Low-Stress Interconnection for Flip Chip BGA Employing Lead-Free Solder Bump

Flip chip bonding technology has been widely used for interconnection in high-end logic LSI employing lead-rich solder bumps. Recently, from an environmental issue, it is desired that the lead-rich solder should be replaced by lead-free solder. However, the stress at the interconnection after flip chip bonding reflow cannot be relaxed with lead-free solder bumps because of their poor creep properties. Since the stress causes delamination of the low-k layer under bumps and electrical open errors, the improvement of the solder bump material and the flip chip bonding process have been necessary for stress relaxation. In this study, we investigated the creep properties of Sn-0.7Cu and Sn-3.5Ag bumps by the indentation method. As a result, it was found that the creep properties of Sn-0.7Cu bumps was more suitable for stress relaxation than those of Sn-3.5Ag. Moreover, we confirmed that a low-stress interconnection had been achieved by employing Sn-0.7Cu bumps. The stress at the interconnection was less than the delaminating stress of the low-k layer. In addition, when the flip chip bonding was carried out by the reflow with the post-annealing, in which the temperature was held for a period of time at 200degC during reflow cooling part, the maximum stress in the low-k layer has been reduced by more than 36% in comparison with the low-k delaminating stress. Furthermore, it was found that the stresses at the flip chip joints were relaxed because of the increase of the creep rate which was caused by the reflow with the post-annealing.