Principal mechanisms that determine the local stress distribution in stacked chips mounted by flip chip technology

The mechanism determining the local distribution of residual stress in stacked chips mounted by flip chip technology was analyzed using a finite element method. The average residual stress in two stacked chips changes according to the distance from a bending neutral axis of the stacked structure. The local residual stress also varies drastically depending on the relative position of bumps between the upper and bottom interconnection layers. The final residual stress distribution in stacks of more than two chips is determined by three independent factors, caused by the different properties of the materials used in the stacked structure. It is very important, therefore, to optimize not only the thickness of a chip and the other structural factors mentioned above, but also the development of bumps or underfill materials to minimize the amplitude of the periodic stress.