Investigation into Impact of Redistribution Layer Design on Thermomechanical Stress in Embedded-Die Package

Abstract

Thermomechanical stress generation on a silicon chip in an embedded-die package (EDP) was investigated. Change in thermomechanical stress on the chip with temperature was evaluated using piezo-resistance gauges fabricated on the chip. To investigate the impact of a redistribution layer (RDL) on stress generation, an EDP where filling materials facing the top and bottom sides and the periphery of the embedded die were entirely removed was fabricated. The removal of filling materials was carried out by using the CO2 laser ablation technology. RDL having diagonal paths from the substrate to the chip was designed and fabricated in addition to the conventional orthogonal RDL. RDLs were made of copper. Temperature tests were carried out in the range from 260 degrees C to 100 degrees C. The experimental results indicate that, while the origin of thermomechanical stress is a mismatch in coefficients of thermal expansion (CTE) between the chip and the organic substrate, RDL plays a significant role in generating the thermomechanical stress on the chip. The results also show that the diagonal RDL design effectively reduces thermomechanical stress from the orthogonal RDL design owing to its spring characteristic.