Anisotropic Conductive Adhesive Film Joints Research Articles

Recent advances in mechanical properties of anisotropic conductive adhesive film for microelectronic packaging

Purpose– This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the significant research and development work for the mechanical properties of ACF material and joints, which helps to the development and application of ACF joints with better reliability in microelectronic packaging systems.Design/methodology/approach– The ACF material was cured at high temperature of 190°C, and the cured ACF was tested by conducting the tensile experiments with uniaxial and cyclic loads. The ACF joint was obtained with process of pre-bonding and final bonding. The impact tests and shear tests of ACF joints were completed with different aging conditions such as high temperature, thermal cycling and hygrothermal aging.Findings– The cured ACF exhibited unique time-, temperature- and loading rate-dependent behaviors and a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrained the ratcheting strain in subsequent cycling with lower mean stress or stress amplitude. The impact strength and adhesive strength of ACF joints increased with increase of bonding temperature, but they decreased with increase of environment temperature. The adhesive strength and life of ACF joints decreased with hygrothermal aging, whereas increased firstly and then decreased with thermal cycling.Originality/value– This study is to review the recent investigations on the mechanical properties of ACF material and joints in microelectronic packaging applications.

Thermal cycling aging effects on the ratcheting behavior of anisotropic conductive film

Purpose – The purpose of this paper is to determine: how the thermal cycling aging affects the ratcheting behavior of anisotropic conductive adhesive film (ACF); how the loading conditions and loading history affect the ratcheting strain and strain rate of ACF with different thermal cycling aging histories. Design/methodology/approach – The ACF of CP6920F was cured at 190°C in an electro-thermal vacuum drying apparatus for 30 s. The cured specimens were put into the thermal cycling chamber (−40-150°C) for aging to 25, 50, 100, 200 and 500 cycles. A series of uniaxial ratcheting tests of aged ACF after different thermal cycles was carried out under stress control at 80°C. Findings – The ACF subjected to larger number of thermal aging cycles exhibits less ratcheting strain under the same loading conditions. The ACF with the same thermal cycling aging history shows more ratcheting strain and a higher ratcheting strain rate when loaded under a larger mean stress or stress amplitude or a lower loading rate. The ratcheting behavior of aged ACF is found to be more sensitive to the lower loading rate. The higher mean stress (or stress amplitude) enhances the deformation resistance and consequently restrains the ratcheting strain of subsequent cycling with a lower mean stress (or stress amplitude). The prior lower loading rate accelerates the plastic deformation more significantly than the higher one. Originality/value – The influencing trends of thermal cycling aging, loading condition and loading history on ratcheting behavior of ACF are obtained, which is important for the design and safety assessment of ACF joints.

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.

Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads

The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints.

Anisotropic Conductive Adhesives for Flip-Chip Interconnects

Anisotropic conductive adhesive films consist of an epoxy adhesive with dispersed conductive particles. They are used as electrical–mechanical interconnecting materials for flip-chip to flexible substrates and flip-chip to glass substrates. Contact resistance and adhesion strength are two important features of anisotropic conductive adhesive film joints. Contact resistance is affected by the curing degree of the adhesive, the bump characteristics, the reflow process and the environmental application conditions. Adhesion strength is affected by bonding temperature, bonding pressure, bubbles in the joints and particle characteristics. To assess reliability, anisotropic conductive adhesive film joints were tested under thermal cycling tests, autoclave tests and mechanical shock tests.

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.

Bonding Parameters of Anisotropic Conductive Adhesive Film and Peeling Strength

The effects of different bonding parameters-temperature, pressure, curing time, bonding temperature ramp and post-processing on the adhesive strengths of Anisotropic Conductive Adhesive Film (ACF) interconnection were investigated. The test results showed the adhesive strength increased as the bonding temperature increase. The curing time had great influence on the adhesive strength of ACF joints. The adhesive strengths increased as the bonding pressure increasing, but decreased if the bonding pressure was over 0.25MPa. The effects of different Teflon thickness on the pressure header and post-processing on adhesive strengths performance of ACF joints were studied. It was shown that the 90o peeling strength became deteriorated as the Teflon thickness increase. Different post-processing conditions showed that the specimens kept in 120oC chamber for 30 minutes had the best performance of the ACF interconnection. The environmental experiments of the thermal cycling (-40 - 125oC) and the high temperature/humidity (85oC, 85%RH) aging were used to evaluate the reliability of the specimens with different bonding parameters. It was shown that the high temperature/humidity was the harshest condition to the ACF bonding. The optimum bonding parameters were determined to obtain better peeling strength.

