Effect of Green EMCs on Fatigue Reliability of Molded Cu Wirebond System

Abstract

Replacement of gold (Au) wires by copper (Cu) wires is being widely studied owing to the advantages of copper over gold. Researchers in the past have correlated reliability issues in the Cu wirebonds with higher ionic contamination in the epoxy molding compound (EMCs). New EMCs with 5ppm contamination or less are being developed to address this issue. Green EMCs often have very high filler content which reduces coefficient of thermal expansion (CTE) and increases the modulus (E) of EMC. Higher CTE mismatch between EMC and Cu wires can cause accelerated fatigue failures. In this paper, a novel approach is presented to address this issue. Quad flat no leads (QFN) packages bonded with Cu wires were molded with variety of EMCs. The parts then were scanned using X-ray CT system and the scanned data was converted into FE platform. This 3D model includes all the details and represents true geometry of the package. Thermal cycling analysis was performed on the packages to study effect of different EMCs on the reliability of Cu wirebonds. Plastic equivalent strain $( \varepsilon _{\mathrm {p}})$ was extracted in each case to assess the damage accumulated on the critical wire due to thermal loading. The new methodology presented in the paper allows true assessment of the accrued damage using the true 3D geometry.