Trend of solder-less joint in flip chip bonding

Abstract

Information devices have recently shown incredible advancements in miniaturization and increased functionality. This has been supported by high density surface mounting technology, build-up technologies for multi-layer boards, the miniaturization of mounted components, advancements in chip mounting equipment, and other innovations. This paper looks specifically at flip chip bonding technology for mounting bare chips directly to the substrate. There are two types of flip chip bonding. One utilizes a metallic bond for the electrical connection, most commonly some sort of solder joint, but always using a metal-to-metal bond such as Sn-Au, conductive paste, or an ultrasonically welded Au-Au joint. Using solder, however, has problems associated with environmental concerns, as well as technical limitations on the usable pitch. The second general method of flip chip bonding uses bumps and an adhesive. In this paper, we term this "mechanical bonding." It is forecast that the usage ratio of mechanical bonding will increase as pitches become finer and the drive to eliminate underfill increases. Mechanical bonding can be divided into four types: ACF (anisotropic conductive film), NCF (nonconductive film), ACP (anisotropic conductive paste), and NCP (nonconductive paste). Advanced ACFs are now in wide general use and are used with many types of electronic equipment. This is a direct result of improvements in reliability and the adoption of ACF in the LCD industry. An example of the reliability of ACF manufactured by Sony Chemicals Corporation is given. This paper also introduces other applications of ACF in the electronics industry.