Anisotropic Conductive Adhesive Research Articles

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.

Reliability of ACF Interconnections on FR-4 Substrates

The use of anisotropic conductive adhesives (ACA) in flip chip interconnection technology has become very popular because of their numerous advantages. The ACA process can be used in high-density applications and with various substrates as the bonding temperature is lower than that in the soldering process. In this paper, six test lots were assembled using two anisotropic conductive adhesive films (ACF) and four different FR-4 substrates. FR-4 was chosen as it is an interesting alternative for making low-cost high-density interconnections. Some of the chips were thinned to study the effect on reliability. To study the effect of bonding pressure, four different pressures were used in every test lot. The reliability of the assembled test samples was studied in a temperature cycling test carried out between temperatures of -40°C and 125°C for 10 000 cycles. A finite element model (FEM) was used to study the shear stresses in the interconnections during the test. Marked differences between the substrates were seen. The substrate thinning and also the chip thinning increased the reliability of the test samples. From the FEM, it was seen that both decreased the shear stress in the adhesive, which is assumed to be the reason for the increased reliability. A significant difference was seen in the reliability between the ACFs. This was probably caused by differences in the conductive particle materials and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> values and of the ACFs. In addition, the bump material used with the ACFs varied, which most likely affected the reliability of the test samples.

An experimental methodology for the study of co‐planarity variation effects in anisotropic conductive adhesive assemblies

Purpose – Non‐planarity of assemblies and co‐planarity variation effects on anisotropic conductive adhesive (ACA) assemblies have been a concern for ACA users since the materials are first devised. The primary objective of this paper is to introduce a new experimental method for studying co‐planarity variation effects on ACA assemblies.

Characterization for coating processes of imidazole powders using an ultrasonic atomizer

Imidazole-curing accelerator powders were coated with stearic acid to increase the pot life of anisotropic conductive adhesive (ACA) formulations. To accomplish an efficient coating process, the coating was tested using an ultrasonic atomizer after mixing imidazole powders with a molten coating agent. Design of experiments analysis was organized to elucidate the effect of process parameters and to deternine the most crucial parameter. The final formulation incorporating well-processed imidazole loaded powders indicated longer pot life, higher shear strength, and excellent highly accelerated stress test (HAST) reliability. Results show that the coating process using an ultrasonic atomizer is effective in increasing the pot life of ACA formulations.

Metal Coated Polymer Particles for Electronic Packaging

Anisotropic conductive adhesives (ACA) are widely used as interconnect materials in the manufacturing of LCD screens. To be integrated in a broader range of interconnect applications several technical and economic issues still need to be addressed. One significant challenge is to encapsulate polymer particles with well controlled modal diameter and size distribution into continuous, compact, and uniform conductive metallic shells. This presentation describes a method to deposit layers of nickel with different thickness (30 – 120 nm) onto monosized polymer particles ranging in size from 1 to 10 micrometers. The novelty of the approach consists in a pre-treatment of the epoxy functionalized polymer particles with linear polymeric amines. We show that by increasing amine chain length the distribution of the palladium catalyst, and consequently the adhesion and distribution of nickel metal deposited, can be improved dramatically. The effect of several plating parameters is discussed and illustrated.

Advanced 3D Packaging of Miniature Biomedical Sensors

In this paper, a novel 3D packaging technique for miniature implantable wireless biomedical sensors for intraocular pressure (IOP) sensing in mice is introduced. Due to the limited size of a mouse eye, the packaging of the sensor and control integrated circuit (IC) is very challenging. The overall size of the packaged sensor must be less than 12 mils cubed. In order to achieve the desired thickness, a magnetically aligned Z-axis anisotropic conductive adhesive (ACA) is used to create the vertical interconnects and micro-vias in the packaging material to distribute signals vertically to limit the eventual area of the device. The first step is to demonstrate the layer-to-layer interconnection between a silicon IC and a liquid crystal polymer (LCP) layer using the Z-axis ACA. The total thickness of the IC and the packaging layer is less than 6 mils. The measured resistance through vertical interconnection is 1.15 ohms on average for 3 mil × 3 mil pads. The second step is to demonstrate 3D transitions through 0.8 mil via holes in a LCP layer. A transition from an antenna layer through the LCP to a rectifier circuit on the above layer is demonstrated. RF power received by a loop antenna on the bottom LCP layer is rectified and generates 5 volts of DC voltage. This miniature 3D packaging technique enables extremely tight integration of all the sensor's components in a small form factor package, which can be implanted into mice eyes for wireless monitoring of the intraocular pressure.

