Anisotropic Conductive Film Joints Research Articles

Reliability of adhesive interconnections for application in display module

The reliability of adhesive interconnections using anisotropic conductive film (ACF) and non-conductive film (NCF) was evaluated by measuring connection resistance during 500 cycles of thermal shock testing. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit (FPC). The connection resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mΩ at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the thermal shock testing, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.

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

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

Characterization of Failure Behaviors in Anisotropic Conductive Interconnection

The effects of bonding forces and reflow process on the failure behaviors of anisotropic conductive film (ACF) interconnections were analyzed. Conventional reflow process was employed with peak temperatures of 220 � C for soldering of Sn-37Pb and 260 � C for soldering of Pbfree Sn-Ag or Sn-Ag-Cu. Two kinds of main failure mode were detected after double reflows at 220 � C: formation of a conduction gap between conductive particles and Ni/Au-plated Cu pad, and delamination of the adhesive matrix from the plated Cu pad on the flexible substrate. The determination of the failure mode was mainly affected by the variation of the bonding force. The main failure mode of the reflowed ACF joints was conduction gap for the joints with lower bonding forces and adhesive matrix delamination for the joints with higher bonding forces. However, only adhesive matrix delamination was observed after reflows at 260� C. A theoretical calculation was also conducted to predict the connection resistance of the ACF joint before and after reflows. The calculation showed that the optimum bonding forces are between 65 and 70 N. [doi:10.2320/matertrans.48.1070]

Thermal and hygroscopic reliability of flip-chip packages with an anisotropic conductive film

Thermal and hygroscopic reliability of anisotropic conductive film (ACF) joints in relation to flip-chip bonding force was evaluated by thermal shock and constant temperature/humidity testing. The failure mode by thermal shock testing varied with increasing bonding force, i.e., (1) formation of a conduction gap between conductive particles and Au bump or Ni/Au plated Cu pad at low bonding force and (2) delamination of adhesive matrix from the plated Cu pad on the flexible substrate at high bonding force. The delamination initiated as a crack under a conductive particle and propagated sideward resulting in a complete delamination of the ACF from the Cu pad. However, delamination was observed between the ACF and Au bump on the chip side after the constant temperature/humidity testing for 500 h. A theoretical calculation was also conducted to predict the connection resistance of the ACF joint before and after reliability tests. The calculation showed the importance of the bonding gap between the electrodes.

Effect of bonding force on the reliability of the flip chip packages employing anisotropic conductive film with reflow process

Thermo-mechanical reliability of flip chip packages employing anisotropic conductive film (ACF) was investigated in terms of the effect of bonding forces on the failure mode of the packages. Conventional reflow process was conducted to evaluate the reliability of the package. Two kinds of failure modes were detected in this study. The first one is formation of a conduction gap between conductive particle and Ni/Au plated Cu pad, while the second one is delamination of the adhesive matrix from the plated Cu pad on flexible substrate. The determination of the failure mode was mainly affected by the variation of the bonding force. In case of the ACF joints with lower bonding forces, a conduction gap was the main failure mode of the reflowed joints, while the delamination of the adhesive matrix was frequently observed in case of the joints with higher bonding forces. The difference in failure mode was discussed in the main text.

Effects of bonding pressure on the thermo-mechanical reliability of ACF interconnection

Effects of reflow process on the thermo-mechanical reliability of the anisotropic conductive film (ACF) joints were investigated. Cross-sectional studies were carried out to investigate the effects using a scanning electron microscope (SEM). Swelling of ACF matrix in z-direction and formation of a conduction gap were observed in the reflowed ACF joints which were bonded in lower flip chip bonding pressures than 60 N. However, a delamination as well as the formation of a conduction gap was occurred between the ACF and Cu pad on flexible printed circuits (FPC) when ACF joints bonded in higher bonding pressures than 70 N were reflowed. This difference of failure mode was mainly due to the divergence of adhesive matrix thickness between the Au bump on Si chip and Cu pad on FPC. Finite element analyses were also conducted to interpret the failure behaviors of the ACF joints.

Effects of hygrothermal aging on epoxy-based anisotropic conductive film

The epoxy-based anisotropic conductive film (ACF) joints have the potential of being exposed to a hygrothermal environment and susceptible to moisture sorption. The long-term hygrothermal aging will induce the irreversible damages of epoxy resins system due to susceptibility of the polymer to hydrolysis, oxidation, etc. In this study, the hygrothermal environment test was used as an accelerator for the degradation of ACF joints in Chip-on-Glass (COG) assemblies. The effects of aging on the adhesion strengths were measured by shear mode tests and the changes of molecular conformation were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). Results show that the ACF adhesion strengths decrease with aging and there are some chemical modifications to the aged ACF joints.

