An investigation into the fracture of silicon die used in flip chip applications

Abstract

In this investigation, the fracture strength of silicon has been measured as a function of die thickness, crystal orientation and die surface treatment using a four-point bend test method. The influence of minute surface flaws or divots generated from a die singulation process has also been quantified. The amount of silicon surface damage sustained in typical IC post-fabrication processes is then estimated using a simplified fracture mechanics approach. Results show that fracture strength does not depend on crystal orientation or thickness, but that it is highly dependent on the amount of surface damage present. In addition to the fracture strength measurements, finite element models have been employed to predict the amount of stress generated in a flip chip die for a given design. A parametric study has been performed to look at the influence of die thickness, substrate thickness and underfill properties (elastic modulus and coefficient of thermal expansion) on maximum die stress. Results show that the level of stress in die assembled to organic substrates is much greater than in die assembled to ceramic substrates. Stress levels determined from the finite element models are then compared to the silicon fracture strength for a given backside treatment in order to predict the likelihood of fracture.