Chemo-rheological characteristics of a self-assembling anisotropic conductive adhesive system containing a low-melting point solder

To achieve the excellent wettability of a solder in an anisotropic conductive adhesive (ACA) in micro-packaging technology, not only the removal of the oxidized layer of the solder but also the chemo-rheological phenomena of the thermally induced self assembly of low-melting temperature solder powders were investigated for ACA applications. Both the optimum kind and amount of reductant were determined in this study to remove the oxidized layer formed on the solder surface in ambient environment. The solder wetting test was performed to observe the chemical and rheological compatibility between the solder, polymer matrix, and reductant. The thermal analysis was done using differential scanning calorimetry (DSC) analysis and rheometer test. Several chemical reactions by the reductant were proposed, and the mechanisms were postulated. It is believed that the reductant reacts with the oxidized layer, amine, and epoxide group, respectively. The relation between conversion and glass transition temperature of the polymer matrix was also obtained from DSC.

Effects of novel carboxylic acid-based reductants on the wetting characteristics of anisotropic conductive adhesive with low melting point alloy filler

A low viscosity epoxy resin with carboxylic acid-based novel reductants was introduced to improve the process ability and reliability of an anisotropic conductive adhesive (ACA) resin with a low melting point alloy (LMPA) filler system. The curing degree of the ACA resin and the melting of the LMPA filler were investigated using differential scanning calorimetry (DSC) conducted in the dynamic mode in order to control the curing conditions such as the reaction temperature and a melting temperature of solder. The temperature-dependent viscosity characteristics of the ACA resin were investigated using a rheometer. The compatibility between the viscosity of a polymer matrix and a melting temperature of solder was characterized to optimize the processing cycle. Three different types of carboxyl acid-based reductants were added to remove the oxide layer on the surfaces of the filler particles and the conductive pad. Good wetting properties were achieved between the LMPA filler and the Cu pad in the epoxy resin using these carboxylic acid-based reductants.

Micropolar interface model of thin-layered interconnection components with heterogeneous internal microstructures

Being a wide variety of thin-layered interconnection components in electronics packaging with relatively small scale and heterogeneous materials, conventional numerical methods may be time consuming and even inefficacious to obtain an accurate prediction for the interface behavior under mechanical and/or thermal loading. Rather than resort to a fully spatial discretization in the vicinity of this interface zone, an interface model was proposed within the framework of micropolar theory by introducing discontinuous approximation. A fracture description was used to represent the microscopic failure progress inside the interface. The micropolar interface model was then numerically implemented with the finite element method. As an application, the interface behavior of a packaging system with anisotropic conductive adhesive (ACA) joint was analyzed, demonstrating its applicability and great efficiency.

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 π-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 π-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.

Effect of substrate material and thickness on reliability of ACA bonded flip chip joints

PurposeThe purpose of this paper is to investigate the effect of substrate material and thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA).Design/methodology/approachFour test lots are assembled using three different substrates. Two of the substrates are made of FR‐4. The thicknesses of these substrates are 600 and 100 μm. The third substrate is made of liquid crystal polymers (LCP) and is flexible. With the thicker FR‐4 substrate two test lots are assembled using both normal and two‐step bonding profiles to study how the bonding profile affects the deformation of the substrate. Four different bonding pressures are used to study the effect of pressure on reliability and the failure mechanism of the ACA joints. The reliability of the test samples is studied using a temperature cycling test.FindingsThe reliability of the test lot with the LCP substrate is considerably better than that of the test lots with the FR‐4 substrates. Additionally, the thinner FR‐4 substrate has better reliability than the thicker FR‐4 substrate. The failure mechanisms found varied among the test lots. The effect of the two‐step bonding process on the deformation of the substrate is found to be minor compared with the effect of the glass fibres.Originality/valueThe work shows that the thermal cycling reliability of ACA flip chip joints is markedly influenced by the thickness and material of the substrate. It is also seen that the substrate used influences the failure mechanisms formed during thermal cycling testing.

