Anisotropic Conductive Adhesive Film Research Articles

Reliability of ACF Interconnections on FR-4 Substrates

The use of anisotropic conductive adhesives (ACA) in flip chip interconnection technology has become very popular because of their numerous advantages. The ACA process can be used in high-density applications and with various substrates as the bonding temperature is lower than that in the soldering process. In this paper, six test lots were assembled using two anisotropic conductive adhesive films (ACF) and four different FR-4 substrates. FR-4 was chosen as it is an interesting alternative for making low-cost high-density interconnections. Some of the chips were thinned to study the effect on reliability. To study the effect of bonding pressure, four different pressures were used in every test lot. The reliability of the assembled test samples was studied in a temperature cycling test carried out between temperatures of -40°C and 125°C for 10 000 cycles. A finite element model (FEM) was used to study the shear stresses in the interconnections during the test. Marked differences between the substrates were seen. The substrate thinning and also the chip thinning increased the reliability of the test samples. From the FEM, it was seen that both decreased the shear stress in the adhesive, which is assumed to be the reason for the increased reliability. A significant difference was seen in the reliability between the ACFs. This was probably caused by differences in the conductive particle materials and the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> values and of the ACFs. In addition, the bump material used with the ACFs varied, which most likely affected the reliability of the test samples.

Dual-side and three-dimensional microelectrode arrays fabricated from ultra-thin silicon substrates

A method for fabricating planar implantable microelectrode arrays was demonstrated using a process that relied on ultra-thin silicon substrates, which ranged in thickness from 25 to 50 µm. The challenge of handling these fragile materials was met via a temporary substrate support mechanism. In order to compensate for putative electrical shielding of extracellular neuronal fields, separately addressable electrode arrays were defined on each side of the silicon device. Deep reactive ion etching was employed to create sharp implantable shafts with lengths of up to 5 mm. The devices were flip-chip bonded onto printed circuit boards (PCBs) by means of an anisotropic conductive adhesive film. This scalable assembly technique enabled three-dimensional (3D) integration through formation of stacks of multiple silicon and PCB layers. Simulations and measurements of microelectrode noise appear to suggest that low impedance surfaces, which could be formed by electrodeposition of gold or other materials, are required to ensure an optimal signal-to-noise ratio as well a low level of interchannel crosstalk.

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.

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.

Evaluation of the Delamination in a Flip Chip Using Anisotropic Conductive Adhesive Films Under Moisture/Reflow Sensitivity Test

Anisotropic conductive adhesive films (ACFs) have been used for electronic assemblies such as the connection between a liquid crystal display panel and a flexible printed circuit board. ACF interconnection is expected to be a key technology for flip chip packaging, system-in-packaging, and chip size packaging. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. Moisture concentration after moisture absorption was obtained by the finite element method. Then, the vapor pressure in the flip chip during solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. It is found that the delamination is well predicted by the present methodology

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.

Microwave preheating of anisotropic conductive adhesive films for high-speed flip chip on flex bonding

Anisotropic conductive adhesive film (ACF) can be preheated by microwave (MW) radiation in order to reduce the bonding time for flip-chip technology. Due to sluggish and nonuniform curing kinetics at the beginning of the curing reaction, thermal curing of epoxy is more time consuming. Therefore, MW radiation may be more effective, due to its uniform heating rate during the cycle. In this paper, MW preheating (for 1–4 sec) of ACF prior to final bonding has been applied to determine the electrical and mechanical performance of the bond. Powers of 80 and 240 W MW were used to study the effect of the MW preheating. A final bonding time of 6–7 sec can be used for flip chip on flex bonding instead of 10–15 sec (standard time for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (as low as 0.01) is low in these samples, whereas the standard resistance is 0.017 ohm (bonded at 180°C for 10 sec without prior MW preheating). The shear forces at breakage were satisfactory (0.167–0.183 KN) for the samples bonded for 6–7 sec with MW preheating. This is very close and even higher than the standard sample (0.173 KN). For MW preheating power of 80 W and sweeping time of 2 sec, final bonding at 6 sec can also be used because of its low contact resistance (0.019 ohm). Scanning electron microscope (SEM) investigation of microjoints and fracture surface shows uneven distribution of conductive particles and thick bond lines in samples bonded for 5 sec (with MW preheating). Samples treated with MW radiation (80 W and 2–3 sec time) serve as evidence that well-distributed particles along with thin bond lines cause low contact resistance and high joint strength.

