High-Density Flexible Substrate Technology with Thin Chip Embedding and Partial Carrier Release Option for IoT and Sensor Applications

The paper describes the fabrication of high-density flex circuits based on wafer level redistribution technology. The systems are build up by sequential processing of polyimide layers and semi-additive structured metal layers on a temporary carrier wafer. With the final removal of the carrier only 20-50 µm thick flexible circuit layers are generated. The technology allows multi-layer routing with up to three levels having line pitches of 30 µm and 35 µm vias in staggered configuration using the standard process by photo structuring of the polymer. With an advanced process, line pitches of 14 µm with vias of 10 µm diameter in stacked configuration are possible using laser structuring of the polymer. As additional features, thin active ICs can be embedded into the flex layers and with a special release sequence a partial rigidness of the flex can be obtained. A 24 GHz radar module was fabricated as first demonstrator of the described flex technology featuring two embedded only 20 µm thin transceiver ICs per module. Due to a partial carrier release a rigid-flex configuration was obtained, which allows the antenna arrays of the module to be tilted relative to the center part of the module where the transceiver ICs and IOs are located. A vibration and stress sensor module was fabricated as second demonstrator with two only 20 µm thin stress and acceleration sensor chips integrated into a flex circuit. The flex with the acceleration sensor overhangs the rigid part of the module, which allows the detection of oscillations with high sensitivity.