A Quantitative Characterization of the Crack Driving Force of the Popcorn-Type Failure in Mold-Underfilled Package Under Hygro-Thermo-Vapor Loads by Finite Element Based Fracture Mechanics

This study investigates the combined effects of hygroscopic expansion, thermal expansion and vapor pressure on the reliability of molded underfill flip-chip (MUF FC) packages using finite element analysis. The moisture concentration distributions during MSL 1 preconditioning and the reflow process were first determined. Moisture diffusion under varying temperature conditions during reflow was simulated using the internal source approach. Fracture behavior at the MUF/die interface, specifically at the interface crack tip and the die corner, was analyzed for different crack lengths based on fracture mechanics. It has been found that vapor pressure loading becomes the dominant driving force for interfacial crack propagation and deflection at the die corner when the crack extends to a certain extent. Under vapor pressure loading, the Mode I driving force is predominant at the interface crack tip, while under moisture and thermal loads, the crack driving force is influenced by both Mode I and Mode II components. In addition, the direction of crack deflection at the die corner, which is predicted using the maximum tangential stress (MTS) criterion under the vapor pressure loading condition, is significantly different from that under moisture and thermal loads. In practical engineering, the cracking angle can serve as a key factor in analyzing the primary types of loads that lead to structural damage and device failure. The conclusions derived from the study enhance the understanding of popcorn failure mechanisms in electronic packages and provide valuable insights for research related to integrated circuit (IC) reliability.