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.

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.

Reliability and thermodynamic studies of an anisotropic conductive adhesive film (ACAF) prepared from epoxy/rubber resins

A thermoset type anisotropic conductive adhesive film (ACAF) comprising epoxy resin, natural butyl rubber (NBR), and micro-encapsulated imidazole as the binder and Ni/Au coated polymer bead as a conductive particle, has been studied. These films have been prepared to respond to requirements such as improved contact resistance, pitch interval of less than 80 μm and reliability. They showed good physical properties after being pressurized and heated to electrically and mechanically bond the ITO glass and the flexible circuit board. The curing conditions for the connection were 40, 20 and 15 s at 150, 170 and 190 °C, respectively. The initial contact resistance and adhesion strength were 0.5 Ω/sq. and 0.4 kg/cm under the condition of 30 kgf/cm 2, respectively. After 1000 thermal shock cycling tests between −15 and 100 °C, the contact resistance was maintained below 0.7 Ω/sq. Durability against high temperature (80 °C) and high humidity (85% RH) was also tested to confirm long-term stability (1000 h) of the conduction.

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.

A continuous contact resistance monitoring during the temperature ramp of anisotropic conductive adhesive film joint

In this study, a systematic experimental work was performed to evaluate the reliability of the anisotropic conductive adhesive film (ACF) joint at high temperature for flip-chip-on-flex (FCOF) assemblies. A four-point probe method was developed to measure the contact resistance at high temperature. Measurement was also conducted along the length of the chip. The correlation between the increased resistance and the failure mechanism was investigated using scanning electron microscopy (SEM). Initially, the contact resistance increased linearly with rising temperature, but later, it increased abruptly. This changeover was related to the glass-transition temperature (Tg) of the ACF matrix. The coefficient of thermal expansion (CTE) is very high at temperatures above Tg; thus, the ACF swells too much, reducing the mechanical contact of the particles with the bump and/or pad. Again, as the adhesive strength becomes weaker at temperatures above the glass transition, it is unable to resist the thermal stress of the flex. The cumulative thermal stress at the edges dislodges the particles from the interconnection. Even below Tg, the thermal stress at the edges is higher than the middle point. Thus, the contact resistance varied from the middle joint of the chip to that of the corner at the same high temperature. To reduce the contact resistance at the corner joint of the FCOF packages bonded by ACF, a square-shaped chip instead of a chip with a higher aspect ratio should be used. It was also suggested to use an adhesive with a higher glass-transition temperature and lower CTE.

Effect of microwave preheating on the bonding performance of flip chip on flex joint

A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW preheating. It has been found that 170 °C can be used for flip chip bonding instead of 180 °C (standard temperature for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015–0.025 Ω) is low in these samples where the standard resistance is 0.017 Ω (bonded at 180 °C without prior MW preheating). The shear forces at breakage were satisfactory (152–176 N) for the samples bonded at 170 °C with MW preheating, which is very close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 °C can also be used because of its low contact resistance (0.022–0.032 Ω), but the bond strength (137.3–145 N) is somewhat inferior to the standard one.

