Mechanical integrity evaluation of low-k device with bump shear

The mechanical integrity of low-k dielectric films has brought many process challenges in both front-end integration and back-end assembly, mostly due to possible interfacial delamination and fractures within the low-k films. From a packaging point of view, it is important to have an assessment of the integrity of the low-k stack before the device is fully assembled and the time-consuming full package evaluation is started. Some of the methods that are presently used to evaluate devices with low-k films either do not reflect the real stress situation in a package (such as 4-point bend), or introduce a mixed die-solder failure mode (such as die pull), which makes the results hard to interpret. In this paper, an evaluation method using solder bump shear is introduced. The solder joints are electroplated with a Cu stud as part of the under bump metallization. When the testing parameters are carefully optimized, bump shear can induce a failure in the low-k stack. By analyzing the maximum load of the shear test and the characteristics of the load curves, die with different interlayer dielectric materials and locations on the die with different interconnect metal densities can be effectively differentiated. A finite-element model is established and fracture mechanics methodologies are utilized to interpret the results of the bump shear.