Anisotropic Conductive Film Package Research Articles

Effects of bonding position on bending behavior of flexible anisotropic conductive film packages considering neutral surface

A theoretical investigation is performed into the bending behavior of FOF-ACF (Flex-On-Flex bonded by Anisotropic Conductive Film) packages bonded at different positions relative to the neutral surface position. The stresses and strains produced in the upper and lower regions of the ACF layer are investigated using Mode I fracture mechanics theory. The results show that the region of the ACF layer beneath the neutral surface of the FOF-ACF package experiences a negative stress, which prevents pre-existing cracks in the conductive particles from propagating further. However, at larger distances from the neutral surface, the compressive stress increases to such an extent that it may result in crumbling of the conductive particles. By contrast, the region of the ACF layer above the neutral surface experiences a positive stress, which promotes crack tip propagation. Moreover, the tension stress (i.e., the crack propagation effect) increases with an increasing distance from the neutral surface. The optimum bonding position for the ACF layer is found to be coincident with the neutral surface since, under this condition, the stresses and strains acting on the compressed particles present their minimum values. Consequently, the risk of fracture failure is reduced and the reliability of the ACF package is correspondingly improved.

Electric conduction failure of ACF packages based on pad array aspect ratio consideration

A mathematical model based on the V-shaped curve method is proposed for estimating the electric conduction failure probability of Anisotropic Conductive Film (ACF) packages with constant I/O number but different pad array aspect ratios. The overall failure probability takes account of both electrical open (opening event modeled using a Poisson function) and short circuit (bridging event modeled using a box model), and is calculated in accordance with the Inclusion---Exclusion principle of combinatorial mathematics. The results show that for a given set of pad geometry parameters, the failure probability decreases monotonically with an increasing pad array aspect ratio. However, the optimal particle volume fraction (i.e., the particle volume fraction which minimizes the overall probability failure) remains approximately constant as the aspect ratio is increased. Moreover, for an ACF package with a specified I/O number and a given conductive particle size, the failure probability can be reduced by decreasing the pad height, increasing the pad side length, increasing the pad distance, and setting the ACF volume fraction equal to that corresponding to the tip of the V-shaped curve.

Estimation of ACF packaging failure probability for IC/substrate assemblies with different pad array dimensions

An analytical model based on the V-shaped curve method is proposed for estimating the failure probability of anisotropic conductive film (ACF) packages with different pad array dimensions. In analyzing the failure probability of the ACF package, the probability of an opening event in the conductivity direction is modeled as a Poisson distribution, while the probability of a bridging event in the insulation direction is modeled using the enhanced box model. The overall failure probability of the package is then calculated in accordance with the inclusion–exclusion principle of combinatorial mathematics. The results presented in this study suggest that the failure probability of ACF packages with a high pad array dimension can be improved by reducing the pad height, decreasing the conductive particle size, increasing the pad pitch, increasing the pad size, or setting the ACF volume fraction close to the tip of the V-shaped curve.

Effects of Pad Array Dimensions and Misalignment Offsets on Optimal Fraction of Conductive Particles in Anisotropic Conductive Film Packages

The failure probability of anisotropic conductive film (ACF) packages is critically dependent on the volume fraction of conductive particles within the adhesive resin. In this study, the V-shaped curve method is used to determine the optimal volume fraction of conductive particles as a function of the bonding geometric parameters, the pad array dimension, and the misalignment offset between the upper and lower pads. In evaluating the corresponding failure probability of the ACF package, the probability of an opening failure is determined in accordance with a Poisson function model, while the probability of a bridging failure is derived using a box model. In computing the opening and bridging probabilities, the two models are modified to take account of the effects of package misalignments on the effective conductive area between opposing pads and the bridging path length between neighboring pairs of opposing pads, respectively. The opening and bridging probabilities are then combined using probability theory to establish the overall failure probability of the ACF package. In general, the results show that, for given bonding geometric parameters and misalignment offset, the optimal volume fraction of conductive particles remains approximately constant as the pad array dimension is increased. However, for given bonding geometric parameters and pad array dimension, the optimal volume fraction of conductive particles increases with an increasing misalignment error. Overall, the results show that, for any given values of the bonding geometric parameters, pad array dimension, and misalignment offset, the failure probability of the ACF package can be minimized by setting the volume fraction of conductive particles equal to the value corresponding to the tip of the V-shaped curve.

