(A) study on the suppression of conductive particle movement in anisotropic conductive films (ACFs) for ultra fine pitch assembly

Abstract

Fine pitch interconnection technology has become a very important part in packaging industry since the demands of smart electronic devices such as tablet personal computers and smart phones have risen dramatically. The packaging industry requires more competitive technology to fulfill customer needs in electronic devices with high performance, multi-functionality, and compactness. As a consequence, more input/output pins are required within the limited space resulting in the narrower pitch and bump space for further miniaturization of electronic devices. However in ultra fine pitch interconnections, there are electrical problems with the conventional ACFs bonding, as the pitch and space between bumps and electrodes have become finer and finer. Bumps shape with longer length and narrower width causes less number of conductive particles captured between the bump and the electrode resulting in high contact resistance problem. Moreover, narrow space between the bumps becomes a serious problem, because flown out conductive particles are agglomerated between bumps causing short circuit failures. As a solution for these problems, many solutions were introduced including nanofiber ACFs. In chapter 2, a modified concept of nanofiber ACFs called “Nanofiber-sheet (NS) ACFs” is introduced. In this chapter, the nanofiber ACFs are modified where conductive particles are located in a very thin layer of nanofiber-sheet with top and bottom surfaces exposed. For this novel NS-ACFs, an electrical path is created through captured conductive particles without melting the nanofibers. As a result, broader selectivity of polymers materials with higher tensile strength and higher suppression effect can be selected. Throughout this chapter, fabrication processes of the novel NS-ACFs are introduced. In addition, the effects of NF on conductive particle movement are investigated and compared with the conventional ACFs. Chapter 3 introduces another polymer structure to suppress the movement of conductive particles in ACFs. In this chapter, fabrication process of anchoring polymer layer (APL) is introduced which is a simplified fabrication process of exposing surfaces of conductive particles. It is important to simplify the process steps because it can facilitate the mass production of cost-effective ACFs. And then, theoretical and experimental results of conductive particle movement in APL ACFs are discussed. Since the APL structure remains unmelted after a thermo-compression COG bonding, it is possible to observe the actual movement of conductive particles of APL ACFs. Therefore, quantitative measurements are analyzed and compared with the theoretical prediction. In chapter 4, effects of conductive particle movement on the electrical stability and reliability in ultra fine pitch COG interconnection are investigated. ACFs joint properties of conventional ACFs, conventional nanofiber ACFs, NS-ACFs, and APL ACFs are characterized and compared by a cross-sectional analysis and electrical conduction tests. Effects of exposed surfaces of conductive particles in NS-ACFs and APL ACFs are determined and compared with the conventional ACFs and conventional nanofiber ACFs. Finally, effects of suppression of conductive particle movements on interconnection reliabilities are discussed.