Polymeric die-attach adhesives have been widely utilized for attaching the sensor die to the lead-frame substrate in the packaging of micro-electro-mechanical systems (MEMS). Although much study has been performed regarding viscoelastic behaviors of packaging materials, less literature has addressed the issue of post-cure shrinkage, which occurs in isothermal storage and in accelerated tests. However, the influence of post-cure shrinkage cannot be ignored in characterizing long-term viscoelastic properties or evaluating long-term reliability for highly sensitive automotive sensors.
In this study, a non-contact digital image correlation (DIC) method was used to characterize the post-cure aging shrinkage on a commercial epoxy-based die attach adhesive. The shrinkage strain of bulk specimens was monitored over a period of isothermal aging. Analysis indicates that the process of postcure shrinkage can be expressed in terms of exponential time functions. Using hygroscopic shrinking as an analogous process, this aging was introduced into a time-dependent finite element (FE) simulation in ANSYS®15.0. FE case studies combining both stress and dimensional relaxation indicate that certain differences can occur in the long-term finite element analysis if the shrinkage strain is ignored.
