Optimal Design of Passivation/UBM Openings for Reducing Current Crowding Effect Under Electromigration of Flip-chip Solder Joint

Abstract

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 of dimensions 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 under bump metallization (UBM) consumption 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, since the MTTF may be doubled when the local current density is reduced to half of its original value. 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. Several dimensions, radius of Al pad, passivation opening and width of Al trace, in this model were varied to analyze the current density distribution. The UBM opening is 60 mu m. The width of Al trace, the radius of Al pad and the passivation opening were varied from 50 mu m to 100 mu m by 10 mu m, 60 mu m to 120 mu m by 10 mu m, and 12 mu m to 48 mu m by 3 mu m respectively. The most effective design to relieve the current crowding effect which was found in this study is to fix the ratio of the radius of passivation opening and UBM opening between 0.5 to 0.6. The ratio is relative to the width of Al trace and the radius of Al pad. The ratio is close to 0.6 with a larger radius of Al pad and a narrower Al trace. On one hand, the ability of relieving current crowding effect is only relative to the radius of passivation opening when the ratio is less than 0.3. On the other hand, the ability of relieving current crowding effect is also limited by the radius of the Al pad. The wider the Al trace is, the lower the current crowding will be. When the radius of Al pad is larger than 100 mu m, the change of Al trace width makes no influence on relieving current crowding effect. The influence of these dimensions, width of Al trace, radius of Al pad and passivation opening show saturated condition. This approach facilitates the systemic study of optimized design to relieve the current crowding effect and thus increase the EM resistance of solder joints. In addition, the results provide a guideline for optimal design for solder joints with a specific UBM structure.