RLC effects in fine pitch anisotropic conductive film connections

PurposeThe resistance, capacitance and inductance of anisotropic conductive film (ACF) connections determine their high frequency electrical characteristics. The presence of capacitance and inductance in the ACF joint contributes to time delays and cross‐talk noise as well as simultaneous switching noise within the circuit. The purpose of this paper is to establish an experimental method for estimating the capacitance and inductance of a typical ACF connection. This can help to provide a more detailed understanding of the high frequency performance of ACF assemblies.Design/methodology/approachExperiments on the transient response of an ACF joint were performed using a digital oscilloscope capable of achieving the required ns resolution. An equivalent circuit model is proposed in order to quantify the capacitance (C) and inductance (L) of a typical ACF connection and this model is fitted to the experimental data. The full model consisted of two resistors, an inductor, and a capacitor.FindingsThe capacitance and inductance of a typical ACF connection were estimated from the measured transient response using Kirchhoff's voltage law. The method for estimation of R, L, and C from the transient response is discussed, as are the RLC effects on the high frequency electrical characteristics of the ACF connection.Research limitations/implicationsThere was decay time deviation between the calculation and the experiment. It may have resulted from the skin effect in the high frequency response and the adhesive surrounding joint as well. The main reason may be the capacitance zctric lost. Further research work will be done to determine more accurately the dielectric losses in anisotropic conductive adhesive (ACA) joint.Originality/valueThis paper presents a new method to characterise the high frequency properties of ACA interconnections and will be of use to engineers evaluating the performance of ACF materials in high frequency applications.

Reliability of anisotropic conductive joints for bumpless flip-chip on flex packages under thermal stresses

The reliability of ACF (Anisotropic conductive film) interconnection is a serious concern especially under thermal loading condition. This paper focuses on the online contact resistance behavior of the ACF joint for bumpless flip-chip on flex packages during different thermal cycling conditions. In this work, flip chips of 11×3 mm2 having bare aluminum pad were used. Real time contact resistance (i.e. live measurement contact resistance variation with temperature) was measured by four points probe method when the packages were inside thermal shock chamber. Tests for three different thermal cycling profiles (125°C to −55°C, 140°C to −40°C and 150°C to −65°C) were carried out. The samples bonded at temperature 180°C and pressure of 2.42Mpa was used. The initial contact resistance of the bumpless samples was 0.4Ω. Contact resistance increased with the number of thermal cycles, however the effect was severe when the temperature variation was above the glass transition temperature (Tg) of the ACF matrix (131°C). Differences in co-efficient of thermal expansion (CTE) between the chip and the substrate generated thermal stresses during temperature fluctuation, which caused the pad of the substrate to slide over the Al pad of the chip. Thus variation of the contact resistance was also observed along the interconnection position in the package, i.e. corner joint showed higher value of increase in contact resistance than the middle position. Even though flex substrate was used in this study; the sliding effect was severe at the corner Al pads of the chip, where cumulative forces generated due to the thermal stress. Results show that for thermal cycling profile 140°C to −40°C, online contact resistance increased to 1.2 Ω in corner joint, whereas for the middle joints the contact resistance just increased to 0.5 Ω. Glass transition temperature (Tg) of the ACF material plays an important role on the high temperature contact resistance. For every thermal cycling profile, there is an incubation period that would have significant impact in the application of ACF. After the incubation period the contact resistance increases rapidly and the joints are no longer reliable.

Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.

Reliability of Anisotropic Conductive Film Joints Using Bumpless Chip—Influence of Reflow Soldering and Environmental Testing

This paper presents the reliability of anisotropic conductive film (ACF) joint tested under reflow soldering and environmental test effect. The ACF joint behaved differently under different reflow soldering profiles. The lower reflow temperature resulted in more reliable ACF joints by maintaining low contact resistance. By contrast, high contact resistance was found in assemblies treated with higher reflow. Under humidity aging (85°C/85%RH), bumpless chips proved to be unreliable due to corrosion mechanism. Moreover, ACF had shown degradation in chemical and physical properties, including modulus reduction, Tg depression, polymer hydrolysis, and surface swelling after exposing to humidity aging. The deterioration in reliability of aged ACF joint during reflow process was mainly caused by hydroscopic swelling-induced stress of ACF.

