Abstract
The root causes of pad cratering may be divided into two categories, namely, overloading induced pad cratering and fatigue induced pad cratering. The former occurs when the interfacial stresses between the epoxy resin and the pad exceed the failure criterion based on the monotonic loading test. The latter happens under cyclic loading conditions, typically with interfacial stresses much lower than the former. Therefore, it is desirable to define an effective failure criterion for the life assessment of pad cratering based on fatigue induced pad cratering. In this research, a fatigue failure criterion for pad cratering is proposed with a correlation between the board level cyclic bending test and repetitive drop test. Time scaling is applied to bridge the cyclic bending test and repetitive drop test, and finite element analysis (FEA) is conducted for pad cratering loading matching to seek the equivalent bending strain of various G-level drop impacts. With the experimental study of cyclic bending tests and pad cratering failure detection, the strain-number of cycle (S-N) curve is able to be defined. This SN curve can also be verified by repetitive board level drop tests, and it means this curve is able to be defined as a common failure criterion for fatigue induced pad cratering. Thus, a universal life prediction method for fatigue induced pad cratering is built. This method can be applied in the design for reliability (DfR). In addition, the repetitive drop test can be replaced by the cyclic bending test under the equivalent loading condition for pad cratering evaluation. It not only provides the faster and simpler characterization of pad cratering fatigue behavior, but also makes the failure able to be real-time monitored.