Development of inclined conductive bump for flip-chip interconnection

In this paper, inclined conductive bumps (ICBs) are proposed as a substitute for the anisotropic conductive film (ACF). The new interconnection method ICBs can provide controlled bump deformation, uniform electrical conductivity, and fine pitch interconnection without short circuit. The ACF is widely used in the chip-substrate bonding process in today's display and semiconductor industries. This is due to the fact that the ACF has various advantages, such as low bonding temperature, low cost, and small packaging sizes, when compared with those of wire bonding. However, as the bump pitch decreases, the short-circuit problem can occur by lumped conductive particles of the ACF between adjacent bumps. In addition, the electrical conductivity of package varies due to the number of the trapped ACF particles between bonded bumps. Various alternatives, such as coating a thin insulating layer at the outside of the particle and utilizing an array of metal pillars instead of the random distributed particles have been attempted to overcome these shortcomings of the conventional ACF. As another alternative, the ICBs, which are the inclined and hollow copper bumps directly fabricated on electrodes, are suggested. In addition, in this paper is the finite element analysis, which has been conducted to predict the elastic-plastic deformations of the ICBs and their reliability issues. Then, the fabrication processes of the ICB is explained. In such process, the ICBs are fabricated on a test wafer with the inclined angles of 70° and 80°. The ICBs are selectively formed on a target pad at a pitch of 30 μm. A singulated chip with ICBs is assembled on an organic substrate using thermo-compressive bonding. After bonding, the ICBs show reasonable contact resistances of 12-27 m Ω.