the evaluated ACFs with different viscosites. Also, the results showed that the ACFs have no differences in moisture absorption rate, die adhesion strength, and degree-of-cure. In scanning electron microscopy images, the lower ACF viscosity resulted in the smoother ACF fillet shape and the higher fillet height. From the results of PCT, the ACF flip chip assembly with the smoother fillet shape showed better reliability in terms of contact resistance changes. After 130 h of PCT, the flip chip assembly with lower ACF viscosity also showed a lesser degree of delamination at the ACF/chip interface.     

Electrical Contact Resistance Theory for Anisotropic Conductive Films Considering Electron Tunneling and Particle Flattening

This study models the electrical contact resistance (ECR) between two surfaces separated by an anisotropic conductive film. The film is made up of an epoxy with conductive spherical particles(metallic) dispersed within. In practical situations the particles are often heavily loaded and will undergo severe plastic deformation and may essentially be flattened out. In between the particles and the surfaces there may also be an ultra-thin insulating film (consisting of epoxy) which causes considerable electrical resistance between the surfaces. In the past this effect has been neglected and the predicted ECR was much lower than that measured experimentally. This added resistance is considered using electron tunneling theory. The severe plastic deformation of the spherical particles is modeled using a new expanded elasto-plastic spherical contact model. This work also investigates the effect of compression of the separating epoxy film on the electrical contact resistance. The model finds that the high experimental ECR measurements can be accounted for by including the existence of a thin insulating film through the electron tunneling model     

Anisotropic Conductive Film Bonding by Making Use of a High-Power Diode Laser

Anisotropic conductive film (ACF) bonding between liquid crystal displays (LCDs) and driver integrated circuits (ICs) is one of the key technologies for developing high-resolution LCDs. The bonding pitch between LCD and tape carrier package (TCP), which influences the total reliability of LCD modules, depends on the characteristics and bonding conditions of ACF used. So, the bonding process between TCP and a glass panel with ACF using a high-power diode laser as a heat source for curing is preliminarily tested in this experiment. Also, laser transient thermal simulation was performed to analyze the thermal response of the assembly process for a package using ACF. The temperature on the ACF layer goes up to 180 degC (ACF curing temperature) within 1 s after exposure to laser light. This paper reports an effective bonding method using a diode laser, which accomplishes a fine-pitch ACF bonding and determines the optimum ACF bonding condition effectively     

ACF在LCD中的应用与发展

在瞬息万变的信息时代,LCD已经成为人机信息交换不可缺少的产品。影响其结构（降低厚度）及技术（提高精密度）的关键材料是ACF与FPC。通过对ACF和ACF在FPC应用的研究,着重论述ACF与FPC在液晶显示器LCD的应用与发展。     

A Study of Process-Induced Deformations of Anisotropic Conductive Film (ACF) Assembly

The warpage of glass substrate in chip-on-glass (COG) assemblies were believed to be the main cause of the so-called “Mura” phenomenon in thin-film transistor liquid crystal display (TFT–LCD) panels. The main objective of the study is to characterize the process-induced deformations of an anisotropic conductive film (ACF) type of COG assembly during the ACF bonding process. For effectively modeling the process-dependent thermal–mechanical behaviors of the ACF assembly, a process-dependent simulation methodology that integrates both thermal and thermal–mechanical finite element (FE) analyses and a “death–birth” meshing scheme is proposed. In the investigation, two various types of mirobump bonding technologies, i.e., the Au alloy and composite bumps, are explored. To substantiate the validity of the proposed methodology, the modeled results are extensively compared against experimental data. Moreover, the alternative goal of the study is to provide a design guideline through the exploration of the individual or combined effect of some essential parameters on the process-induced deformations. Both the modeled and experimental results reveal that in addition to the coefficient of thermal expansion (CTE) mismatch between the die and substrate, the process-induced temperature gradient appears to be the key factor causing the deformation of the assembly, and the deformed shape of the substrate is totally opposed to that obtained from isothermal modeling. Evidence also suggests that a preheated substrate could effectively ease the maximum process-induced warpage of the substrate, while the ACF fillet would unfortunately enlarge it. Besides, it is not surprising to find that an increase of the die length would enhance the maximum process-induced warpage of the substrate, but what surprises many is that an increase of the die width would diminish it.