Thermo-mechanical analysis of flexible and stretchable systems

This paper presents a summary of the modeling andtechnology developed for flexible and stretchableelectronics. The integration of ultra thin dies at package level, with thickness in the range of 20 to 30 μm, into flexible and/or stretchable materials are demonstrated as well as the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. These technologies can achieve mechanically bendable and stretchable subsystems.The base substrate used for the fabrication of flexiblecircuits is a uniform polyimide layer, while siliconesmaterials are preferred for the stretchable circuits. The method developed for chip embedding andinterconnections is named Ultra Thin Chip Package(UTCP). Extensions of this technology can be achievedby stacking and embedding thin dies in polyimide,providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the relatively rigid polyimide by a soft and elastic silicone material. We have shown through finite element modeling and experimental validation that an appropriate thermo mechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages.