Elimination of polyimide stress buffer on integrated circuits using advanced packaging materials

Polyimide is typically used as the final layer in silicon technology process integration. Its primary purpose is to protect the topside structures and relieve the interface of stresses introduced during and after encapsulation. However, developments in mold compound technology as well as in wafer fabrication techniques have caused the industry to re-evaluate the need for a polyimide stress buffer. Mold compound fillers have become finer and more spherical, reducing particulate pressure loading from the fillers on the die top surface. Additionally, the use of CMP in wafer fabrication reduces topographical variations, which result in less stress points on the die surface., This paper presents the evaluations that were conducted to assess the continued use of polyimide, and the effort made to eliminate if from some microelectronic packages. Moisture characterization data for several MAPBGA packages are included and package performance without polyimide is assessed. It was concluded that the removal of polyimide from these devices does not significantly affect the yields, but more work needs to be done to realize the limitations.