3D integration of ultra-thin functional devices inside standard multilayer flex laminates

Nowadays, more and more wearable electronic systems are being realized on flexible substrates. Main limiting factor for the mechanical flexibility of those wearable systems are typically the rigid components - especially the relatively large active components - mounted on top and bottom of the flex substrates. Integration of these active devices inside the flex multilayers will not only enable for a high degree of miniaturization but can also improve the total flexibility of the system. This paper now presents a technology for the 3D embedding of ultra-thin active components inside standard flex laminates. Active components are first thinned down to 20-25 mum, and packaged as an ultra-thin chip package (UTCP). These UTCP packages will serve as flexible interposer: all layers are so thin, that the whole package is even bendable. The limited total package thickness of only 60 mum makes them also suitable for lamination in between commercial flex panels, replacing for example the direct die integration. A fan-out metallization on the package facilitates easy testing before integration, solving the KGD issue, and can also relax the chip contact pitch, excluding the need for very precise placement and the use of expensive, fine-pitch flex substrates. The technology is successfully demonstrated for the 3D-integration of a Texas Instrument MSP430 low-power microcontroller, inside the conventional double sided flex laminate of a wireless ECG system. The microcontrollers are first thinned down and UTCP packaged. These packages are then laminated in between the large panels of the flex multilayer stack and finally connected to the different layers of the flex board by metallized through-hole interconnects. The thinning down, the UTCP packaging and the 3D-integration inside the commercial flex panels did not have any affect on the functionality of the TI microcontroller. Smaller SMD´s were finally mounted on top and bottom of the integrated device.