Anisotropic Conductive Joints Research Articles

Behavior of Anisotropic Conductive Joints under Hygrothermal Conditions

The long-term hygrothermal aging will induce the irreversible damages of epoxy-based anisotropic conductive film (ACF) joints. In this study, hygrothermal environment tests were used to accelerate the degradation of ACF joints in Chip-on-Glass (COG) assemblies. The effects of aging on adhesion strengths were measured by shear mode tests and changes of molecular conformation of ACF material were analyzed by Fourier Transform Infrared Spectroscopy (FTIR). Also, the fracture surfaces of ACF joints were investigated by Scanning Electronic Microscope (SEM). The results show that the ACF adhesion strengths decrease with aging; some chemical modifications occur in the aged ACF joints; the fracture failure mechanisms change with the hygrothermal aging.

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.

The effect of thermal cycling on the contact resistance of anisotropic conductive joints

Purpose – Anisotropic conductive film (ACF) offers miniaturization of package size, reduction in interconnection distance and high performance, cost‐competitive packaging and improved environmental impact. However, a major limitation for ACF is the instability caused by thermal warpage. The purpose of this paper is to study the effects of thermal warpage on contact resistance in real time i.e. make online measurements of contact resistance fluctuations while the assembly undergoes thermal shock.Design/methodology/approach – The ACF assemblies are subjected to thermal cycling with different temperature profiles that have peak temperatures either below or above the glass transition temperature (Tg) of the ACF. The flex substrate used was made of polyimide film, with Au/Ni/Cu electrodes and a daisy‐chained circuit matched to the die bump pattern. The ACF used was based on epoxy resin in which nickel and gold‐coated polymer particles are dispersed. A comparative study was carried out on the results obtained.F...

Effect of bump characteristics and temperature variation on the online contact resistance of anisotropic conductive joints

Anisotropic conductive film (ACF) suffers a major drawback in regard to reliability even though it has merits, such as reduction in interconnection distance, high performance, and environmental friendliness. The factor of thermal warpage may lead to a highly unreliable electrical connection in the assembly. The work presented in this paper focuses on the online contact-resistance behavior of the ACF joint during thermal shock and compares the results of two different types of dies (Au/Ni bump and bumpless). For this work, we used a flip chip of 11 × 3 mm2 in dimension. The flex substrate used was made of polyimide film with an Au/Ni/Cu electrode and daisy-chained circuit for a matching die-bump pattern. The ACF that was used is an epoxy resin in which nickel and gold-coated polymer balls are dispersed. 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 a temperature of 180°C, and a pressure of 80 N was used. The initial contact resistances of Au/Ni bump and bumpless samples were 0.25 ω and 0.4 ω respectively. A comparative study was carried out from the results obtained. The results showed that for the flip-chip-on-flex (FCOF) packages having an Au/Ni bump, the increase in online contact resistance is higher than that of the FCOF packages having bumpless chips. For example, in the thermal-cycling profile of 140°C to −40°C, the online contact resistance for the Au/Ni bump raised to 4.6 ω after 180 cycles, whereas it was only 1.3 ω for the bumpless sample. The bump height and bump materials were found to be the main factor for such variation. Results show that, above the glass-transition temperature (Tg), the ACF matrix becomes less viscous, which reduces its adhesive strength and lets the higher bump height of the chip result in a higher standoff of the package and thus sliding is easier to take place. The responses by the assemblies in hot and cold conditions are examined, and in-chamber behavior of the assembly is studied and explained.

Dynamic strength of anisotropic conductive joints in flip chip on glass and flip chip on flex packages

The work presented in this paper focuses on the behavior of anisotropically conductive film (ACF) joint under the dynamic loading of flip chip on glass (COG) and flip chip on flexible (COF) substrate packages. Impact tests were performed to investigate the key factors that affect the adhesion strength. Scanning electron microscopy (SEM) was used to evaluate the fractography characteristics of the fracture. Impact strength increased with the bonding temperature, but after a certain temperature, it decreased. Good absorption and higher degree of curing at higher bonding temperature accounts for the increase of the adhesion strength, while too high temperature causes overcuring of ACF and degradation at ACF/substrate interface––thus decreases the adhesion strength. Higher extent of air bubbles was found at the ACF/substrate interface of the sample bonded at the higher temperature. These air bubbles reduce the actual contact area and hence reduce the impact strength. Although bonding pressure was not found to influence the impact strength significantly, it is still important for a reliable electrical interconnect. The behaviors of the conductive particles during impact loading were also studied. From the fracture mode study, it was found that impact load caused fracture to propagate in the ACF/substrate interface (for COG packages), and in the ACF matrix (for COF packages). Because of weak interaction of the ACF with the glass, COG showed poor impact adhesion.

Corrosion study of anisotropic conductive joints on polyimide flexible circuits

In application, ACF joints often undergo thermal foray and since under most circumstances moisture absorption exists at the same time, the combined effects are, therefore, important. Autoclave test was used in this study as a stimulator for stress-corrosion cracking in the strong presence of hygroscopic swelling-induced stress. SEM/EDX analysis showed evidences of metal oxidation and stress-corrosion cracking at ACF joints. Apart from the physical damages, results of FTIR spectroscopy showed chemical modification to aged ACF joints under the effect of autoclave test condition. ACF joints lost their epoxy functionality after 288 h and hydrolysis was becoming obvious after 384 h of the autoclave test.

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.

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%.