Evaluation of probing process parameters and PAD designs: Experiments and modelling correlations for solving mechanical issues

Electrical Wafer Sort is known to induce stress in the pad structure and can lead to mechanical failures. In the present work, both EWS process (over drive and number of passes) and die pad design (thicknesses and interconnect layer architectures) parameters are evaluated through actual tests and Finite Element Modelling. Thus, for the experimental tests, a dedicated design of experiment is set up. With the help of pad integrity tests and comprehensive failure analysis, including novel method for 3D imaging reconstruction, the effect of the parameters is evaluated and the interconnection structures are ranked according to their mechanical robustness. Hence, the root causes and locations of the cracks are found. Secondly, a 3D dynamic model is developed to reproduce the probing test. After a preliminary calibration step, process test and pad design parameters are weighted. Through stress analysis, a ranking according to the mechanical robustness of the pad is also established and compared to experimental results. A good agreement between experimental and numerical results is found for most of the parameters showing the ability of the numerical model to reproduce EWS and forecast and lower failures. Thus, the increase of the overdrive is shown to increase the fracture hazard. On the contrary, the increase of the aluminium and pad open oxide thicknesses allows reducing the crack occurrence. The metal design also has a significant effect on the pad robustness. Nevertheless, the current numerical model has few limitations, whose reasons are described, particularly for the prediction of the effect of the number of passes and for specific metal designs. Finally, the modelling has shown its ability to fairly reproduce the probing. Thus, this paper is helpful to reduce the reliability issues in the pad structures and increase yield. Indeed guidelines for pad structures and fine tuning of probing process parameters are proposed.