Investigating moisture diffusion in Mold Compounds (MCs) for Fan-Out-Wafer-Level-Packaging (FOWLP)

Abstract

Fan-Out Wafer-Level-Packaging (FOWLP) has an increased interest because of its lower cost substrate-less and lower footprint driven by the need for higher-density, higher-bandwidth chip-to-chip connections. However, many challenges are faced during process steps, such as reconstructed wafer warpage. It occurs mainly during polymers curing and when temperature changes during the process, due to the mismatch in the coefficient of thermal expansion (CTE) of the constituent materials. Furthermore, when moisture diffuses into polymeric materials, it induces hygroscopic swelling strains and therefore it changes the original stress condition and warpage of the wafers. Large warpage of the reconstructed wafers leads to difficulties in wafer handling and tool limitations.

This study aims to investigate moisture induced warpage of two Mold Compounds MCs. weight measurements were performed to assess moisture during humidity absorption and desorption tests. These tests showed a dual stage moisture diffusion. A Sequential Dual Fickian (SDF) model where Fick's law was assumed to act in two subsequent stages was used and showed a good agreement with the experiment.

Moreover, hygroscopic swelling of the MCs was investigated by measuring warpage of a silicon-MC bilayer samples during moisture absorption experiment. Finite Element analysis of moisture absorption using the so-called thermal-moisture analogy and coupled thermal-structural simulation to predict warpage due to CTE mismatch and moisture. The coefficient of moisture expansion was estimated using warpage measurement data.