Prediction of Crack Initiation at Die Corner of Molded Underfill Flip-Chip Packages Under Thermal Load by New Criteria

The present study provides solutions for the stress and strain fields near the junction formed at the intersection of the inclusion and matrix interfaces under thermal load. It further explores how the non-singular stress terms influence the stress field near the corner of the interface between the molded underfill (MUF) and the die in a typical flip-chip (FC) package subjected to thermal load. The results obtained indicate that achieving a comprehensive understanding of the stress field requires consideration of the dominant singular stress terms, the regular stress term (the first non-singular stress term) and the I-stress term (the second non-singular stress term), so as to offer a theoretical basis for accurate evaluation of the reliability of integrated circuit packages.

I. Introduction

The past decade has seen rapid development and adoption of advanced packaging technologies to achieve the heterogeneous integration (HI) of separately manufactured components and devices into a sophisticated assembly system with enhanced functionality and operation performance. In advanced electronic packaging, polymeric materials have been increasingly used to facilitate the process, improve the reliability of package structures and reduce the manufacturing cost, such as the underfill in flip-chip (FC) and ball grid array (BGA) packages, polyimide in redistribution layers (RDLs) for fan-out wafer-level package, and epoxy molding compound (EMC) as encapsulation of chips and packages. Obviously, electronic packages typically comprise bonded materials with different thermal and mechanical properties. Sharp interface corners are often present in the package structures, which can generate high-stress gradients owing to a change of temperature or under mechanical loading, leading to microcracking and eventual fracture [1], [2], [3], [4]. Heretofore, research on the interface reliability of electronic packages has mainly focused on the cracking problems at the corners of the package structures assuming the presence of cracks at the potential failure corner of the model (structure) and applying fracture mechanics theory to calculate the fracture parameters (or the driving force of crack propagation and fracture, such as the strain energy release rate, stress intensity factor and J-integral) as a basis for selecting materials and optimizing the design of the packages [5], [6], [7], [8]. However, in the case that the interface bonding is strong, cracks may initiate from the bulk material rather than at the interface. This requires a thorough stress analysis of the entire neighborhood of the interfacial corners. Thus, accurate formulation and characterization of the stress fields at these corners are critical to accurately evaluate the reliability of electronic packages. It should be noted that the corner points represent stress singularities near which the finite element method (FEM) often encounters convergence issues. Solving the problem requires the adoption of specialized techniques to improve these solutions.