Miniaturized Sensor Modules for under Water Applications realized by Printed Circuit Board Embedding Technology

Autonomous robots for survey and control tasks are increasingly used in environments which can not or not easily be explored by humans. The autonomy of such platforms is based on representation or mappings of the local environment into which the robot is immersed. Advanced autonomous systems in natural environments typically record, evaluate and finally use a multitude of sensor data, e.g. for navigation or sample retrieval. Depending on the type of parameter a single sensor may be sufficient, but more often arrays of the same sensor type are distributed over the robot in order to provide the necessary data. As an example of a distributed sensor network we describe in the present paper the skin of an autonomous underwater vehicle. The vehicle is inspired by a manta ray: size and locomotion are the bionic copy of a ray. The skin of the vehicle is a textile which is permeable for water and spanned over the internal skeleton of the vehicle. The aim of the project "Roboskin" is to equip that skin with an array of sensor nodes to detect touch, nine axis-orientation, and pressure. The touch sensors, three on each wing, form the human machine interfaces [HMI] for the control of the vehicle by a diver. Orientation sensors are used for the self-representation of the robot movement and pressure sensors do monitor pressure distribution inside the vehicle while moving. Additional large area coils around the circumference of both wings are used for the detection of metals, which are related to the use case of the robot. The skin is equipped with a four wire bus structure stitched onto the skin of the robot. Two wires are for data (controller area network (CAN)-bus) and two for energy transmission. Touch sensors and metal detectors are likewise realized by stitching respective coils onto the skin. At defined positions "sensor housings" are joined up in the bus structure by plug connectors and mechanically fixed to the skin. They contain a connector structure for the miniaturized sensor nodes which are mounted into the housing and subsequently encapsulated with a lid. In order to realize as small as possible and highly robust sensor nodes, the electronic build up was realized using embedding technology of components into a printed circuit board. This allows the highest possible miniaturization of a heterogeneous system (i.e. non-monolithic), a high flexibility for design/fabrication modifications and an optimized protection of the electronic components. The resulting sensor node can easily be integrated into the housing, which is finally flooded with a protective grease to prevent water do diffuse into the electronics. The system specification of the device requires high density routing in a four layer printed circuit board build-up. Layer interconnects therein are a mixture of mu-vias and plated through holes. A single embedded layer contains more than 20 passive components with various package types and a number of active components like mu-controller, orientation sensors, and CAN-transceivers. All components are soldered onto the core prior to embedding into the printed circuit board by lamination. The embedded components were positioned with smallest possible spacing and are thus packed very densely. A particular challenge therefore is the configuration of the prepreg stack around the components, having different heights depending on component type, prior to the embedding. After lamination through holes and blind vias are drilled and plated, followed by photolithographic structuring of the copper. Finally onto the top side of the embedded module the pressure sensor is mounted as a single component on the outside of the embedded module. The yield of a typical fabrication run of the embedded sensor nodes is after ramp up above 90 %. Summarized this paper shows the potential of embedding technology towards maximum miniaturization and high robustness for a highly challenging application. Design rules for highest integration are provided and a brief description of target application including sensor data gained is given.