Conductive Adhesive Interconnects Research Articles

Effects of low melting point SnBi58 alloy on the properties of isotropic conductive adhesives

Adequate mechanical and electrical properties and long-term reliability are critical in conductive adhesive interconnects. In order to improve the electrical and mechanical properties of isotropic conductive adhesives (ICAs), reduce contact resistance and enhance the aging resistance of ICAs, the effects of low melting point SnBi58 alloy on the properties of ICAs were studied. The low melting point SnBi58 alloy formed metallurgical bonds between conductive fillers, as well as between the ICAs and a Cu substrate and not only improved the electrical conductivity and mechanical properties of ICAs, but also reduced the contact resistance shift of ICAs effectively. It was found that when the mass ratio of SnBi58 alloy in conductive fillers was maintained at 20 wt%, the bulk resistivity of ICAs reached 4.8 × 10−4 Ω cm, the shear strength of ICAs was 27.71 MPa, and the contact resistance shift of ICAs was 7% after aging. In addition, better bulk resistivities and higher shear strengths of ICAs were achieved after aging at 85 °C/85%RH.

Conductive adhesive based shingled solar cells: Electrical degradation under cyclic loading

Shingled solar cells based on electrically conductive adhesive (ECA) interconnection have emerged as a commercially viable option for photovoltaic (PV) modules with attractive attributes including no soldering process (lead-free), tight packing with no gap between cells, and resulting efficiency gains. Reliability of such modules is, however, less certain due to the lack of long-term outdoor performance data. Here we report numerical simulation of thermo-mechanical stress in shingled solar cells using finite element modelling, and identify stress concentration in silicon in the interconnect region not anticipated from simpler analytical models. We also present design and fabrication of samples using ECA interconnection with geometry and characteristics that closely match production modules for realistic fatigue testing under cyclic loading. The setup is combined with real-time electrical measurements that permit monitoring of fatigue-induced changes in the electrical resistance of ECA joint. Detailed measurements are presented using samples with different types ECA and varying patterns of the glue-line aimed at evaluating reduced material usage for cost-reduction. This study demonstrates that the sample structure and experimental setup presented here can be effectively used to evaluate electrical and mechanical degradation of shingled PV modules to guide their design process.

Three-terminal III–V/Si tandem solar cells enabled by a transparent conductive adhesive

Three-terminal tandem solar cell with conductive adhesive interconnect and back-contacted bottom cell delivers 27.3% efficiency.

ACA Curing Process Optimization Based on Curing Degree Considering Shear Strength of Joints

This paper presents an optimized curing process for the shear strength achievement and retention for anisotropic conductive adhesive (ACA) flip-chip interconnections. The curing kinetics of ACA is first presented and then a curing degree $(\alpha)$ relationship between curing temperature $T$ and curing time $t$ is established. The influence of different curing degree states on the interfacial shear strength is then reported, including the effect of degree of curing on shear strength both before and after hygro-thermal aging.

Low-temperature Interconnection of PVD-Aluminium Metallization

Aluminium, thermally evaporated with physical vapor deposition (PVD), is considered a cost efficient rear metallization for crystalline silicon heterojunction or tunnel oxide passivated solar cells. Owing to the temperature-sensitivity of the solar cell structure, potential routes of low temperature interconnection methods for PV module integration are assessed. Textured wafers with a full area 2μm PVD-Al metallization are divided into two groups: The first is capped with sputtered 100nm Ti and 400nm Ag for testing solderability. The second is capped with 400nm Ag for electrically conductive adhesive (ECA) interconnection. Low temperature soldering with Sn43Bi57 and Sn41Bi57Ag2 coated ribbons and ribbon interconnection with two ECAs are evaluated in terms of peel strength, contact resistivity and accelerated aging properties. It is found that peel strength of soldered interconnections on the Al/Ti/Ag achieve between 1.5N mm-1 to 2N mm-1 whereas glued interconnections on Al/Ag between 0.5N mm-1 to 1N mm-1. The contact resistivity is 2.6 × 10-3 mΩ cm2 to 3.6 × 10-1 mΩ cm2 for both interconnection technologies. Soldered samples show a stable contact resistivity when tested in 1000h in damp heat conditions or 200 thermal cycles. The contact resistivity of glued interconnections increases to 1 mΩ cm2 to 10 mΩ cm2 along with an observed disintegration of the Al-layer and an ablation of the Ag-capping from the Al-layer.

