New approach for assessment of dielectric layer adhesion strength, demonstrated on eWLB FO-WLP interfaces

In our research, stud pull and shear tests are employed to obtain a quantitative measure of thin film adhesion between mold compound fan-out area and dielectric films in embedded wafer level ball array (eWLB) ICs. This package design uses thin dielectric films as stress buffers to reduce the stress between the second level connections and both the silicon passivation layer and mold compound surfaces. The interface strength or adhesion is typically a difficult physical property to quantify, and in most cases it is not specified by the material manufacturer. Due to the discrepancy of material properties between the different interlayers, adhesion failure is a reliability problem. Therefore, characterizing the influence of different dielectric formulations and process conditions on the interfaces strength is fundamental for a robust package and process. In order to assess these two tests, a set of factors is changed and its impact on film adhesion verified. In the end, the selected methods have been successfully used in the field and showed to be a valuable process instrument for both product development and process monitoring.