Adhesion and Reliability of Anisotropic Conductive Films (ACFs) Joints on Organic Solderability Preservatives (OSPs) Metal Surface Finish

The effect of final metal finishes of Cu electrodes on the adhesion and reliability of anisotropic conductive film (ACF) joints was investigated. Two different metal surface finishes, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs) coated on Cu, were selected in this study for ACF bonding. The adhesion strength of ACF/OSP joints was higher than that of ACF/bare Cu and ACF/ENIG joints. The fracture sites of the ACF/bare Cu and ACF/ENIG joints were ACF/metal interfaces, while those of ACF/OSP joints were inside the ACF. Transmission electron microscope (TEM) and Fourier-transform infrared (FT-IR) analyses showed that the OSP coating layer on the Cu electrodes reacted with the epoxy resin of the ACFs but still remained at the bonding interface. According to the in-depth X-ray photoelectron spectroscopy (XPS) analysis, additional C-N bonds formed after the OSP-epoxy reaction and the outermost nitrogen of the OSP layer participated in curing of the epoxy resin of the ACF. Therefore, the OSP layer acted as an adhesion promoter to ACFs. Furthermore, this role of the OSP layer enhanced the reliability of the ACF/OSP joints under high temperature and humid environments, as compared to the ACF/ENIG joints.     

环境对各向异性导电胶膜性能参数的影响

各向异性导电胶膜(ACF)的玻璃转化温度T_g是它的一个重要性能参数,用差示扫描热示计(DSC)分别测定商用各向异性导电胶膜固化前和固化后的玻璃转化温度,并确定不同的固化时间对它的玻璃转化温度的影响,以及高温高湿(85℃,85%RH)环境对它的玻璃转化温度影响,得到各向异性导电胶玻璃转化温度下降是其粘接强度下降的原因之一。     

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 (T_g), 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.     

Research on the contact resistance,reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications

Although there have been many years of development, the degradation of the electrical performance of anisotropically conductive adhesive or film (ACA or ACF) interconnection for flip-chip assembly is still a critical drawback despite wide application. In-depth study about the reliability and degradation mechanism of ACF interconnection is necessary. In this paper, the initial contact resistance, electrical performance after reliability tests, and degradation mechanisms of ACF interconnection for flip-chip-on-flex (FCOF) assembly were studied using very-low-height Ni and Au-coated Ni-bumped chips. The combination of ACF and very-low-height bumped chips was considered because it has potential for very low cost and ultrafine pitch interconnection. Contact resistance changes were monitored during reliability tests, such as high humidity and temperature and thermal cycling. The high, initial contact resistance resulted from a thin oxide layer on the surface of the bumps. The reliability results showed that the degradation of electrical performance was mainly related to the oxide formation on the surface of deformed particles with non-noble metal coating, the severe metal oxidation on the conductive surface of bumps, and coefficient of thermal expansion (CTE) mismatch between the ACF adhesive and the contact conductive-surface metallization. Some methods for reducing initial contact resistance and improving ACF interconnection reliability were suggested. The suggestions include the removal of the oxide layer and an increase of the Au-coating film to improve conductive-surface quality, appropriate choice of conductive particle, and further development of better polymeric adhesives with low CTE and high electrical performance.