Corrosion testing of anisotropic conductive adhesive interconnections on FR4, liquid crystal polymer and polyimide substrates

Anisotropic conductive adhesive films (ACF) have been widely studied for numerous applications. However, their resistance to corrosion in highly corrosive environments has been studied only very little. This study investigated the reliability and behaviour of ACFs in corrosive salt spray environment. ACF was used to attach flip chip (FC) components on FR4, liquid crystal polymer (LCP) and polyimide (PI) substrates and the FC packages were subjected to a salt spray test lasting 3000h. The FC packages had daisy chain structures which were measured continuously in real time during testing. After testing cross sections of the tested packages were examined using an optical microscope and a scanning electron microscope (SEM). Most components failed during the test and the results showed significant differences between the various substrate materials. The LCP substrate performed considerably better than the other substrates and the PI substrate proved to have the poorest reliability. Corrosion of the pads on the substrates as well as open joints was seen in all substrate materials. The corrosion behaviour as well as the differences between the substrates showed that the substrate structure and material are critical factors in corrosive environments and should be carefully considered. The reliability of the ACF FC package with the LCP substrate was found to be good, as the test was very severe and no failures occurred during the first 625h of testing and only 20% failed during the first 1000h.

Reliability Study on Adhesive Interconnections in Flex-to-Flex Printed Electronics Applications Under Environmental Stresses

The reliability of conductive adhesive interconnections in flex-to-flex printed electronics applications under thermal cycling and cyclic bending tests is investigated in this paper. The components under study consisted of a backplane, having screen-printed silver wiring on polyethylene terephthalate film, fabricated in a roll-to-roll process, and a flexible functional component mimic stacked on top of the backplane. Each test component contained four daisy-chained conductive adhesive interconnections, and their reliability was monitored in situ with a four-point dc resistance measurement during tests. In addition, the effect of supportive nonconductive adhesives on structure reliability was studied in various configurations. The accelerated life test results proved that the long-term reliability of the studied components can be enhanced by adding supportive adhesive between the flexible layers. In both thermal cycling and cyclic bending tests, the most reliable method was found to be the configuration with supportive adhesive around the top layer sides, attaching the two layers totally together, whereas the configuration with no supportive adhesive at all showed the worst reliability. The failure analysis emphasized that the critical factor, in terms of reliability, was silver conductor delamination from the backplane surface at the interconnection area.

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125°C, 1000 cycles) and high-temperature and high-humidity tests (85°C, 85% RH, 1000h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu6Sn5 (η-phase) and Cu3Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.

Reliability of SMD interconnections on flexible low-temperature substrates with inkjet-printed conductors

This work dealt with potential reliability issues in inkjet-printed electronics. The aim was to increase our understanding of how failures occur in hybrid electronic structures that utilize inkjet printing. A future goal in this field will be to enable expansion of this product area from low-life-expectancy and -performance products to high-life-expectancy and -performance products.In this work the reliability of isotropically conductive adhesive interconnections between 0402- and 0805-sized ceramic resistors and three types of flexible substrates was investigated. The use of low-temperature ink material (curing temperature 150°C) allows new, inexpensive substrates to be introduced into the process. In addition, the interconnection reliability of a more complex low-temperature cofired ceramic module attached to a flexible substrate with ball grid array interconnections was investigated.Reliability testing was carried out in a thermal cycling test chamber in a 0–100°C temperature range. Daisy-chain resistances of the samples were monitored with a datalogger during the cycling. Besides optical inspections, the interconnections were analyzed with X-ray microscopy. Also scanning acoustic microscopy was used to observe crack propagation. Finally, scanning electron microscopy was used to detect failure mechanisms.The two main failure mechanisms of the chip resistor components were loss of adhesion at the ICA/component terminal cladding interface and cracking of matte tin cladding of the component. For the tested LTCCs it was noted that breakdowns of interconnections occurred because of fractures on the module side of the interconnection—in most cases in the solder/intermetallic compound interface—and not because of ICA fractures or ink delamination on the substrate side.

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.

Mechanics of Adhesively Bonded Flip-Chip-on-Flex Assemblies. Part I: Durability of Anisotropically Conductive Adhesive Interconnects

