Reliability Study of the Bump on Flexible Lead for Wafer Level Packaging

摘要

The Bump on Flexible Lead (BoFL) is a chip-to-substrate interconnect technology which uses flexible structures tornaccommodate the CTE mismatch between the chip and PCB substrate and therefore eliminates the need for underfill.rnThe concept of the BoFL is based on wafer level packaging processing methods, such as photolithography, electroplatingrnand wet-etching. The flexible lead consists of a copper redistribution layer embedded in a polyimidernbridge which is located over an air gap. For the BoFL only 3 additional processing steps are necessary, comparedrnto the conventional redistribution technology for wafer level chip size package (WL-CSP). Prototype chips (10 mm xrn5 mm) with different lead shapes were fabricated and assembled on low cost FR-4 boards. The assembled chipsrnwere subjected to thermal cycling (-55℃/+125℃). A big influence of the shape and the orientation of the BoFL onrnthe board-level reliability can be seen. The best results were obtained with the banana-shaped BoFL.