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Reliability Study of the Bump on Flexible Lead for Wafer Level Packaging
Keywords: wafer level packaging, flexible, BoFL
The Bump on Flexible Lead (BoFL) is a chip-to-substrate interconnect technology which uses flexible structures to accommodate the CTE mismatch between the chip and PCB substrate and therefore eliminate the need for underfill. The concept of the BoFL is based on wafer level packa¬ging processing methods, such as photolithography, electroplating and wet-etching. The flexible lead consists of a copper redistribution layer embedded in a polyimide-bridge which is located over an air gap. For the BoFL only 3 additional process steps are necessary, compared to the conventional redistribution technology for wafer level chip size package (WLCSP). Prototype chips (10 mm x 5 mm) with different lead shapes were fabricated and assembled on low cost FR-4 boards. The reliability evaluation of the BoFL covers thermal cycling tests (-55°C/+125°C) and FEM simulations. A big influence of the shape and the orientation of the BoFL on the board-level reliability can be seen. The best results were obtained with the banana-shaped BoFL.
Ines Kolb,
Technical University of Berlin
Berlin G-12555,
Germany


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