This is Part I of a two-part paper on adhesively bonded flip-chip-on-flex (FCOF) microelectronic assemblies. This paper examines the use of anisotropically conductive adhesives (ACAs), and the companion paper (Part II) addresses the use of non-conductive adhesives (NCAs). Two types of FCOF dies, bonded with ACAs, were subjected to temperature cycling and repeated temperature shock durability tests. The specimens have Au-plated bumps and the ACA contains Au-plated polymer particles. Specimens were fabricated with different bonding pressures. Scanning Electron Microscopy (SEM) investigations were performed to characterize the distribution and shape of the conducting particles. Contact resistance measurements, conducted throughout the temperature cycling durability test for 1000 temperature cycles between 20°C and 115°C, showed that resistance varied cyclically with each temperature cycle, but there was no increase in the average resistance over the duration of the test. In contrast, cyclic thermal shock tests between –50°C and 115°C produced failures within 200–2500 cycles, depending on the specimen configuration. Studies were conducted to examine the effect of specimen configuration and temperature on: (i) interconnect resistance, based on electro-thermal modeling; and (ii) mechanical contact stresses, based on thermomechanical modeling. Modeling techniques used detailed finite element analysis (FEA) as well as simpler rapid-assessment models based on 2D variational methods (for example, the Raleigh–Ritz method) and 1D electrical resistance networks. These models were used to parametrically investigate the influence of interconnect design, bonding pressure, temperature and interconnect degradation mechanisms, on the contact resistance and contact stresses. Results suggested that the thermal expansion of the ACA during temperature cycling did not cause any changes in the contact resistance at the contact interface, thus suggesting the possibility of bonding at Au–Au interfaces. During thermal shock, the expansion mismatch stress between the die and Printed Wiring Board (PWB) at the low temperature (–50°C) was probably sufficient to fracture this bond. The issue of gold-to-gold bonding will be addressed in a future paper.

Experimental Analysis of Mechanical and Electrical Characteristics of Metal-Coated Conductive Spheres for Anisotropic Conductive Adhesives (ACAs) Interconnection

An accurate characterization for the deformation behavior of conductive particles is important: 1) to understand the anisotropic conductive adhesive (ACA) interconnection and 2) to optimize the ACA bonding parameter. This paper introduces an experimental technique, which has been developed to allow continuous monitoring of deformation characteristics of a single conductive particle. The load-deformation curve of a single conductive particle is measured, which provides the quantitative estimation of the mechanical and electrical characteristics of metal-coated polymer spheres used in ACAs. Based on the load-deformation result of a single conductive particle and the number of trapped particles on a bump, equivalent spring models are used to predict the deformation degree of conductive particles after flip chip assembly. For two kinds of conductive particles with different polymer cores, the mechanical and electrical characteristics of ACA interconnection were studied. Such results are used to further achieve a more sophisticated approach of the ACA bonding process and contact reliability

Electrical Characterization of Flip-Chip Interconnects Formed Using A Novel Conductive-Adhesive-Based Process

Using conventional microfabrication techniques, we have developed a new, low-cost wafer bumping process that enables a high degree of control over patterning of conductive adhesive interconnects. This approach obviates the need for development of dispensing and scraping head equipment that may otherwise be required for mass fabrication of lithographically patterned adhesive bumps. Flip-chip interconnects formed using this new process offer better electrical performance as compared to those formed by squeegee-based definition techniques. This is inferred in this paper by experimentally demonstrating lower contact resistance with the polished bumps as compared to the squeegeed bumps. Furthermore, in order to study the high-speed electrical performance characteristics of these conductive adhesive bumps, a 10-GHz 1.55-mum p-i-n photodetector fabricated in the antimonide material system was used as case study. The results from the bandwidth characterization of the polymer flip-chip-integrated detector showed minimum degradation in the high-speed performance characteristics of the detector

Interfacial Adhesion of Anisotropic Conductive Adhesives on Polyimide Substrate

There has been steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. Simple processing, low processing temperature and fine pitch capability are the major advantages of conducting adhesive technology. A new and innovative connection technology geared towards achieving increased functionality at a lower total system cost is anisotropic conductive adhesive (ACA) interconnection. ACAs, when used for flip chip assembly, provide electrical as well as mechanical interconnections for fine pitch applications. This work deals with adhesion issues between ACA on polyimide materials. The paper presents a reliability assessment of adhesive joints using ACA on polyimide substrate that was conducted by testing samples at various aging temperature, high humidity and high pressure environments. The effects of high temperature and high humidity on the bond strength of ACA joints were measured using 90 deg peel test as well as by microstructural examination. It was found that aging generally caused a decrease in peel strength, especially the results of scanning electronic microscopy showed that the pressure cooker test could most effectively reveal the adhesion behavior.

Failure Mechanism of Anisotropic Conductive Adhesive Interconnections in Flip Chip ICs on Flexible Substrates

In this work we study how the endurance performance of electrically conductive adhesive interconnections of flip chip integrated circuits with a pitch of 200 and 150/spl mu/m on flexible substrates is affected by varying environmental conditions. This is accomplished by comparison of offline and online control measurements that are carried out to monitor the electrical resistance in temperature-humidity and temperature cycling stress tests. From the gradual degradation of the resistance with time and the failure analysis, it is conjectured that periodic absorption and desorption of moisture forms one of the more important failure mechanisms in this type of assembly.

