Polymer elastic bump formation through photodefinable thermal reflow process for system in foil package

Abstract

In this study, hemispherical polymer elastic bumps with 200 μm diameter were formed using a highly heat-resistant photodefinable polymer to realize a system in foil package. Because polymers have a more temperature-dependent modulus than a metal, a dynamic mechanical analysis (DMA) test was performed to analyze the modulus of the polymer. The results showed that the storage modulus of the cured photodefinable polymer used for producing the polymer elastic bumps was ~3.5 GPa at room temperature, and the glass transition temperature (Tg) was 210 °C–220 °C. A single polymer elastic bump package was modeled by using the modulus values obtained from the DMA test, and the deformation under loads was analyzed using the finite element method and compared with the results for conventional copper bumps. The results showed that the bump coplanarity can be corrected easily in hemispherical polymer elastic bumps because they show large z-axis deformation under loads. The test also showed that the stress on an ultrathin chip (thickness: 20 μm) is less than half that on copper bumps. In addition, the deformation behavior depending on the metal wiring configuration on the polymer elastic bumps was analyzed under different temperature and load conditions by using a microtribometer. The analysis of the contact resistance on a single polymer elastic bump verified that the polymer can be used as a material for bumps.