In flip-chip package, the mismatch of thermal expansion coefficients between the silicon die 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 many interfaces on several levels of structures, in various length scales from tens of nanometers to hundreds of micrometers. A major challenge to model flip-chip packages is the huge variation of length scales, the complexity of microstructures, and diverse materials properties. In this paper, we simplify the structure to be silicon/substrate with wedge configuration, and neglect the small local features of integrated circuits. This macroscopic analysis on package level is generic with whatever small local features, as long as the physical processes of interest occur in the region where the concentrated stress field due to chip-packaging interaction dominates. Because it is the same driving force that motivates all of the flaws. Therefore, the different interface cracks with same size and same orientation but on different interfaces should have similar energy release rates provided that the cracks are much smaller than the macroscopic length. We calculate the energy release rate and the mode angle of crack on the chip-package interface based on the asymptotic linear elastic stress field. In a large range of crack length, the asymptotic solution agrees with finite element calculation very well. We discuss the simplified model and results in context of real applications. In addition, we find that the relation of energy release rate G and crack length a is not power-law since local mode mixity is dependent of crack length a. Therefore, the curve of G~a can be wavy and hardly goes to zero even if crack length a goes to atomically small. The local mode mixity plays an important role in crack behavior.

Abstract

The International Conferences on Nonlinear Mechanics (ICNM-x) have been regarded as important series conferences in mechanics circles. The previous four meetings in the series were successfully held in Shanghai and Beijing in 1985, 1993, 1998 and 2002, respectively. In recent years, new achievements in this field have been made. Therefore, it is appropriate to organize a new conference on this vitally important area of applied mathematics and mechanics. The Fifth International Conference on Nonlinear Mechanics (ICNM-V) will be held in Shanghai. The Conference aims to provide an international forum for presenting the latest results and stimulating wider academic exchange for experts in the related fields all over the world.

In an inaugural article in the latest issue of PNAS, George C Schatz writes on Using theory and computation to model nanoscale properties. Here is the abstract of the article:

I had known Liviu since his early days in the Engineering Science and Mechanics Department at Virginia Tech when I was just beginning my own academic career. I had received my PhD from this department in 1981 in an area (composite materials) that at the time was at the cutting edge of high technology. In 1985 I had come back to VA Tech from the industry to continue working in this exciting area in which the ESM Department excelled world-wide. Liviu had arrived shortly thereafter with an already established reputation as a top-notch scientist.

Active polymers are being developed to mimic a salient feature of life: movement in response to stimuli. Large deformation can lead to intriguing phenomena; for example, recent experiments have shown that a voltage can deform a layer of a dielectric elastomer into two coexistent states, one being flat and the other wrinkled. This observation, as well as the needs to analyze large deformation under diverse stimuli, has led us to reexamine the theory of electromechanics.

In the recent issue of Science, researchers from UCLA (Chung et al) report on an ambient pressure synthesis (using arc melting) of a compound, namely, rhenium diboride, which is superhard. Apparently, the material leaves scratch marks on the surface of diamond. Here is the abstract of the paper:

Recently Henry talked about software that could be used to simulate explosions and introduced CartaBlanca. Luming asked whether anyone had used the software, how good it was, and whether one needed Java to implement models into CartaBlanca.

Here is a video from the Seed magazine called Science in Silico. The video shows results from large scale simulations (and visualization) of fractals, microscopic dynamic processes in ribosomes, structure of viruses, bacterial flagellum, turbulence, explosions, and the modelling of cosmological events.