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.     

Effect of Conductive Particle Properties on the Reliability of Anisotropic Conductive Film for Chip-on-Glass Applications

This paper describes how the material properties of conductive particles in anisotropic conductive films (ACFs) affect the electrical conductivity and the reliability of ACF interconnections for chip-on-glass (COG) applications. For the conductive particles, Au/Ni-coated polymer particles with a 5-diameter were used. Two different types of conductive particles were characterized with respect to their mechanical and electrical properties, such as ball hardness, recovery behavior, and electrical resistance. In addition, two ACFs were fabricated in the form of a double-layered structure, in which the thickness of the ACF and a nonconductive film (NCF) layer were optimized to have as many conductive particles as possible on the bump after COG bonding. The electrical contact resistance of an ACF interconnection in a COG structure depends mainly on the electrical properties of conductive particles in the ACF. The electrical reliability of an ACF interconnection in a COG structure also depends more on the electrical properties than the mechanical properties of conductive particles under a high-temperature and humid condition. Conductive particles with a lower electrical resistance, higher mechanical hardness, and lower recovery rate show better reliability than conductive particles with a higher electrical resistance, lower mechanical hardness, and higher recovery rate. Cross-sectional scanning electron microscopic (SEM) pictures of a COG interconnection show the deformation of two different conductive particles after the reliability tests. The ACF interconnections in the edge or corner of a driver IC show less reliable joints due to high absorption of moisture.     

Electrical Interconnection with a Smart ACA Composed of Fluxing Polymer and Solder Powder

The interconnection mechanisms of a smart anisotropic conductive adhesive (ACA) during processing have been characterized. For an understanding of chemorheological mechanisms between the fluxing polymer and solder powder, a thermal analysis as well as solder wetting and coalescence experiments were conducted. The compatibility between the viscosity of the fluxing polymer and melting temperature of solder was characterized to optimize the processing cycle. A fluxing agent was also used to remove the oxide layer performed on the surface of the solder. Based on these chemorheological phenomena of the fluxing polymer and solder, an optimum polymer system and its processing cycle were designed for high performance and reliability in an electrical interconnection system. In the present research, a bonding mechanism of the smart ACA with a polymer spacer ball to control the gap between both substrates is newly proposed and investigated. The solder powder was used as a conductive material instead of polymer‐based spherical conductive particles in a conventional anisotropic conductive film.     

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。     

各向异性导电胶互连技术的研究进展

随着微电子封装技术的发展,各向异性导电胶作为一种绿色的连接材料,广泛应用于电子产品中。文中主要介绍各向异性导电胶互连器件的粘接原理和影响其可靠性的各种因素,如粘接工艺参数、外界环境的干扰、各向异性导电胶的物理特性等。     

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.     

Enhanced Electrical Properties of Anisotropic Conductive Adhesive With $pi$ -Conjugated Self-Assembled Molecular Wire Junctions

We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer-sized (~500 nm in diameter) silver (Ag) particle as conductive filler with the effect of pi-conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability (~3400 mA) than those with micro-sized Au-coated polymer particles (~2000 mA) and Ag nanoparticles (~2500 mA). More importantly, by construction of pi-conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.     

Electrical and mechanical characterization of an anisotropic conductive adhesive with a low melting point solder

For the highly reliable interconnection in a micro-packaging technology requiring an excellent electrical and mechanical performance, the new anisotropic conductive adhesive (ACA) system with a low melting point solder was designed and characterized. An optimum flip-chip bonding cycle considering the chemo-rheological properties of a polymer matrix and solder was proposed. The bonding mechanism of the new ACA system was experimentally observed by the optical microscope. The electrical properties such as electrical resistance of about 5.6 mΩ and current density of 10,000 A/cm² were measured by the 4-point probe test. The measured shear strength was 304 MPa after bonding process.Electrical and mechanical performances were measured and compared before and after a pressure cooker test (PCT). In order to get a more stable ACA system during processing, the polymer matrix mixed with a reductant and a low melting point solder powder will be continuously developed in the near future.     

