High Density Thin Film Flex Technology for Advanced Packaging Applications

The paper provides a latest technology review for fabrication of high-density thin film flex and related application examples. The fabrication approach utilizes the well-established processes from wafer level packaging and redistribution technology. Multi-layer routing structures are built by sequential processing of polyimide and semi-additive metal on a carrier wafer with a dedicated release layer. Depending on the number of copper routing layers with typical thickness of 3-5 μm and the required polymer inter dielectric thickness the final multi-layer stacks can have total thicknesses between 20-60 μm. A laser assisted process allows an easy detach of the thin film flex circuits from the carrier wafer. The technology allows high density routing with lines and spaces of 7/7 μm over up to four layers, custom specific front and back side contacts of Au, Cu, NiAu or solder (only front side) to enable different assembly technologies, embedding option for ultra-thin ICs, option for partial rigidness by utilizing the carrier as permanent stiffener as well as option for arbitrary shaped outlines.Three application examples for the high-density flex technology are shown. The first one is a thin film flex with embedded ICs, which is a technology demonstrator for usage as system core of cardiac monitoring patches. The second example includes rigid flex demonstrators with embedded ICs for implementation of 24/60 GHz radar sensor devices. And as a third application case thin film flex is used as polymeric substrate with front and back side contacts. Those thin substrates can be used as interposers or can be stacked on top of each other to enable a multiplication of routing layers. All details regarding the different technology options as well as application examples will be discussed in this paper.