The Effect of Filler on the Solder Connection for No-Flow Underfil

Abstract

Miniaturizing and lightening of flip chip packages are now being evaluated because high performance and high density of semiconductors are rapidly advancing. Furthermore, various assembly processes are evaluated because conventional assembly process has its limitation for narrow gap and fine pitch. One of these processes is no-flow underfill (NUF) process. Fluxing NUF is applied on the substrate prior to bonding of the IC with local reflow by flip chip bonder (FCB). High density/narrow gap/fine pitch are possible in the area around the chip because NUF is applied on the substrate before bonding IC. In addition, bump connection and material cure can be done at the same time on NUF process, and also the process can be simplified. Underfill is generally blended with inorganic filler such as silica filler due to its high reliability. It is not an exception on NUF either. Blending filler is indispensable. But as for NUF, defect of solder connection by filler penetration into solder is considered. This paper presents the evaluation of the filler effect on the solder connection for NUF. At first solder connection was confirmed using NUF with/without silica filler. Even if it contained the filler, we found out that good connection was obtained. After grinding the surface, we observed the connection between the solder and the pad. Filler existed inside the solder connection. Furthermore when the amount of filler loading is increased into NUF, the amount of remaining filler at the connection is increased, too. Next we focused on the remained filler on the connection between the solder and the electrode; we confirmed the effect of interface strength by filler left on the contact. As the amount of remained filler was increased, there was a tendency that shear strength of the connection was degraded. We found that controlling filler is important for getting good connection and high shear strength of connection. So we evaluated the effect of shear strength of connection and connectivity by filler size and surface treatment of filler. Finally, the relation between the quantity of remained filler and reliability of connection was evaluated by reliability test. As a result, condition of including filler with NUF was optimized.