<title>Fracture mechanics approach for flip chip BGA design</title>

Abstract

Die cracking and interfacial delamination are of major failure modes in IC packages. To these crack-related problems, design approaches based on traditional strength theories, are inadequate and insufficient. To achieve a more reliable and robust design, a fracture mechanics approach is needed. In this paper, fracture mechanics is applied to flip chip BGA design, assumed that the major potential failure mode is cracking of die from its backside. Fracture mechanics is integrated with the finite element analysis (FEA) and design of virtual experiments to analyze the effects of location and length of a die crack, and the effects of some key material properties and package dimensions on flip chip BGA reliability in terms of die cracking. The stress intensity factor and the strain energy release rate, are used as the design indices. The FEA is used as a numerical tool to calculate the fracture parameters. And the virtual DOE is employed to determine contributions of each design parameters to die cracking and their acceptable design windows. The investigation consists of three parts. The first is relations of length and location of a die crack with the fracture parameters. The relations are established through sweeping along crack length for a crack located in the center of die backside, and along the die backside surface. The critical crack length is determined. The second is the virtual DOE based on fracture mechanics. Several key material properties and cracking are calculated. From it, some generic design guidelines are made. The third part compares the virtual DOE design results between using the maximum normal stress (MNS) theory and using the fracture mechanics approach. The comparison gives a clear picture of the applicable range of the MNS theory. It is concluded that design optimization be a must in order to achieve a robust package design. Substrate and die thicknesses are the two most significant factors to die cracking of flip chip BGA. Increasing substrate thickness and reducing die thickness are the most effective measures to design a package with high resistance to die cracking. The fracture mechanics approach will produce more accurate design result than the MNS theory.