The increasing demand for high performance integrated circuit devices has led to the development of multi-die stacking in a single package to improve functionality without increasing package form factor. Furthermore, there is tremendous pressure to decrease overall package height even with the additional dies stacking. Stacking same-sized dies directly on top of each other provides a unique challenge since there can be no clearance for the lower die bond wires. The solutions include (1) dummy silicon-as-spacer, (2) die attach paste-as-spacer and (3) die attach film-as spacer. Key requirements for the die attach (DA) paste or film is excellent adhesion to the passivation materials of adjacent dies. Also, a uniform bond-line thickness (BLT) is required for a large die application. In addition, high cohesive strength at high temperatures and low moisture absorption are preferred for good reliability. This paper focuses on the key DA materials challenges and resolution associated with the same-sized dies stacked packages qualification. The DA materials assessment includes both paste and film-based adhesive. Packages were built to understand how these DA adhesives would work in-package and study its reaction to the assembly process conditions.
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