Application of no-flow underfill for a reliable high performance Flip Chip Flex BGA

A Flip-Chip Flex BGA with commercial reflowable (no-flow) underfill is developed in this work for applications in the domain of 700 I/Os, 5 GHz frequency bandwidth and 20 W power dissipation. An encouraging package reliability performance is achieved mainly through package design, selection of appropriate reflowable underfill and optimization of the flip chip assembly process. We focus on a detailed discussion of the various considerations and methodologies adopted in the underfill selection and assembly optimization process. A reflowable or no-flow underfill was selected for this package after a balanced consideration of its moisture test and thermal cycling reliability potential, processability and cycle time. Details of the considerations and characterization methodology are reported. Through Design of Experiments (DOE), a process window for this underfill was identified. The effects of various key process parameters, such as reflow temperature ramp-up rate, die placement force and substrate preheating temperature, are also discussed.