Failure Probability Estimation of Anisotropic Conductive Film Packages With Asymmetric Upper-to-Lower Pad Size and Misalignment Offsets

Anisotropic conductive films (ACFs) are widely used in the packaging of flat panel displays and liquid crystal displays and for attaching bare chips to both flexible and rigid substrates. This paper utilizes the V-shaped curve method to analyze the failure probability of ACF packages with an asymmetric upper/lower pad size and misalignment offsets. In the proposed method, the probability of opening failures is modeled using a Poisson function, modified to take into account the effects of the pad-width difference and misalignment offset on the effective conductive area between opposing pads. Meanwhile, the probability of bridging failures is evaluated using an enhanced bridging model based on the distance between the neighboring pad pairs in the array. The failure probability of the pad array is evaluated as a function of both the difference in width of the upper and lower pads and the degree of misalignment between the opposing pads in the array. The results show that the V-shaped curve method provides the means to predict the ACF volume fraction which minimizes the failure probability of the ACF assembly given a knowledge of the pad-width difference and the misalignment offset. In addition, it is shown that when the misalignment offset is greater than the pad-width difference, the minimum failure probability reduces as the pad-width difference increases due to the corresponding increase in the effective conductive area between opposing pads. Conversely, when the misalignment offset is less than the pad-width difference, the minimum failure probability increases with an increasing pad-width difference due to the corresponding reduction in the effective conductive area between the pads.

Moisture induced interface weakening in ACF package

One interesting degradation mechanism in flip–chip interconnections using anisotropic conductive film (ACF) is delamination between the adhesive and the sandwiched material. To understand the delamination process, an attempt was made to quantitatively measure the interfacial fracture toughness of both silicon/ACF and flame retardant 4 (FR4)/ACF sandwiched specimens exposed to various humidity conditions using a 4-point bending test. For moisture absorption, specimens were stored inside a humidity chamber (85 °C, 85% RH) until they were fully saturated, and a pressure cooker test (PCT) was additionally performed to expose the specimen to harsher environmental conditions. Delamination occurrence after moisture absorption was checked by means of a scanning acoustic microscope (SAM). Based on the experimental results, we have determined the energy release rate of each interface under different humidity conditions and have located the weaker region where crack initiation would be likely to occur under service conditions in interconnections using ACF. Evidence of crack propagation along the interface was checked by cross section optical images and scanning electron microscopy (SEM). By comparing the results of the specimens under different humidity conditions, interface weakening by moisture absorption was observed and quantitatively analyzed. Interestingly, we found that ACF can show severe degradation of adhesion strength caused by moisture absorption without any delamination occurring.

Failure Analysis of Anisotropic Conductive Film Packages With Misalignment Offsets

This paper utilizes the V-shaped curve method to analyze the failure probability of anisotropic conductive film (ACF) packages with various degrees of IC/substrate misalignment. In evaluating the failure probability of the ACF package, the probability of an opening failure in the vertical gap between the pads is determined in accordance with a Poisson function, while the probability of a bridging failure between the pads in the pitch direction is computed using a bridging model. In computing the opening and bridging probabilities, the Poisson function and bridging model are modified to take account of the effects of package misalignments on the effective conductive area between opposing pads and the bridging-path length between neighboring pairs of opposing pads, respectively. The opening and bridging probabilities are then combined using probability theory to establish an overall failure prediction model for the IC/substrate assembly. It is shown that, for any given value of the IC/substrate misalignment, the modified V-shaped curve method enables not only the failure probability of the ACF package to be reliably predicted but also an estimate to be made of the optimal ACF volume fraction. The results show that the semilogarithmic failure probability increases approximately linearly with the volume fraction for both uni- and bidirectional misalignments of the ACF package. The optimal volume fractions for ACF packages with misalignments not considered in this paper can be derived from the current results via a process of interpolation.