Deformation mechanism and its effect on electrical conductivity of ACF flip chip package under thermal cycling condition: An experimental study

In this study, experimental works are performed to investigate the deformation mechanism and electrical reliability of the anisotropic conductive adhesive film (ACF) joint subjected to temperature cycling for flip chip on organic board (FCOB) assemblies. This paper presents some dominant deformation parameters governing the electrical degradation in an ACF joint between a chip and a substrate when flip chip assembly is heated and cooled. The deformation mechanism of ACF flip chip assemblies during the temperature cycling are investigated using in situ high sensitivity moiré interferometry. A four-point probe method is conducted to measure the real-time contact resistance of ACF joint subjected to the cyclic temperature variation. As the temperature increases below T g of ACF, the bending displacement of assembly decreases linearly. At the temperature higher than T g of ACF, there is no further change in bending behavior and in-plane deformations of a chip and a substrate become approximately free thermal expansion. It is because that soft-rubbery ACF at the temperature above T g cannot provide the mechanical coupling between a chip and a substrate. The effect of bump location on the temperature dependent contact resistance is evident. A characteristic hysteresis in bending curves is observed and discussed. The contact resistance of the corner bumps increases with increasing temperature at a higher rate when compared to that of the middle. Failure analysis is performed to examine the ACF interconnections before and after thermal cycling test. The results indicate that during the thermal loading, the shear deformation is more detrimental to the electrical degradation of ACF joints than normal strain.

Effect of drop impact energy on contact resistance of anisotropic conductive adhesive film joints

The contact resistances investigated in this study of anisotropic conductive adhesive film joints using Au/Ni bumps and flexible substrates are found to be increased by the drop impact energy and also by the combined effect of heat/humidity and the impact energy. The samples humidified at 85 °C/85% RH for 384 h, on which impact energy of 50 J was induced, exhibit the most severe results. The contact resistance increases by 700%, which had been about 0.062 Ω in the as-bonded condition. The samples without humidification showed a sluggish and gentle increase in contact resistance with induced drop impact energy. The contact resistance was found to be increased by 400% after absorbing 90 J energy. Scanning electron microscopy images show particle deformation due to abrasion and friction between the contacting surfaces resulting from the sudden impact. Joints are also observed with no connections, which signify open circuits. Almost 25% of circuits were found open in the samples (after 384 h in a humid environment), which have suffered severe mechanical shock. Breaking of the conductive layer of the particle and exposing the underlying polymeric portion was also observed.

Effect of autoclave test on anisotropic conductive joints

This paper reports that the stress-corrosion cracking induced by autoclave test condition reduces the mechanical strength of anisotropic conductive joints and also increases the contact resistance by allowing more moisture to reach the aluminium metallization. The use of anisotropic conductive joints with bumpless chips allows a reduction in the costs of the flip chip bonding process. The epoxy-based anisotropic conductive adhesive film (ACF) absorbs moisture and experiences hygroscopic swelling, hence degrading adhesion strength and elasticity in hazardous environments, e.g. if moisture at high vapour pressure, and test temperature near to its glassy temperature ( T g) are applied. Contact resistances also show an increasing trend that is similar to that typical of a corrosion process. It is most probably due to the formation of an oxidation layer on top of the aluminium metallization and to the hygroscopic swelling of the ACF. The elastic properties of ACF joints reduce by about 50% after 384 h test time. In this study, 336 h of autoclave test is the critical test duration to affect the electrical and mechanical properties of an ACF joint.