Stress analysis of spacer paste replacing dummy die in a stacked CSP package

Stacked die CSP (SCSP) package has been a popular choice for low cost, low profile devices, especially in integration of memory and memory controller in mobile devices. In some wire-bonded stacked die packages, a dummy die as a spacer is required to provide enough standoff for wire bonding at the bottom die. This design not only introduces extra assembly cost but also creates reliability issues due to the severe CTE mismatch of the dummy die to surrounding components. Henkel Loctite has pioneered using spherical particles filled die attach adhesives as the spacer to replace the dummy die. In contrast to the dummy die design, this invention reduces assembly steps and improves reliabilities. In this paper, stresses in a stacked die package were. studied with finite element analysis. The simulation results showed that with spacer pastes replacing dummy die, stresses on die attach were greatly reduced. Furthermore, the delamination issues of die attach related to molding compound underfilling the recessed area between the stacked die were also reduced.