Plasma cleaning of the flex substrate for flip-chip bonding with anisotropic conductive adhesive film

Advanced integrated circuit packaging processes require good bondability and reliability between various mating surfaces. A key factor affecting this requirement is surface cleanliness. Plasma cleaning is the most suitable process for optimum surface cleanliness. An investigation of O2, Ar, and O2/SF6 plasma cleaning was carried out on a flexible substrate to study the adhesion of anisotropic conductive adhesive film for flip chip bonding. Surface roughness was found to increase substantially after the plasma treatment. Adhesion strength was evaluated by 90° peeling tests both for untreated and plasma-treated flex. A higher adhesion strength of anisotropic conductive film (ACF) bond was observed after plasma cleaning. The surface morphology of plasma treated and untreated flex substrate before bonding, as well as the fracture surfaces after the peel test for both cases, was characterized by secondary electron image techniques of scanning electron microscopy (SEM). Based on the detailed SEM findings, extensive comparisons were made between the plasma treated and the untreated samples. Mechanical interlocking is found to be responsible for higher peel strength of the plasma treated flex bonding. It was also proposed to select the right flexible substrate for highly reliable, ACF bonded flip chip on flex substrate.

Reducing bonding cycle time of adhesive flip chip process

Flip chip assembly using anisotropic conductive adhesives offers an interesting alternative for making high‐density interconnections. The use of conventional organic printed circuit boards makes this technique even more attractive. However, a low‐cost adhesive flip chip bonding process will require a reduced bonding cycle time or the use of multi‐head joining equipment. Adhesive flip chip bonding is characterized by a long bonding cycle time due to the relatively long curing time of adhesives and the need for simultaneous application of pressure during the curing process. In soldered flip chip techniques, the bonding time per assembly is shorter, because all the chips on the substrate can be soldered in a reflow oven at the same time. In this study, the minimum pre‐curing time needed to make a reliable adhesive joint was determined using one commercial anisotropic conductive adhesive film used on FR‐4 substrates. The results are promising, since bonding time reduction from 40 s to 10 s does not reduce the joint reliability.

Curing kinetics of anisotropic conductive adhesive film

Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications, such as chip-on-glass, chip-on-flex, etc. Among all the conductive-adhesive materials, anisotropic conductive adhesive film (ACF) is an attractive interconnect material because of its fine pitch capability. Anisotropic conductive-adhesive film is a thermosetting, epoxy matrix impregnated with a small amount of electrically conductive particles. During component assembly, the epoxy resin is cured to provide mechanical connection, and the conducting medium provides electrical connection in the z direction. The thermal cure process is critical to develop the ultimate electrical and mechanical properties of the ACF. In this paper, the curing reaction of ACF was studied with a differential scanning calorimeter (DSC) under isothermal conditions in the range of 120–180°C. An autocatalyzed kinetic model was used to describe the curing reaction. The rate constant and the reaction orders were determined and used to predict the progress of the curing reaction. A good agreement is found between the proposed kinetic model and the experimental reaction-rate data. The reaction-rate constants were correlated with the isothermal temperature by the Arrhenius equation. The glass-transition temperature also has been studied as a function of cure degree and moisture absorption.

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.

813 異方性導電樹脂接合部の吸湿リフロー試験時のはく離評価

813 異方性導電樹脂接合部の吸湿リフロー試験時のはく離評価

Anisotropic conductive film (ACF) prepared from epoxy/rubber resins and its fabrication and reliability for LCD

A thermoset type anisotropic conductive adhesive film (ACAF) comprising epoxy resin and natural butyl rubber (NBR) as the binder, micro‐encapsulated imidazole as the curing agent, and Ni/Au coated polymer bead as a conductive particle has been studied. These films have been prepared to respond to requirements such as improved contact resistance, current status less of than 60 μm and reliability. These films can also be used for connection between the ITO glass for LCD panel and the flexible circuit board. The curing conditions for the connection were 40, 20 and 15 seconds at 150, 170 and 190 °C, respectively. The initial contact resistance and adhesion strength were 0.5 O/square and 0.4 kg/cm under the condition of 30 kgf/cm2, respectively. After completing one thousand thermal shock cycling tests between ‐15 °C and 100 °C, the contact resistance was maintained below 0.7 O/square. Durability against high temperature (80 °C) and high humidity (85 % RH) was also tested to confirm long‐term stability (1000 hrs) of the conduction.