A comparison of thin film polymers for Wafer Level Packaging

Polymers are a key building block for all WLP and related technologies like IPD (integrated passives devices) and 3D-SiP (system in Package). 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. A list of commercially available polymers will be compared in this paper with the focus of processing, material properties and reliability. Curing and the resulting shrinkage play an important role due to temperature sensitive devices and the topography of the metallization which have to be passivated. Test structures have been designed to measure planarization with respect to different feature sizes. In addition Cu compatibility is very important to polymer processing to avoid Cu migration. The mechanical properties have a strong influence on the reliability of non-underfilled WLP. There are different failure modes using different polymers for WLP on FR4 boards. Most important material properties are elongation to break and tensile strength. This is much less important for WLP if an underfiller is used.