Investigation of hygrothermally induced failures in multilayer ceramic capacitors during thermal reflow process

Abstract

The reliability issues of hygrothermally induced defects during thermal reflow of multilayer ceramic capacitor was investigated to determine the root causes and propagation mechanism of the defects. Samples of the capacitor package were subjected to JEDEC preconditioning Level 1 (85 °C/85 %RH/168 h) with 5 times thermal reflow at 270 °C. The reflowed capacitors were inspected under scanning electronic microscope (SEM) for possible evidence of defects. The reliability test was consecutively simulated using the XFEM framework, which is validated using the experimental findings. From the results, two reliability defects, voids and interfacial delamination, were observed at the Cu/Cu-epoxy interface of the capacitor assembly. The voids width sizes were in the range of 0.15 μm and 0.55 μm, and the interfacial delamination ranges from 0.05 mm to 0.21 mm in length. The biggest crack recorded is about 3.56 μm thick and it was obtained at post-reflow stage, when the defects had propagated. Two root causes, moisture contamination and thermal mismatch, were established for the defects. The absorbed moisture contamination, during preconditioning, generated 6.23 MPa vapour pressure, which exerts stresses that exceed the 6 MPa adhesion strength of the adhesive/substrate interface. Also, thermal mismatch due to the high difference, of approximately 5 times, in the CTE of Cu and Cu-epoxy layers will result in uneven deformation rates of their layers during thermal expansion, thereby leading to de-bonding or voiding of the layers' interface. Generally, the deformation and stresses along the boundary of the formed crack increases with the size of the initial microvoid. The numerical simulation predictions agree very well with the experimental results obtained through SEM inspections, which validates the research methods as an effective prediction scheme for investigating the reliability of capacitor package.