Anisotropic Conductive Adhesive Joints Research Articles

Hygrothermal Aging of an ACA Attached PET Flex-on-Board Assembly

In this paper, accelerated temperature and humidity test methods were used to study the long-term reliability of a flex-on-board assembly attached with an anisotropically conductive adhesive (ACA). Test methods included constant 65°C/90% relative humidity (RH) (65/90) and 85°C/85%RH (85/85) tests, 10°C-65°C/90%RH cycling test with moisture condensation (65/90 cycling), and 85°C/85%RH humidity cycling test (85/85 cycling). The resistance of the ACA joints was measured in real time during testing. On basis of the resistance measurement results, failures occurred in the 65/90 cycling test due to ACA delamination and cracking. In addition, failures were also observed in the constant 85/85 test and 85/85 cycling test. However, these failures were mostly related to the properties of the polyethylene terephthalate flex and its embrittlement or latent defects. In addition, a 90° peel strength test was used to study the changes in the mechanical strength of the assembly. The peel strength results showed that with all the test methods, the largest drop in the mechanical strength occurred at the start of the test. After this, the peel strength in 65/90 and 65/90 cycling tests remained almost unchanged, whereas a more significant drop was observed with the 85/85 cycling test. The 85/85 test caused the flex to become brittle, which prevented peel testing. Based on the failure analysis of the peel tested samples, similar failure modes were observed with all the test methods used. With untested samples, failure mostly occurred at the interface between the adhesive and the flex, although cohesive as well as adhesion failures at the ACA-printed circuit board interface were also observed. After hygrothermal aging, the failure location changed toward the ACA-flex interface as testing progressed and, in most cases, finally resulted in the detachment of conductor traces from the flex.

Reliability Analysis of RFID Tags in Changing Humid Environment

Radio frequency identification (RFID) tags with anisotropic conductive adhesive (ACA) joints are used in various applications where the environmental conditions may impair their reliability. Therefore, it is essential to investigate the effects of different environmental stresses on reliability and to analyze failure mechanisms. The purpose of this paper was to study the effects of changing humidity conditions on the performance of a passive ultrahigh frequency RFID tag with ACA joints. The tags were tested in a humidity cycling test where the humidity varied from 85%RH to 10%RH and temperature from 85 °C to 25 °C. Tags with four different sets of bonding parameters were tested. Significant differences in reliability between tags with different bonding parameters were observed. Different failure times, failure modes, and failure mechanisms were observed. Full-wave electromagnetic antenna simulations were used to support the analysis on the failure mechanisms manifesting themselves through changes in the common tag performance parameters. Changes in impedance matching between the chip and the antenna were found to be the most likely failure mechanism. The impedance matching may have changed due to cracks formed in the antenna during testing or due to chemical and mechanical changes in the ACA joints. It was observed that several failure factors might act simultaneously.

A Shear Strength Degradation Model for Anisotropic Conductive Adhesive Joints Under Hygrothermal Conditions

Hygrothermal environments can degrade anisotropic conductive adhesive (ACA) joints by weakening the shear strength of adhesive interface. In this paper, the shear strength degradation model of ACA joints under hygrothermal conditions was formulated through experimental testing and theoretical modeling. The shear strength degradation data were obtained from different hygrothermal aging tests and the ACA moisture properties were characterized for the corresponding hygrothermal conditions. Theoretical models considering the hygrothermal factors of T (temperature), RH (relative humidity), and t (time), were used to fit the shear strength degradation data. It was found that the inverse exponential law was the best candidate model to predict the degradation data. The shear strength degradation model of ACA joints under hygrothermal conditions was proposed, where the relationship of the S (shear strength) and the hygrothermal factors (T, RH, and t) was expressed in an analytical model. The degradation model was validated by experiments, and the model predictions agreed well with the test results.

Reliability Testing and Modeling of Anisotropic Conductive Adhesive Joints Under Temperature Cycling Test

