Design of Al pad geometry for reducing current crowding effect in flip-chip solder joint using finite-element analysis

Electromigration of flip-chip solder joints has been studied extensively in recent years. It was investigated in plenty of studies that the current crowding effect takes place at the corner near the traces due to huge differences in cross-section area between traces and solder joint. The local high current density, which has been known as a serious reliability issue, causes the failure such as void formation and the consumption under bump metallization (UBM) to occur much earlier than expected in the current-crowding region in solder bump. As a result, to relieve the current crowding effect can significantly increase the mean-time-to-failure (MTTF) of solder bump. On the base of the Blech's equation, the MTTF may be extended to four times of the original value when the local current density is reduced to half of its original one if the n value is taken as 2. Therefore, finding a robust design of geometry, which is effective and of low cost, has turned into a popular issue. However, there is still no technology can observe the current density directly in a current stressed sample. In order to obtain more precise observation, a three-dimensional finite element model (3D-FEM) was performed to simulate the current density distribution in solder bump. In this study, several voids are designed in the Al pad and distributed as concentric circle shape encircling the passivation opening. With these well defined voids, the maximum current density in solder joint is reduced significantly. For flip-chip structure with 1.5µm thick Al pad, the concentric circular voids could reduce the maximum current density in solder joint by more than 60%. The crowding ratio decreases from 4.03 to 1.72. Even if the Al pad is 12µm thick, the concentric circular voids also reduce the maximum current density by about 35%. The crowding ratio decreases from 2.30 to 1.46. The simulation results indicate the design is effective to relieve the current crowding effect and reduce the maximum current density in flip-chip solder joints. To understand how concentric circular voids influence the thermal distribution of flip-chip structure, a thermal-electric multiphysics model is also performed in this study. The thermal-electric simulation results indicate that the concentric circular void also disperse the local Joule heating effect, which comes from the current crowding effect. This approach facilitates the systemic study of optimized design to relieve the current crowding effect and thus increase the electromigration resistance of solder joints. In addition, the results provide a guideline for optimal design for solder joints with a specific UBM structure.