Thermal Cycling Reliability and Delamination of Anisotropic Conductive Adhesives Flip Chip on Organic Substrates With Emphasis on the Thermal Deformation

One of the most important issues whether anisotropic conductive film (ACF) interconnection technology is suitable to be used for flip chip on organic board applications is thermal cycling reliability. In this study, thermally induced deformations and warpages of ACF flip chip assemblies as a function of distance from neutral point (DNP) and ACF materials properties were investigated using in situ high sensitivity moire´ interferometry. For a nondestructive failure analysis, scanning acoustic microscopy investigation was performed for tested assemblies. To elucidate the effects of ACF material properties and DNP on the thermal cycling reliability of ACF assembly, Weibull analysis for the lifetime estimation of ACF joint was performed, and compared with thermal deformations of ACF flip chip assembly investigated by moire´ interferometry. Results indicate that the properties of ACF have a significant role in the thermal deformation and reliability performance during thermal cycling testing. Therefore, optimized ACF properties can enhance ACF package reliability during thermal cycling regime.

Study of anisotropic conductive adhesive joint behavior under 3-point bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.

Effects of Solder Reflow on the Reliability of Flip-Chip on Flex Interconnections Using Anisotropic Conductive Adhesives

This work describes the work of an investigation of the effects of solder reflow process on the reliability of anisotropic conductive film (ACF) interconnection for flip-chip on flex (FCOF) applications. Experiments as well as computer modeling methods have been used. The results show that the contact resistance of ACF interconnections increases after the reflow and the magnitude of the increase is strongly correlated to the peak reflow temperature. In fact, nearly 40 percent of the joints are open when the peak reflow temperature is 260/spl deg/C, while there is no opening when the peak temperature is 210/spl deg/C. It is believed that the coefficient of thermal expansion (CTE) mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. To understand this phenomenon better, a three-dimensional (3-D) finite element (FE) model of an ACF joint has been analyzed in order to predict the stress distribution in the conductive particles, adhesive matrix and metal pads during the reflow process. The stress level at the interface between the particle and its surrounding materials is significant and it is the highest at the interface between the particle and the adhesive matrix.

Contraction stress build‐up of anisotropic conductive films (ACFs) for flip‐chip interconnection: Effect of thermal and mechanical properties of ACFs

AbstractThe important mechanical mechanism for the electrical conduction of anisotropic conductive films (ACFs) is the joint clamping force after the curing and cooling processes of ACFs. In this study, the mechanism of shrinkage and contraction stress and the relationship between these mechanisms and the thermomechanical properties of ACFs were investigated in detail. Both thickness shrinkages and modulus changes of four kinds of ACFs with different thermomechanical properties were experimentally investigated with thermomechanical and dynamic mechanical analysis. Based on the incremental approach to linear elasticity, contraction stresses of ACFs developed along the thickness direction were estimated. Contraction stresses in ACFs were found to be significantly developed by the cooling process from the glass‐transition temperature to room temperature. Moreover, electrical characteristics of ACF contact during the cooling process indicate that the electrical conduction of ACF joint is robustly maintained by substantial contraction stress below Tg. The increasing rate of contraction stresses below Tg was strongly dependent on both thermal expansion coefficient (CTE) and elastic modulus (E) of ACFs. A linear relationship between the experimental increasing rate and E × CTE reveals that the build‐up behavior of contraction stress is closely correlated with the ACF material properties: thermal expansion coefficient, glassy modulus, and Tg. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2634–2641, 2004

Fundamental understanding of ACF conduction establishment with emphasis on the thermal and mechanical analysis

This paper presents the thermal and mechanical contribution of anisotropic conductive films (ACFs) to the electrical conduction establishment of ACF joint. The conduction mechanism of ACF joint strongly depends on the thermal and mechanical properties of ACF. Therefore, it is important to understand the relationship of thermal and mechanical properties of ACF in a bonding process with the electrical conduction establishment. In this study, ACF flip chip process was fully designed based on the material characterization and in situ process monitoring. Moreover, the effect of degree of cure on the ACF conduction establishment was investigated in a bonding process window. The important mechanical mechanism of ACF conduction for good bonding quality is the joint clamping force due to curing and cooling-down processes of ACFs. The build-up behavior of z-axis shrinkage stress in ACF joint during curing and cooling-down processes of ACF materials was experimentally investigated with thermo-mechanical measurement of ACF. These results reveal that shrinkage stress in ACF joint developed during bonding process is the important parameter to establish the electrical conduction of interconnects using ACF material.