Experimental and theoretical characterization of electrical contactin anisotropically conductive adhesive

Electrical conduction through anisotropically conductive adhesive (ACA) is caused by deformation of metal fillers under pressure and heat. In this work, the hardness of the electrical particles under various deformation degrees was determined by nano-indentor measurements and the electrical resistance of the electrical contacts was measured under various deformation degrees. Theoretical model and simulation have been developed for the microscopic mechanism of the electrical conduction through metal fillers in the anisotropically conductive adhesive. By comparing with experimental data it is concluded that the deformation of the metal filler in our ACA is plastic even at rather low external load. Further theoretical simulation reveals two important aspects of the conductance characteristics. The conductance is improved by increasing the external load but the dependence of the conductance on the spatial position of the metal filler becomes stronger. Design and optimization of the ACA with respect to the absolute value of the electric conductance and its dependence on the spatial position of the metal filler are of essential importance for the electronics packaging application of the anisotropically conductive adhesives     

倒装芯片封装材料-各向异性导电胶的研究进展

介绍了两种新型各向异性导电胶ACA(Anisotropic Conductive Adhesive)结构,分析了邦定压力和导电颗粒特性对常用ACA互连接触电阻的影响,综合叙述了环境因素、邦定参数、误对准、凸点高度等对ACA互连可靠性影响的研究进展.     

Research on the contact resistance,reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications

Although there have been many years of development, the degradation of the electrical performance of anisotropically conductive adhesive or film (ACA or ACF) interconnection for flip-chip assembly is still a critical drawback despite wide application. In-depth study about the reliability and degradation mechanism of ACF interconnection is necessary. In this paper, the initial contact resistance, electrical performance after reliability tests, and degradation mechanisms of ACF interconnection for flip-chip-on-flex (FCOF) assembly were studied using very-low-height Ni and Au-coated Ni-bumped chips. The combination of ACF and very-low-height bumped chips was considered because it has potential for very low cost and ultrafine pitch interconnection. Contact resistance changes were monitored during reliability tests, such as high humidity and temperature and thermal cycling. The high, initial contact resistance resulted from a thin oxide layer on the surface of the bumps. The reliability results showed that the degradation of electrical performance was mainly related to the oxide formation on the surface of deformed particles with non-noble metal coating, the severe metal oxidation on the conductive surface of bumps, and coefficient of thermal expansion (CTE) mismatch between the ACF adhesive and the contact conductive-surface metallization. Some methods for reducing initial contact resistance and improving ACF interconnection reliability were suggested. The suggestions include the removal of the oxide layer and an increase of the Au-coating film to improve conductive-surface quality, appropriate choice of conductive particle, and further development of better polymeric adhesives with low CTE and high electrical performance.     

ACF键合中导电粒子捕捉及变形的影响机理与实验

ACF键合工艺下导电性能的提升需要对导电粒子捕捉与变形的影响机理进行深入了解。通过实验研究得到了不同键合工艺下导电粒子捕捉与变形规律,并对规律进行了理论分析。实验结果表明:随着键合压力增加,导电粒子捕捉数呈现出减少的趋势,导电粒子的变形呈现出增大的趋势。随着键合温度增加,导电粒子变形量呈现出先增大后减小的趋势,当键合温度达到230℃时,导电粒子捕捉数骤升。     

The contact resistance and reliability of anisotropicallyconductive film (ACF)

The effect of bonding pressure on the electrical and mechanical properties of anisotropic conductive film (ACF) joint using nickel particles and metal-coated polymer ball-filled ACFs was investigated. The contact resistance decreases as the bonding pressure increases. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrates is the main conduction mechanism in ACF interconnection. In addition, environmental effects on contact resistance and adhesion strength such as thermal aging, high temperature/humidity aging and temperature cycling were also investigated. Interestingly, the contact resistances of the excessively bonded samples deteriorated more than those of optimally bonded ones. Increasing contact resistance and decreasing adhesion strength after harsh environmental tests were mainly due to the loss of contact by thermal stress effect and moisture absorption, and also partially due to the formation of metal oxide on the conductive particles     