Homogenization of delamination growth in an ACA flip-chip joint based on micropolar theory

Abstract In the present paper the delamination mechanism of a typical internal structure of the anisotropic conductive adhesive (ACA) interconnect for electronic packaging is modeled on the basis of micropolar theory and computational homogenization. The interface is treated as a finite Representative Volume Element (RVE), across which the macroscopic deformation is expressed in terms of regularized strong displacement and rotational discontinuities. For the microstructure of the RVE, the micro-macro kinematical coupling is considered as a Taylor series expansion in the regularized macroscopic discontinuities, and, connected to that, a discontinuous fluctuation field representing the microstructural variation is included to describe delamination on the microlevel. As to the microlevel delamination modeling, on the basis of the discontinuous fluctuation field, a damage coupled to slip and dilation formulation is used to model the interface degradation. The constitutive relations are established in a thermodynamic setting, where the interfacial free energy involves internal variables of damage and plastic deformation. The parameters of the interface are calibrated so that a predefined amount of fracture energy is dissipated in mode I. In the numerical example, the response of a planar interface is considered when it is subjected to the basic modes I-II and also the non-conventional rotational discontinuity mode. Case studies on fracture and geometry parameters have also been carried out. Finally, an uncoupled thermomechanical analysis of a microsystem involving a representative ACA microstructure has been made for the understanding of the microscopic delamination during a thermal cycling procedure.

Movement of solder fillers because of the unevenness of interfacial tension in self-organization assembly process

Soldering or anisotropic conductive adhesives (ACAs) are used for assembling of devices with high-density area-array terminals. However, solder bump processes are high-cost and interconnects of ACAs are less-reliable. The purpose of this study is to develop a novel method. Self-organization assembly method that uses active resin containing solder fillers may allow reliable interconnects at low-cost. Fundamental process of Self-organization assembly are movement, coalescence, and wetting of molten fillers in resin. The focus of this paper is on the movement of fillers. In-situ observations of coalescence behavior for 40μmϕ molten fillers in the resin revealed irregular movement of the fillers at a velocity of several μm/s. Numerical analysis, using improved volume fraction method, indicated that a 10% degradation of interfacial energy on one side of a 40μmϕ filler could move the filler at a velocity of several mm/s. This degradation of the interfacial energy was resulted the remaining oxide film.

Nanomechanical characterization of single micron‐sized polymer particles

AbstractThe mechanical characterization of single micron‐sized polymer particles is very important for understanding the anisotropic conductive adhesives interconnection. In this article, a nanoindentation‐based flat punch method was employed to investigate the mechanical properties of single polymer particles. A diamond flat tip, instead of a commonly used sharp tip for indentation, was specially designed to deform single polymer particles. The maximum applied load is 10 mN and the linear loading/unloading rate is 2 mN/s. Two types of amorphous polymer particles were examined. The polymer particles display significantly different stress–strain behaviors. The material responses at different strain levels were analyzed and compared. A particle size effect, the smaller the diameter, the harder the particle, on the compression stress–strain behavior, was observed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Self-Organized Interconnection Process Using Solderable ACA (Anisotropic Conductive Adhesive)

In this paper, we discuss our design of a new class of low-melting-point alloy (LMPA)-filled anisotropically conductive adhesives (ACA) and a self-organized interconnection process for using these adhesives, and we demonstrate a good electrical conduction for the process. Flow, melting, coalescence, and wetting characteristics of LMPA fillers in the ACA facilitate this process. In order to exploit good coalescence and wetting characteristics of LMPA fillers, the polymer matrix has a sufficient fluxing capability against oxide films of both LMPA fillers and electrode materials. Furthermore, it is essential that the polymer have a low viscosity level around the melting point of the incorporated LMPA to achieve a good electrical conduction path. In order to study coalescence and wetting characteristics, we formulated two types of ACA with different volume fractions of LMPA filler (10 vol% and 40 vol%). Our test board had an 18-mm thick Cu line-type pattern (10 mm � 0:1 mm) and an area array-type pattern (square type: 0:1 mm � 0:1 mm, circle type: ’, 0.1 mm) with five different pitches (50 mm to 250 mm). We measured thermal properties of the ACA by differential scanning calorimetry (DSC), and we determined a temperature profile for the interconnection process. We then monitored coalescence and wetting characteristics of LMPA fillers and morphology of conduction path in ACA by using a microfocusing X-ray inspection system and an optical microscope. We determined that the developed LMPA-filled ACA have good coalescence and wetting characteristics regardless of pattern types. In addition, we were able to achieve a good electrical conduction path because of the coalescence and wetting characteristics of the LMPA fillers in the ACA. [doi:10.2320/matertrans.MF200918]

Solder Filler Motion Driven by Interfacial Tension in Self-Organization Assembly Process

Soldering and anisotropic conductive adhesives (ACAs) are widely used for assembling electronic devices with narrow-pitch terminals. Solder bump methods need a number of processes, while interconnects of ACAs are less reliable. The purpose of this study is to develop a novel assembling method. The Self-organization assembly method using active resin containing solder fillers may allow reliable interconnects by simple process. The focus of this paper is on the movement of fillers, one of the fundamental processes of the Self-organization assembly method. In-situ observations on the behavior of 40 µmφ molten fillers in the resin revealed irregular movement of the fillers at a velocity of several µm/s. Numerical analysis, using improved volume of fluid method, indicated that even 10% difference of interfacial energy between half sides of a 40 µmφ filler could move the filler at the velocity of several mm/s. The inhomogeneous distribution of the interfacial energy is thought to be caused by the remaining surfactant elements.