Current-carrying capacity of anisotropic-conductive film joints for the flip chip on flex applications

The effect of the substrate-pad physical properties (surface roughness and hardness) on the current-carrying capacity of anisotropic-conductive film (ACF) joints is investigated in this work. Flip chips with Au bumps were bonded to the flexible substrates with Au/Cu and Au/Ni/Cu pads using different bonding pressure. It was found that the current-carrying capacity of ACF joints increased to a maximum value with the rise of the bonding pressure; then, it reduced if the bonding pressure continually increased. The maximum average value per unit area of Au/Ni/Cu pad and Au/Cu pad ACF joints is about 93 µA/µm2 and 118 µA/µm2, respectively, at 100-MPa bonding pressure. The variation trend of connection resistance is the opposite of current-carrying capacity. The variation of current-carrying capacity (or connection resistance) of Au/Cu pad joints is larger than that of Au/Ni/Cu pad joints. The current-carrying capacity is related to the variation of the resistance of ACF joints. The connection resistance of ACF joints depends primarily on the particle constriction resistance (Rcoi), Rcoi ∞ 1/a, where “a” is the radius of contact spot. A smaller contact area results in larger joule heat generation per unit volume (Qg), Qg ∞ 1/a4, which preferentially elevates the temperature of the constriction. The raised temperature increases the resistance because of the temperature-dependent coefficient of the metal resistivity. The theory of tribology is used to explain the difference between Au/Cu pad and Au/Ni/Cu pad ACF joints. For the Au/Cu pad ACF joints, the deformation of the particles’ upper and bottom sides is nearly symmetrical; the contact between conductive particles and pad has the character of “sliding contact,” especially under high pressure. For the Au/Ni/Cu pad ACF joint, the contact between particles and pad determined the conduction characteristics of ACF joints. It has the character of “static contact.” Thus, the current-carrying capacity (or connection resistance) of Au/Cu pad joints is more sensitive to the bonding pressure.

Behaviour of anisotropic conductive joints under mechanical loading

Anisotropic conductive films (ACFs) received a great deal of attention in recent years for interconnection applications in electronic packaging. This paper reports the behaviour of ACF joints under various mechanical loading, i.e., die shear and cyclic fatigue in shear. The mechanical behaviour of ACF joints that have been exposed to environmental effects, i.e., high moisture and elevated temperature (autoclave test conditions) has been examined using die shear test. The maximum shear force prior to fracture was determined as 465.0 N, at which the contact resistance is found extremely high and can be considered as open circuits. Epoxy based ACF exhibits insignificant plastic deformation, especially for samples that have undergone autoclave test. Reduction trend was observed in the shear moduli over autoclave test time for ACF joints. Fracture surface of ACF that failed in shear test shows spalling and less plastic deformation after exposed to autoclave test. For cyclic fatigue test, the endurance limit is determined at about 143.5 N and the corresponding calculated endurance ratio is around 32.0%.

Effect of misalignment on electrical characteristics of ACF joints for flip chip on flex applications

The effect of misalignment on the electrical properties of anisotropic conductive film (ACF) joints is investigated in this work. It is found that along with the increase of misalignment, the connection resistance of ACF joints increases. When the misalignment in x-direction is less than 5 μm, the increase rate of connection resistance is quite large. Then, along with the severity of misalignment, the increase rate becomes smaller. Finally, when the misalignment is close to 20 μm, the increase rate rises again. The Holm's electric contact theory is used for understanding the connection resistance variation. On the other hand, with the increase of misalignment in x-direction, the insulation resistance between ACF joints decreases. If the misalignment exceeded 10 μm, the decrease is prominent for the Ni particle ACF joints. This phenomenon can be explained by the effect of dielectric damage of the epoxy. Computer programs are also developed to calculate the variation of the probability of open and shorting after misalignment and predicate the maximum misalignment tolerance. The results show that the open and shorting probability increase abruptly after misalignment. On the view of pad parameters, the open probability is mainly related to the pad area, while the pads gap is critical to the shorting probability. Large pads gap (small pad width) can reduce the shorting probability obviously. On the other hand, enlarging the pad area by increasing pad length decreases the open probability significantly. So comparing to square shape pad, rectangle shape pad can reduce the failure probability greatly.

Flow characterization and thermo-mechanical response of anisotropic conductive films

The paper reports investigations on the chip on glass (COG) bonding process using anisotropic conductive films (ACF). Experimental methods as well as theoretical analyses, by both analytical and numerical means, are applied. The assumptions concerning the thermo-mechanical and rheological properties of the polymer materials involved in the bonding process are characterized for dependence on temperature. The transient development of the temperature field during the bonding process is studied by finite element (FE) analysis for dependence on the upper and lower chuck temperatures. Analytical techniques of fluid mechanics are used to predict the flow of the conductive particles during bonding, treated as dimensionless points embedded in a viscous matrix. This analytical description allows one to estimate the number of conducting particles on a bump of a chip after bonding. Furthermore, numerical calculations are applied to characterize the influence of viscosity gradients on the particle flow. Finally, nonlinear finite element simulations are used to investigate the stress development and stress relaxation process within the ACF joints.