Robust and reliable encapsulation of electronics for underwater applications

Abstract

The market increasingly demands remote monitoring sensors and electronics for safety-related industrial equipment, transport vehicles and also building structures. In these applications the robustness of the electronics and the reliability of the packaging technology are key factors. Reliability requirements are for electronic modules are derived from environmental and operational loads. This leads to general requirements for the system concept to ensure their operation and thus system reliability. Most frequently the focus is set on thermomechanical design issues. The presented work however includes, investigations for a different loading scenario - long-term use under constant media exposure in a maritime environment in seawater. The application scenario is monitoring of the foundation structure for offshore wind turbines. Specific requirements to protect the sensors and sensor electronics from the surrounding medium sea-water and permanent pressure load of 3 to 6 bar in the application of water depth between 20 to 50 meters. Other environmental conditions are occurring minerals and microorganisms which can attack the package system in long term. The permanent installation of electronics requires a system design with lifetimes in the range of the test structure itself, which is set to 10 years. The presented work include characterizations of organic potting compounds for this purpose. Emphasis has been put into the materials resistances to media absorption and diffusion properties. Furthermore, aging effects in sea water and fuel mix and their impact towards mechanical stability of the polymer systems are presented. These include displacements of the characteristic glass transition and the variation of the elastic modulus. Unfilled epoxy material has been successfully modified to achieve increased media resistance. Higher polymer cross-linking has been shown to increase resistance. Dynamic-mechanical measurements have shown, that aging causes softening of material and shift of glass transition towards lower temperatures. The general tendency polar polymers are degrading in polar solvents (EP and PUR) according to contained molecular group. Ester group in basic medium, ether in acid medium. Other properties strongly depend on the individual recipe of the polymer product (e.g. hardener system). The investigated polyurethane has shown to be only conditionally useful for encapsulation. Comparatively the PCB materials have shown very good media resistance properties. Overall a full protection with polymer is hardly possible and measures to adjust layout component placement and pcb design are to be done also to ensure longtime reliability. As potential life-reducing mechanisms unwanted material changes through media storage and path formation within the polymer system have been identified. Possible starting areas for crack initiations have to be minimized. This means the reduction of air inclusions or de-lamination potential at interfaces. Based on the results, it is possible to counteract the formation of cracks in encapsulations and select suitable encapsulation materials to effectively resist the diffusion of the medium and constant pressure. Thus, a sufficient lifetime of the functional electronic modules can be reached.