A Study of Hygrothermal Behavior of ACF Flip Chip Packages With Moiré Interferometry

A primary factor of anisotropic conductive film (ACF) package failure is delamination between the chip and the adhesive at the edge of the chip. This delamination is mainly affected by the thermal shear strain at the edge of the chip. This shear strain was measured on various electronic ACF package specimens by micro-Moire interferometry with a phase shifting method. In order to find the effect of moisture, the reliability performance of an adhesive flip-chip in the moisture environment was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different geometric size specimens in terms of interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between the adhesive and other components in the package, which induces hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated by using the Moire interferometry. From Moire measurement results, we could also obtain the stress intensity factor <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> . Through an analysis of deformations induced by thermal and moisture environments, a damage model for an adhesive flip-chip package is proposed.

Reliability of Au bump flip chip packages with adhesive materials using four-point bending test

In this study, the bending fatigue reliability of Au bump flip chip packages with anisotropic conductive film (ACF) and non-conductive film (NCF) as adhesive materials was evaluated using a four-point bending test. Differential bending between the substrate and the Si chip was the dominant failure driver for the Au bump flip chip packages subjected to the bending test. The bending fatigue life of the NCF joint was longer than that of the ACF joint, regardless of the bending frequency. The main reason for the increased electrical resistance of the packages with ACF and NCF was the delamination between the adhesive material and the electroless Ni-immersion Au (ENIG) pad and between the Au bump and ENIG pad, respectively. For the ACF package, the bending stress was concentrated at the conductive particles between the Au bump and the ENIG pad during the bending fatigue test, resulting in a low bending fatigue strength. The delamination in the packages propagated with increasing bending fatigue cycles and frequency. The Au bump flip chip package with NCF was mechanically robust due to the higher initial strength and the larger bonding area between the Au bump and the ENIG pad.

Effects of Systematic and Stochastic Errors on Estimated Failure Probability of Anisotropic Conductive Film

This paper analyzes the effects of systematic and stochastic errors on the failure probability of anisotropic-conductive-film (ACF) assemblies estimated using the V-shaped-curve method. It is shown that the effect of systematic errors varies as a function of the volume fraction and the volume-fraction bias. The effects of stochastic errors are investigated by using an in-house software program to generate random conductive-particle distributions in the pad and inter pad regions of the ACF package for the given volume fractions and package geometries. The dependences of the coefficient of variation (CV), essentially the degree of uniformity of the particle distribution, and the failure probability on the volume fraction are examined, and the corresponding results are used to derive the correlation between the stochastic error and the CV for a given volume fraction. In general, the current results indicate that the effects of systematic errors on the accuracy of the estimated failure probability can be controlled by improving the accuracy with which the resin and conductive-particle components of the ACF-compound material are weighed during the ACF fabrication process. However, the effects of stochastic errors cannot be controlled and vary as a function of the volume fraction and the degree of nonuniformity of the particle distribution. Nevertheless, the results indicate that the effects of both systematic and stochastic errors can be suppressed by specifying the volume fraction as the value corresponding to the tip of the V-shaped curve when designing the ACF compound.

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.

Failure mechanism study of anisotropic conductive film (ACF) packages

Anisotropic conductive film (ACF) consists of an adhesive polymer matrix with dispersed conductive particles. In flip-chip technology, ACF has been used in place of solder and underfill for chip attachment to glass or organic substrates. The filler particles establish the electrical contacts between the interconnecting areas. ACF flip-chip bonding provides finer pitch, higher package density, reduced package size and improved lead-free compatibility. Nevertheless, the interconnection is different from traditional solder joints, the integrity and durability of the ACF interconnects have major concerns. Failures in anisotropic conductive film (ACF) parts have been reported after temperature cycling, moisture preconditioning and autoclave. The failures have not been well understood and have been attributed to a wide variety of causes. This paper investigates the failure mechanism of ACF using finite element simulation. From a failure-initiation point of view, the response of ACF packages to environmental (temperature and humidity) exposure is very different from standard underfilled packages. These differences cause the ACF package to fail in different ways from an underfilled package. Simulation results have shown that moisture-induced ACF swelling and delamination is the major cause of ACF failure. With moisture absorption, the loading condition at the interface is tensile-dominant, which corresponds to lower interface toughness (or fracture resistance). This condition is more prone to interface delamination. Therefore, the reliability of ACF packages is highly dependent on the ACF materials. The paper suggests a new approach toward material selection for reliable ACF packages. This approach has very good correlation with experimental results and reliability testing of various ACF materials.