Comparable Study for Redistribution Layers in FO POP RDL First and Last (Fan-Out Package on Package)

Redistribution layers (RDL) based fan-out package on package (FO-POP) have been developed for high-end mobile application processor (AP) to meet ultra-thin package height with high bandwidth and improved electrical performance. RDL first fabrication flow, enable RDL processed on carriers to achieve higher yield performance and ensure the placement of known good die on known good RDL interposer. For RDL last, chips are attached on carriers before RDL, no bump reflow and under-fill encapsulation are required for face up dies, thus overall assembly process steps can be reduced. The challenges of RDL process in FO-POP RDL first and RDL last include: optimized bottom and top RDL stack-up structure to optimize wafer warpage behavior (reduce potential stress) and enlarge process window. Besides, the optimized thermal management of bottom/top side RDL layers are critical for good RDL adhesion and robust interconnection for embedded die. In this investigation, an ultra-thin FO-POP Test Vehicle with package size 240 mm^2 and overall package height less than 400 μm (without DDR) is presented. The multiple RDL layers performance including warpage behavior, line width variation, and leakage post bottom fine RDL processes are compared for RDL first and last fabrication flow. The interconnection between embedded die and RDL layer is also studied for both manufacturing process flow. Both RDL first and last study legs have been evaluated by employing package reliability test following JEDEC include thermal cycling test, high temperature storage test and un-bias HAST test. The goal of this investigation is to compare RDL performance of two different FO-POP fabrication flow and package reliability performance to meet high-end mobile application requirements.