The use of anisotropic conductive adhesives (ACAs) in flip-chip interconnection technology has several advantages compared to solders with underfills. However, when ACAs are used with organic substrates, their reliability may cause problems. Temperature cycling testing can be used to study the effect of fluctuating temperatures on the reliability of ACA joints. However, this is fairly time consuming. If modeling can be utilized in the evaluation of reliability, significant time savings can be achieved. In this paper, the prospects of using shear stress modeling in predicting the reliability of two different anisotropic conductive adhesive films (ACFs) with an organic FR-4 substrate in temperature cycling testing are examined. Finite element models were made for the test structures. Stresses and strains during the cycling tests ae calculated. To estimate the validity of the models, experimental testing is also done. Test samples with two different ACFs are tested in a temperature cycling test where the temperatures vary from -40°C to 125 °C. Testing is carried out for 10000 cycles. In addition, failure analysis is conducted on the test samples. To model the test samples accurately, the material properties of the ACFs are tested using a dynamic mechanical analyzer and a thermomechanical analyzer. The results of the modeling are compared with those of the experimental tests by comparing typical failure locations, failure modes, and the failure order of the test samples with different ACFs. According to the modeling, the highest shear stress seems to indicate the failure location. In addition, it seems that shear stress models can be used to rank ACF flip-chip joints in the order of the anticipated failure times. On the other hand, normal stress and strain models seem to describe failure mechanisms different from that observed in the experimental tests, and they are observed to be impractical for our purposes. Delamination, which may be one manifestation of failure mechanisms related to shear stress, is observed in all samples with ACF1. However, as no delamination is found in the samples with ACF2, it cannot be positively stated whether the failure mechanism of ACF2 joints is related to shear stress or not. If the samples with different ACFs have different failure factors, simple stress or strain models seem to be impractical for reliability studies, as the results of different models cannot be easily compared.

Investigation of Electrical Contact Mechanism for Anisotropic Conductive Adhesive Joints After the Thermocompression

Flexible interconnects are needed that meet assembly requirements for future applications in flexible consumer electronics products. This work investigates the electrical contact mechanism of ultrathin chip-on-flex (UTCOF) package using anisotropic conductive adhesive (ACA), which is highly flexible. In this paper, a 3-D nonlinear finite element (FE) model, which integrates analytical models of ACA joints and the thermal-mechanical behaviors of the UTCOF, is presented. The model is then applied to simulate the electrical contact mechanism for various ACA joints after thermocompression. Moreover, a “death-birth” meshing scheme is utilized to determine the effect of ACA resin temperature on contact resistance of the ACA joints. Multiple particle models are also generated using the ANSYS program. To validate the suitability of the proposed FE analytical model, contact resistance is measured to determine the bonding quality for 80-μm -pitch UTCOF test samples. The interfaces between the silicon chip and substrate for samples bonded under different bonding pressures are observed using cross-sectional scanning electron microscopy. The contact resistances obtained from the 3-D FE models were in good agreement with experimental results and can be used to predict the effects of thermocompression loading, pressure unloading, and cooling to room temperature on electrical contact mechanism in ACA joints after thermocompression. Generally, the gold bump and the compliant bump joined with multiple conductive particles reached stable contact resistance by capturing five to nine conductive particles. Contact resistance of an ACA-bonded UTCOF can be estimated using a proposed empirical model equation, particularly under loading conditions of L = 40%-60%. Overall, this work improves the design of flip chips for flexible electronics and the understanding of the electrical contact mechanism for ACA joints.

Utilizing Modeling in a Reliability Study of ACA Joints in Humidity Tests

Radio frequency identification tags (RFID) with anisotropic conductive adhesive (ACA) joints are used in various applications where the environmental conditions may impair their reliability. Thus the effects of different environmental stresses on reliability need to be investigated. The purpose of this work was to study whether a relatively simple shear stress model can be utilized in reliability prediction of anisotropically conductive paste (ACP) joints in an accelerated humidity test on the basis of the information obtained from another humidity test. If modeling gives accurate results when studying reliability, the need for actual testing would decrease and thereby time and cost savings could be achieved. In this study, finite element models were made to calculate shear stresses in ACP joints induced by two different humidity tests. Additionally, experimental tests were performed and the results were compared with those of modeling. The test samples were RFID tags whose microchips were attached with ACP. A constant humidity test was used to study the effects of high humidity level and a humidity cycling test was used to examine the effects of constantly varying humidity. In the modeling it was observed that the selection of the stress-free temperature has a significant effect on the results. With three different stress-free temperatures, three different sets of results were obtained. Although the tags saturated in the extreme conditions of the humidity cycling test, according to modeling, the change in relative humidity level in the humidity cycling test did not increase the harshness of the test. However, the temperature change in the humidity cycling test increased the harshness.