Fatigue and failure behavior of silver-filled electronically-conductive adhesive joints subjected to elevated temperatures

The use of adhesives to replace mechanical connectors and other joining methods has enjoyed rapid growth in recent years. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). One of these is the cyclic fatigue behavior of conductive adhesive interconnects under different environmental conditions, in which fatigue failure might occur due either to mechanical or thermal stresses varying in a cyclic manner. This paper addresses the effect of elevated temperatures on the fatigue and failure behavior of ECAs. For this purpose, joints were prepared using stainless steel adherend specimens bonded with a commercial ECA, and tested using monotonic and cyclic loadings, at two elevated temperatures, namely 50°C and 90°C. When the temperature was increased to 90°C, close to the glass transition temperature of the adhesive, we observed consistently parallel fatigue curves at different load ratios (R = P min /P max) for joints, as in the case of 50°C test condition, along with significant reduction in fatigue lives. Joint failure mechanisms were also analyzed using optical techniques, and joint conductivity measurements.

Electrical Conductive Characteristics of Anisotropic Conductive Adhesive Particles

In anisotropic conductive adhesive (ACA) interconnections, the particles are electrical conductors providing current paths in the fine pitch electronic packaging as well as physical parts connecting with the chip bumps and the substrate pads through the mechanical deformation interfaces. The primary object of this fundamental research is to reveal the electrical conductive characteristics of Ni/Au coated resin particles. Such an ACA particle resistance is resulted from two metal coated layers, which are two parallel resistors in the circuit determined by the particle transformation degree. In order to investigate the effect of the particle transformation degree upon the particle resistance, the particle transformation factor is defined. The mathematical electrical resistance function of an ACA particle, an integral function of the transformation factor and the particle geometries, resin diameter, nickel layer thickness, and gold layer thickness, is worked out from the physical model of an ACA particle. To carry out the solutions of the function, MathCAD software is applied. According to the numerical solutions, the deeper the particle transformation, the thicker the metal coated layer thicknesses and the longer the resin diameter are, the lower the particle resistance is. In conclusion, it is stated that the ACA particle resistance is determined by the particle transformation and the particle geometries, however, the transformation and the nickel layer thickness are more sensitive than the resin diameter and the gold layer thickness. Finally, the resistance function will explain the conductive mechanism of the deformed ACA particle.

Study of short-circuiting between adjacent joints under electric field effects in fine pitch anisotropic conductive adhesive interconnects

Anisotropic conductive adhesive film (ACF) has been extensively used in the liquid crystal display (LCD) industry for decades on chip-on-glass (COG) applications. It offers the advantages in terms of fine-pitch capability and more environment compatibility. One of the very important performance requirements of using ACF in fine pitch interconnection is not to create leakage of electric current between adjacent joints as it may lead to abnormal display segments/pixels of the LCD. In this work, the possibility of short-circuiting between adjacent joints in fine pitch ACF interconnections under the effects of electric field was investigated. Insulation resistance measurements of the selected adjacent joints against electric field strength 1 V/μm for a 24 h testing duration were discussed and analyzed. The results showed a strong dependence of curing degree of the ACFs on the chance of short-circuiting between adjacent joints under field effects.

Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly II. Different bonding pressure

The effects of different bonding pressure during flip-chip-on-flex (FCOF) assembly in relation to the performance of anisotropic conductive film (ACF) interconnect were investigated. Two types of ACF were used in this study. ACF 1 is designed to create good interconnection when the connecting bumps and pads are in close contact while ACF 2 can give good connections when the conductive particles are in contact with the bumps and pads, hence the deformation of conductive particles within ACF 2 FCOF packages were less than that within ACF 1 packages. ACF 2 gave much better interconnection performance when compared to ACF 1 indicating that ACF 2 is more flexible and can tolerate a wider range of bonding pressure.

Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly I. Different bonding temperature

The effects of different bonding temperatures during flip-chip-on-flex (FCOF) assembly in relation to the performance of anisotropic conductive adhesive (ACF) interconnect were investigated. Two types of flip chips were used in this study. It was found that Ni bumps formed better interconnections than bumpless FCOF packages. Aluminium oxide was observed and was thought to be the main cause of the increased in contact resistance after the moisture-soak tests. The conductive particles were not fully compressed by the bumps and pads and gaps were observed between the conductive particles and Cu pads in bumpless packages. Conductive particles in the Ni bump FCOF packages were tightly trapped between the bumps and pads and hence gave better connections. The performance of the ACF interconnects were affected by the degree of curing of the ACF, which was determined by the bonding temperature.