Next generation thin film polymers for WLP applications and their mechanical characterization

Thin film polymers are well known in application for wafer level packaging. A couple of different classes of photo-sensitive polymeric materials are available for the integration, for example: Polyimide (PI), Polybenzoxazole (PBO), Benzocyclobuten (BCB), Silicones, Acrylates and Epoxy. There are over 40 different commercially available polymers which have related to their formulation different characteristic like processing, material properties and reliability [1]. In the last years a lot of new polymer formulations are entering the marked. This "next generation polymers" promise optimized properties which are adopted for the advanced WLP and 3D applications, like ultra-thin chips, chips stacking, fan-out packaging, chip embedding and panel level packaging. This paper takes closer view on some of the application-driven polymer trends like low stress, low CTE, low cure temperature, stress buffer effect and high voltage toughness. Based on these trends the characterizatio n of the mechanical and electrical properties of the polymers will be discussed in details. The characterization of the polymer properties is important for assist the polymer developments and also for the implementation of the material into simulation models which is essential for new packages. But the accurate and the reliable estimation is quite challenging because the thin film polymers are typical generated on a carrier. Especial for the mechanical property estimation the rigid silicon substrate could influence the measurement results of the much softer polymer. To avoid that influence a release from the substrate is necessary to measure the free-standing polymer film. The thickness of a polymer film is typically in a range from 5 to 20µm where the handling of free films is a challenge.