Mechanical assessment of an anisotropic conductive adhesive joint of a direct access sensor on a flexible substrate for a swallowable capsule application

Sensor interconnection achieved with Flip Chip (FC) technology and most particularly with Anisotropic Conductive Adhesive (ACA) is a very attractive technique, in achieving a Direct Access Sensor (DAS), in a swallowable diagnostic sensing capsule. This paper describes the work carried out to mechanically characterise the ACA joints when they are inserted in the capsule to determine the smallest capsule diameter that could be used without imparting excessive stress on the interconnect in the DAS integration process for a specific substrate and chip design. Three point inward bending was used to study the effect of the mechanical loading on the joints during the insertion process. The results showed that the insertion force linearly increased and leveled off at a, low friction, constant value. The spring constant of the linear region in a 23mm was 0.2729N/mm and increased to 1.5N/mm for the 15mm diameter hole. It showed that the spring constant decreased linearly as the diameter increased, signifying that for the same distance, less force will be required in a larger diameter hole than in the small diameter holes. During insertion the stress in the assembly, was higher towards the centre of the chip and the window than at the edge of the chip and the ACA fillet. This was characterised by it exhibiting lower voltage values measured in the partial daisy chains that were close to the window rather than the ones that were situated away from the window. The insertion test suggested that the 23mm diameter hole would be the smallest suitable hole for insertion of this assembly. Failure analysis revealed that the depression and the wide crack close to the window imply that the stress was high in this region. The cross sectional analysis showed that failure occurred within silicon/silicon chip pad and that the ACA contacts form a strong joint and was able to withstand the insertion force required to secure the ACA sensor in place before encapsulation.

Bulk Resistance Evaluation of Anisotropic Conductive Adhesive Particles Considering the Current Bending Effect

Because it is difficult to accurately estimate the electric resistance of anisotropic conductive adhesive (ACA) joints, the ACA’s applications in high density packaging field have been greatly limited. The bulk resistance of particles is an essential part of the resistance of ACA joints. For the ACA using solid nickel (Ni) particles, because current flows along the spherical profile of the particles, the bulk resistance of these particles will be underestimated if the current bending effect is neglected. Here we propose a new method, which considers this current bending effect, to accurately evaluate the bulk resistance of Ni particles. First, a mathematical model to calculate the resistance of an arbitrary shaped resistor is deduced on the basis of electromagnetic theory. Second, a numerical model is introduced to calculate the potential distribution in the particles. Finally, the bulk resistance calculated by the new model is compared with the conventional methods. It is shown that the value obtained from this model is much higher than those calculated by other methods. Furthermore, the correlation studies between the bulk resistance and the particle’s diameter, the deformation degree, and the bonding force are carried out. And the results show that these three parameters influence significantly on the bulk resistance. In conclusion, to obtain accurate bulk resistance of ACA particles and make them stable and reliable, it is important to take the current bending effect into consideration and control the particle diameter and the bonding force properly.

Shear strength of anisotropic conductive adhesive joints under hygrothermal aging and thermal cycling

The shear strength behaviors of Anisotropic Conductive Adhesive Film (ACF) joints were investigated experimentally with various environments. The shear tests of ACF joints were performed at different constant temperature conditions. It is found that the shear behaviors of the ACF joints are strongly dependent on testing temperature, and the shear strength decrease and the maximum displacement increases with increasing temperature. The shear strength of ACF joints was also investigated under hygrothermal aging and thermal cycling. The results show that the shear strength of ACF joints gradually decreases at first, then quickly decreases and finally the rate of decrease slows down again with increasing hygrothermal aging time. However the shear strength of ACF joints after thermal cycling increases firstly and then decreases with increasing thermal cycling time.

The Effects of Temperature Profile of Accelerated Temperature Cycling Tests on the Reliability of ACA Joints in RFID Tags

Anisotropic conductive adhesives (ACAs) are used in radio frequency identification tags (RFID tags) because of their advantages compared with conventional solders. However, some reliability problems may occur in the joints when the tags are exposed to high or fluctuating temperatures. Temperature cycling tests and constant temperature tests can be used to study the effects of temperature on reliability. With these tests, the failure modes and mechanisms can be found faster. Nevertheless, if an excessively rigorous test is chosen, the failure mechanisms and modes may not be the primary causes of failure in the operating environment of the tags. In this study, the reliability of a tag with ACA joints was studied with two different temperature cycling tests and two different constant temperature tests. Four different test series with varying bonding parameters were tested. The reliability of the tags was verified in each test by monitoring the daisy chain resistance of the ACA joints. The results from the test with the wider temperature range were achieved fastest. However, the behavior of the test series varied when compared with each other during different temperature cycling tests and differences in failure modes between the two temperature cycling tests were observed. Consequently, the test with the wider temperature range revealed failure mechanisms different from those seen during the less rigorous test. Due to this, the test with the wider temperature range may not have shown the primary failure modes and it is possible that these results do not correctly reflect the differences between the test series under use conditions.

Experimental/Numerical Analysis of Thermally Induced Warpage of Ultrathin Chip-on-Flex (UTCOF) Interconnects

Future applications of flexible displays and wearable electronics will need 3-D stacked flexible interconnects. The interconnection of an ultrathin chip-on-flex (UTCOF) that can provide flexibility is one approach that meets this requirement. Therefore, thermally induced warpage of UTCOF interconnects using anisotropic conductive adhesive (ACA) is investigated. In this paper, the effects of the ACA joint material properties, bonding temperature, and chip thickness on warpage of ACA-bonded UTCOF interconnects are examined experimentally and numerically. Two film types of ACA materials, ACA-P and ACA-F, are assembled under different bonding temperatures to study the effects of bonding temperature on warpage via out-of-plane deformation measurements using a micro figure measurement instrument. Micro Au-bump and compliant-bump assemblies in 80-μm-pitch dummy test vehicles are evaluated. Moreover, ultrathin chips with 25-50 μm thickness were assembled onto polyimide flex substrates to study the effects of chip thickness on thermally induced warpage. The 85°C/85% relative humidity thermal humidity storage test (RH THST) was also conducted for 1000 h for the UTCOF assembled with the selected process parameters. The interfaces between the ultrathin silicon chip and substrate are inspected in cross-sectional scanning electron microscopy (SEM) images. To validate the results of the experiments, a rigorous 3-D finite element (FE) analysis model integrating both thermal and thermal-mechanical behaviors of the UTCOF is established and performed using the ANSYS program. Experimental and numerical results indicate that the warpage of the micro compliant-bump assembly is less than that of the micro Au bump. Furthermore, the averaged warpage of the ACA-P-bonded samples with the Au bump using a 50- μm-thick chip is around 50.3 μm at a bonding temperature of 160°C whereas that of the ACA-F-bonded samples is 64.4 μm at 190°C. Additionally, both thermal expansion mismatch and the thermal gradient between the ultrathin silicon chip and substrate strongly affect the thermal-mechanical behaviors of the UTCOF interconnects. As expected, warpage increases as thickness of an ultrathin silicon chip decreases. A strong correlation exists between FE analysis results and experimental results. The manufacturing technology for high-density and flexible UTCOF interconnects with ACA joints is thus established.

Reliability and Flexibility of Ultra-Thin Chip-on-Flex (UTCOF) Interconnects With Anisotropic Conductive Adhesive (ACA) Joints

The need for flexible interconnects in advanced applications in consumer electronic products is increasing rapidly. The reliability and flexibility of ultra-thin chip-on-flex (UTCOF) interconnects formed using anisotropic conductive adhesive (ACA) are thus investigated. Two films of ACA materials, namely ACA-P and ACA-F, are assembled at different bonding temperatures to study the effect of temperature on the adhesion at the substrate-adhesive and adhesive-chip interfaces using differential scanning calorimetry (DSC) and a 90° peeling test. The contact resistance of a daisy chain with 188 input/output (I/O) is measured to examine the quality of bonding through dummy test samples with an 80-μm pitch. The reliability of the fabricated UTCOF interconnects bonded via selected ACA joints is evaluated by performing an 85°C/85% RH thermal humidity storage test (THST) for 1000 h, and their flexibility is evaluated in static bending and four-point bending tests. The interfaces between the ultra-thin silicon chip and the substrate of failed samples in the THST and four-point bending testing are then investigated by scanning electron microscopy (SEM), which is utilized to obtain cross-sectional images. Finite element analysis is also conducted to elucidate the failure mechanism of the UTCOF interconnects in the four-point bending test. The averaged maximum allowable deflections of the fabricated UTCOF interconnects with ACA-P and ACA-F materials are 26% and 168%, respectively, higher than those of the COF interconnects with a chip thickness of 670 μm. Moreover, the contact resistance remains stable, varying by less than 10%, in the static bending test with a bending radius of 30 mm. According to the results thus obtained, give the appropriate choice of an ACA material and the optimal curing conditions, the UTCOF interconnects with ACA joints reliably serve as flexible interconnects for use in consumer electronic products.

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.

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.

Reliability of Anisotropic Conductive Adhesive Joints in Electronic Packaging Applications

New interconnection materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. In particular, anisotropic conductive adhesives (ACAs) have gained popularity as a potential replacement for solder interconnects. Despite numerous benefits, ACA-type packages pose several reliability problems. During the last 10 years, tremendous research and development efforts have been spent on improving the reliability of ACA joints. In this paper, the effects of the bonding process (including the bonding temperature, bonding pressure, curing conditions and reflow processes) on the reliability of ACA joints are presented. Then the effects of the environmental factors (including high temperature, humidity, thermal cycling, impact load, etc.) on the reliability of ACA joints are discussed. Finally, the effects of the properties of the components (including properties of substrates, ACAs, conductive particles and the bump height) on the reliability of ACA joints are reviewed. Additionally, future research areas and remaining issues are pointed out.