Driver IC and COG Package Design

This paper investigates the interconnection between the driver integrated circuit (IC) and glass substrate via anisotropic conductive adhesive (ACF) of chip on glass package. The conductive particle deformation is evaluated using a novel method, optical microscope (OM) inspection. The proposed method is more convenient than the traditional approach using scanning electron microscopy applied in the manufacturing process. Interconnection performance is easily judged using OM, allowing poor interconnection between the driver IC and glass substrate to be screened out. Several types of driver ICs with different bump area ratios (total input bump area/total output bump area, input/output ratio) and length/width (L/W) ratios are designed in this experiment. The conductive particle deformations are investigated in this study. Driver ICs with L/W ratios larger than 15 have better conductive particle deformation uniformity at each position. The average deformation degree at the driver IC center position is larger than that at the side and edge positions. The deformation degree at the input position with a smaller bump area is better than that at the output position. The conductive resistance increases with the reliability testing time because of the thermal stress effect and softening of the ACF polymer material. The deformation degree is related to the conductive resistance of the interconnection. The conductive resistance is lower at the center and input positions with larger deformation degree.     

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.     

Statistical Characterization of Open Failures in the Fine-Pitch COG Interconnection

Chip-on-glass (COG) interconnection using anisotropic conductive film (ACF) is susceptible to open failures. Open failures can be induced by the absence of conductive particles or an insufficient contact. Experimental results as well as statistical approaches were used to understand the conditions for open failures in COG bonding. The binomial distribution was used to predict the probability of the open failure due to the deficiency of conductive particles. The probability of an open failure decreased with increasing bump area and decreasing particle size. The bump height variation was also an important factor that affected the probability of the open failure together with the bump-to-electrode gap and the particle size. The variation in bump height should be minimized to avoid open failures in fine-pitch applications where a smaller particle size is required.     

Effect of elastic recovery on the electrical contact resistance in anisotropic conductive adhesive assemblies

The successful design of anisotropic conductive adhesive (ACA) assemblies depends mainly on the accurate prediction of their electrical contact resistance. Among the parameters that influence this resistance, the bonding force used to compress the conductive particles against the conductive tracks during the assembly process is very important. This paper investigates how the contact resistance changes as the bonding force is removed at the end of the assembly process when the epoxy resin used to bond the surfaces has cured. The final contact resistance is determined by examining, through theoretical, experimental and numerical analyzes, the evolution of the residual stress as the elastic recovery of the compressed conductive particles and tracks takes place when the bonding force is removed. An iterative algorithm derived from methods found in fracture mechanics analysis is used to determine the relationship between the contact resistance, the adhesive strength and the stiffness of the cured resin. It is shown that smaller values of adhesive strength yield higher contact resistance values; and similarly, smaller values of modulus of elasticity of the resin lead to higher contact resistance values.     

Highly Conductive,Flexible, Polyurethane‐Based Adhesives for Flexible and Printed Electronics

Flexible interconnects are one of the key elements in realizing next‐generation flexible electronics. While wire bonding interconnection materials are being deployed and discussed widely, adhesives to support flip‐chip and surface‐mount interconnections are less commonly used and reported. A polyurethane (PU)‐based electrically conductive adhesive (ECA) is developed to meet all the requirements of flexible interconnects, including an ultralow bulk resistivity of ≈1.0 × 10^?5 Ω cm that is maintained during bending, rolling, and compressing, good adhesion to various flexible substrates, and facile processing. The PU‐ECA enables various interconnection techniques in flexible and printed electronics: it can serve as a die‐attach material for flip‐chip, as vertical interconnect access (VIA)‐filling and polymer bump materials for 3D integration, and as a conductive paste for wearable radio‐frequency devices.     

Resistance Analysis of Spherical Metal Thin Films Combining Van Der Pauw and Electromechanical Nanoindentation Methods

Although micron-sized metal-coated polymer particles are an important conductive filler material in anisotropic conductive adhesives, the resistance of the particles in an adhesive is not well understood. In this study, a van der Pauw method for spherical thin films is developed and applied to determine the resistivity of 30 μm silver-coated poly(methyl methacrylate) (PMMA) particles. The resistivity is used to interpret resistance contributions in single particle electromechanical nanoindentation measurements, which simulate the compression particles undergo in application. The resistivity was found to be coating thickness dependent for thin films in the range 60–270 nm. Estimation of the resistance of the metal shell using the measured resistivity did not account for the total resistance measured in electromechanical nanoindentation. We therefore deduce a significant contribution of contact resistance at the interfaces of the particle. The contact resistance is both coating thickness and particle deformation dependent.