Blog posts

JOM is a monthly publication of TMS--The minerals, metals, and materials society. It covers a wide range of materials topics. I expecially like the overview articles, which, in four or five pages pack lots of information. Further, the historical articles about metallurgy and materials in ancient civilizations will interest those of you who like to read about history in general, and science history, in particular.

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.