High performance anisotropic conductive adhesives for lead‐free interconnects

PurposeTo study the interface properties of anisotropic conductive adhesives (ACAs) and improve the electrical properties of ACA joints as a replacement for Sn/Pb solder in the electronics industry.Design/methodology/approachIn this study, different types of self‐assembled monolayer (SAM) compounds were introduced into the interface between the metal filler and the substrate bond pad. The formation of these SAMs on gold and silver surface and their thermal stability were investigated by measuring the contact angles with water of these SAM coated surfaces. The SAM compounds used had either hydrophilic or hydrophobic tail groups, and included octadecanethiol (ODT), mercpatoacetic acid (MAA), 1,4‐benzenedithiol (dithiol) and malonic acid (acid M). Epoxy resins with two different curing temperatures were used as polymer matrices for the ACA formulations. The electrical properties of ACA joints containing these SAMs were studied by measuring their current‐voltage relationship.FindingsThe results show that SAMs can be successfully coated onto specific metal surfaces, depending on the affinity of their functional groups with the specific metal. The SAM treated ACA joints show much lower resistance at the same applied current than non‐treated joints, and the effect on the low curing temperature epoxy matrices was more significant. Nano‐Ag filled ACAs show more significant improvements in their electrical properties due to the greater surface area and higher surface energy of nano‐particles and consequently their higher thermal stability when coated with SAMs.Originality/valueThis paper is the first to use functional organic monolayers to enhance the interface properties of electrically conductive adhesives and, in particular, for ACAs.

Effects of hygrothermal aging on anisotropic conductive adhesive joints: experiments and theoretical analysis

Epoxy-based anisotropic conductive adhesive film (ACF) joints have been used in a number of interconnect applications, including direct chip attachment, i.e., chip on glass, chip on ceramics, etc. The ACF joints can be subjected to high relative humidity environment and are susceptible to moisture sorption, especially at elevated temperatures. The long-term hygrothermal aging will induce irreversible changes to epoxy resin systems due to susceptibility of the polymer resin to hydrolysis, oxidation, etc. In this study, the hygrothermal environment was used as an accelerator for the degradation of ACF joints in chip-on-glass (COG) assemblies, which were fabricated in the form of single-lap joints. The effects of aging on the epoxy-based ACF joints were characterized by shear tests and scanning electron microscopy (SEM) at accelerated aging times of 125, 250, 375 and 550 h. The results show that the strength of ACF joints decreases and the fracture mechanism gradually changes with hygrothermal aging. In order to further interpret the hygrothermally-induced degradation to the ACF joints, an ACF joint aging model with hygrothermal environment has been developed, introducing a dimensionless parameter A, which was obtained from the interfacial fracture energy.

Enhancement of electrical properties of anisotropically conductive adhesive joints via low temperature sintering

AbstractThe electrical properties of anisotropically conductive adhesives (ACAs) joints through low temperature sintering of nano silver (Ag) particles were investigated and compared with that of the submicron‐sized Ag‐filled ACA and lead‐free solder joints. The nano Ag particles used exhibited sintering behavior at significantly lower temperatures (<200°C) than at the bulk Ag melting temperature (960°C). The sintered nano Ag particles significantly reduced the joint resistance and enhanced the current carrying capability of ACA joints. The improved electrical performance of ACA was attributed to the reduced interfaces between the Ag particles and the increased interfacial contact area between nano Ag particles and bond pads by the particle sintering. The reduced joint resistance was comparable to that of the lead‐free (tin/3.5 silver/0.5 copper) metal solder joints. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1665–1673, 2006

Modelling the effect of voids in anisotropic conductive adhesive joints

Purpose – Anisotropic conductive adhesive film (ACF) is used for very fine pitch applications in the microelectronics industry, such as flip chip (FC) technology. During the bonding process, bumps on the chip and pads on the substrate are first aligned and then heat and pressure are applied so as to apply thermal energy to the ACF for curing and to cause permanent plastic deformation of the conductive particles. Consequently, a permanent electrical and mechanical contact is formed between the bumps and the pads. The purpose of this study is to investigate the effect of the size and location of any voids in the ACF during subsequent solder reflow processes necessary for SMT component attachment. The paper also investigates the use of a protective aluminium cover during such reflow cycles, which reduces the temperature inside the ACF, and therefore, the stresses inside the ACF, especially when voids exist.