3D fracture mechanics analysis of underfill delamination for flip chip packages

Abstract

In flip-chip package, the mismatch of thermal expansion coefficients between the silicon die, copper heat spreader and packaging substrate induces concentrated stress field around the edges and corners of silicon die during assembly, testing and services. The concentrated stresses result in delamination on various interfaces involving a range of length scales from hundreds of nanometers to millimeters. Among these failures underfill delamination is a dominant failure mode. In this paper, a full parametric 3D model of flip chip package with heat spreader is developed with the capability of explicit modeling of 3D cracks. The crack driving force is computed as the functions of underfill properties including coefficient of thermal expansion and Young's modulus, as well as underfill fillet dimensions. The impact of different shapes of crack front is also investigated. The results show that underfill properties need to be optimized to minimize the occurrence of underfill delamination at the die corner. The results also show that there exists an optimal range of underfill fillet height to balance the manufacturability and reliability.

Flip-Chip (FC) and Fine-Pitch-Ball-Grid-Array (FPBGA) Underfills for Application in Aerospace Electronics—Brief Review

“…Predictions of the modulus from their theory were compared with experimentally measured moduli, and excellent agreement has been observed. Xu et al [115] employed fracture mechanics J-Integral method for the evaluation of the possible underfill delamination from the FC; Caruthers et al [116] suggested a rigorous approach for modeling the behavior of glassy polymers used as encapsulants; Adolf et al [117] suggested a nonlinear viscoelastic model for glassy polymers; Zhai et al [118] analyzed possible underfill delamination in FC packages and suggested a way to minimize its possibility; Wan et al [119,120] reviewed advances in modeling the underfill process and have indicated the critical clearance (space) between the adjacent solder bumps as an important feature of a FC design; Zhang et al [121] carried out a 3D fracture mechanics based analysis of underfill delamination; Adolf et al [122] suggested a simplified potential energy clock model for the behavior of glassy polymers; Suhir et al [123] suggested a simple formula for the evaluation of the size of the inelastic zone at the peripheral portions of a soldered assembly; Celina et al [124] (Sandia) analyzed Zymet underfill epoxy material, and Wyatt and Chambers [125] (also Sandia) reported on a comprehensive materials analysis and modeling of underfill materials. The thermal-mechanical properties of three potential underfill candidate materials for FPBGA applications were characterized.…”