Influence of medical sterilization on ACA flip chip joints using conformal coating

Health care is a field in which electronics is increasingly used. If electronics is used in a device that is implanted, it needs to withstand a harsh environment. Moreover, before implantation the device needs to be sterilized, and this should not impair the function or reliability of the device. Since implantable applications are small in size, electronics packaging solutions need to be space saving. Conformal coating is needed to shield the electronics from conditions inside the human body, and on the other hand to protect the body from the materials used in electronics. This paper studies the effect of medical sterilization on the reliability of adhesive joints. Flip chip technology with anisotropically conductive adhesives (ACA) was used and epoxy and parylene C were used as the conformal coating materials. Differential scanning calorimetry (DSC) was used to ascertain the curing degree of the adhesive after bonding. Sterilization was performed using gamma radiation sterilization and the reliability tests were conducted in a constant humidity test, at 85°C and 85%RH. The results clearly indicated that parylene C coating is the most reliable choice and gamma radiation does not affect the reliability of parylene C coated devices. However, the gamma radiation increased the reliability of the non-coated test lot.

Failure modeling of ACA-glued flip-chip on flex assemblies

This paper addresses the reliability of flip-chip on flex (FCOF) assemblies glued with an Ag-particle filled anisotropic conductive adhesive (ACA). As the description of FCOF failure gives still much scope for speculation, a physics of failure based approach is developed here, taking into account the changing thermo-mechanical properties of the ACA under temperature and moisture. A failure hypothesis is formulated based on the loss of contact pressure. Material analysis, material characterisation, finite element (FE) modeling and lifetime tests have been employed to establish correlations to support this failure hypothesis. It was found, that moisture plays the most important role for interconnect failure. The model is able to predict quantitative changes of force as function of loading parameters and correlate them qualitatively to the experimental mean time to failure. New insights are provided about the stress fields at the ACA bump. The model is discussed with respect to a direct prediction of failure versus time.

Electrical and reliability properties of isotropic conductive adhesives on immersion silver printed-circuit boards

Electrically conductive adhesive (ECAs) have been used for many years in both the anisotropic conductive adhesive (ACA) and isotropic conductive adhesive (ICA) forms for microelectronic packaging applications and as an alternative to lead (Pb) soldering technology. Especially, environmentally friendly ICAs offer many advantages over solder, such as simple and low temperature process conditions and better thermo-mechanical performance. However, a number of reliability questions linger, and continue to be studied. In particular, the ICA-to-pad contact resistance can increase with time due to the galvanic corrosion of dissimilar contact and ICA-filler metals (Lu et al., IEEE Trans Electron Packag Manuf 22:228–232, 1999). ICAs are usually silver (Ag) epoxy composites, and the corrosion potential should be completely eliminated by the use of Ag contact pads. To completely eliminate lead (Pb) from electronics, the printed wiring board (PWB) must also change from hot-air-leveled solder (SnPb) to alternative metallic finishes, such as immersion-silver (Ag), immersion-tin (Sn), electroless nickel (Ni)/immersion-gold (Au) and organic solderability preservative (OSP) (Pas, in European institute of printed circuits summer conference, 2005). Especially, immersion-Ag is one of the leading Pb-free final finish choices for many OEMs in the telecommunications, computer, automotive and consumer electronics industries, because of its excellent properties and reasonable cost. This paper presents the electrical properties of Ag epoxy composite ICAs materials on Cu-finished and immersion-Ag finished PWBs, as solder replacements for SMT or flip chip technologies. All PWBs were subjected to 85°C/85% relative humidity (RH) aging testing, with junction resistance monitored for comparison of the immersion-Ag board to the Cu-finished board as a control. We expected that the corrosion potential, which is one of main causes to degrade conductivity between ICAs and PWB, should be eliminated by the use of Ag contact pads with the Ag epoxy composite ICAs materials. Not only is the junction resistance of immersion-Ag finished boards lower than that of Cu finished boards, but its junction resistance changes are smaller than those of Cu-finished boards, as expected.

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/cm 2 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.