Study of de-gate remnant resolution with high reliability performance moulding compound

Abstract

Ever increasing stringent reliability requirements in semiconductor package demands a mold compound with a property of resisting package crack under stress condition of High Temperature Storage (HTS) at 175°C, compatible with package assembly material such as copper interconnect, High Humidity High Temperature Gate Stress (H3TGS) as well as meeting second level reliability test in Thermal Cycle on Board (TCoB). Such property would require a molding compound that has higher adhesion, higher cross link density and storage modulus in order to provide good protection over the chip and interconnect. These properties, however, would mean a higher adhesion molding compound that results in a harder-to-degate mold area, leaving mold remnant on the lead-frame runner after molding operation and leads to assembly yield loses. Material analysis, extended reliability test, design revision on mold de-gating tool and leadframe, molding parameter optimization are conducted in order to minimize molding runner remnant and subsequently improve molding assembly yield. Extended reliability test of H3TGS at 175°C, HTS 175°C and 6000 cycle of TCoB has been performed with package A to identify the right formulation selection for mold compound and impurities level. Several iterations to the DOE are done throughout the studies. Brainstorming session produced a list of molding parameters and mechanism potentially leading to the mold remnant. Using the button shear test at various temperature, adhesion of various mold compounds at various temperature and contact area are identified to have greatest impact. A design of experiements focusing on cooling station design & cooling time are performed. Additionally, contact area of mold compound to leadframe are also being considered to reduce the remnants rate. It reveals that mold remnant occurrence on package A has failure rate of 60%. By using the selected high performance mold compound on package B, the result is much better with almost no mold remnant observed on the runner area due to much lesser contact. Therefore a correlation of contact area versus remnant rate has been established, and package A leadframe is being redesigned together with the de-gating tool and thus this improve the remnant to 1%.