Anisotropic Conductive Film Layer Research Articles

Enhancement of electrical stability of anisotropic conductive film (ACF) interconnections with viscosity-controlled and high T g ACFs in fine-pitch chip-on-glass applications

This paper describes how anisotropic conductive film (ACF) properties including viscosity affect the electrical stability of ACF interconnections for fine pitch chip-on-glass (COG) applications. In this study, new ACFs for COG applications were designed by combining a high viscosity ACF layer and a low viscosity NCF layer to prevent the electrical shortage between bumps. As expected, the viscosity-controlled ACF showed better electrical insulation stability than a conventional ACF in fine pitch COG assemblies. According to the results of thermo-mechanical analysis (TMA) and dynamic-mechanical analysis (DMA), the viscosity-controlled ACF showed the improved thermo-mechanical properties such as lower coefficient of thermal expansion (CTE), higher storage modulus ( E′) at higher temperature region, and higher glass transition temperature ( T g ) than the conventional ACF. Furthermore, hot air reliability test and pressure cooker test (PCT) results showed that the viscosity-controlled ACF with higher T g had better hot air test and PCT reliabilities than the conventional ACF.

Ultrasonic Bonding Using Anisotropic Conductive Films (ACFs) for Flip Chip Interconnection

In this paper, a novel anisotropic conductive film (ACF) flip chip bonding method using ultrasonic vibration for flip chip interconnection is demonstrated. The curing and bonding behaviors of ACFs by ultrasonic vibration were investigated using a 40-kHz ultrasonic bonder with longitudinal vibration. In situ temperature of the ACF layer during ultrasonic (U/S) bonding was measured to investigate the effects of substrate materials and substrate temperature. Curing of the ACFs by ultrasonic vibration was investigated by dynamic scanning calorimetry (DSC) analysis in comparison with isothermal curing. Die adhesion strength of U/S-bonded specimens was compared with that of thermo-compression (T/C) bonded specimens. The temperature of the ACF layer during U/S bonding was significantly affected by the type of substrate materials rather than by the substrate heating temperature. With room the temperature U/S bonding process, the temperature of the ACF layer increased up to 300degC within 2 s on FR-4 substrates and 250degC within 4 s on glass substrates. ACFs were fully cured within 3 s by ultrasonic vibration, because the ACF temperature exceeded 300degC within 3 s. Die adhesion strengths of U/S-bonded specimens were as high as those of T/C bonded specimens both on FR-4 and glass substrates. In summary, U/S bonding of ACF significantly reduces the ACF bonding times to several seconds, and also makes bonding possible at room temperature compared with T/C bonding which requires tens of seconds for bonding time and a bonding temperature of more than 180degC.

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.

Macro‐micro modelling of moisture induced stresses in an ACF flip chip assembly

PurposeThis paper discusses the use of modelling techniques to predict the reliability of an anisotropic conductive film (ACF) flip chip in a humid environment. The purpose of this modelling work is to understand the role that moisture plays in the failure of ACF flip chips.Design/methodology/approachA 3D macro‐micro finite element modelling technique was used to determine the moisture diffusion and moisture‐induced stresses inside the ACF flip chip.FindingsThe results show that the ACF layer in the flip chip can be expected to be fully saturated with moisture after 3 h at 121°C, 100%RH, 2 atm test conditions. The swelling effect of the adhesive due to this moisture absorption causes predominately tensile stress at the interface between the adhesive and the metallization, which could cause a decrease in the contact area, and therefore an increase in the contact resistance.Originality/valueThis paper introduces a macro‐micro modelling technique which enables more detailed 3D modelling analysis of an ACF flip chip than previously.

Anisotropic conductive films (ACFs) for ultra-fine pitch Chip-On-Glass (COG) applications

This paper describes the development of anisotropic conductive films (ACFs) for ultra-fine pitch Chip-On-Glass (COG) application. In order to have reliable COG interconnects using ACF at fine pitch, the number of conductive particles trapped between the bump and substrate pad should be enough and less conductive particle between adjacent bumps. The ACF in this paper has double-layered structure, in which ACF and nonconductive film (NCF) layer thickness is optimized, to have as many conductive particles as possible on bump after COG bonding. In ACF layer, non-conductive particles of diameter 1/5 times smaller than the size of conductive particles were added to prevent electrical short between the bumps of COG assembly. The conductive particles are naturally insulated by the non-conductive particles even though conductive particles are flowed into and agglomerated in narrow gap between bumps during COG bonding. Also, flow property of the conductive particles is restrained due to increased viscosity of ACF layer with non-conductive particles, and the number of the conductive particles is constantly maintained. To ensure the insulation property at the level of 10 μm gap, insulating coated conductive particles were used in ACF layer composition. The double-layered ACF using low temperature curable binder system was also effective in reducing the warpage level of COG assembly due to low modulus and low bonding temperature.