Fabrication and Reliability Demonstration of 3 µm Diameter Photo Vias at 15 µm Pitch in Thin Photosensitive Dielectric Dry Film for 2.5 D Glass Interposer Applications

In this paper, the authors report on the development of a novel epoxy-based photosensitive dielectric dry film material (PDM). The PDM has two key features: 1) Low CTE value of 30 ppm/°C resulting from the concentration optimization of nano-sized fillers in the material composition, 2) Low Stress of the PDM films, attributed to low-temperature (180 °C) processing of the polymer dielectric which is lower than most of the known advanced dielectric materials. Improvements reported on these two key features will significantly enhance the reliability of high-density packages. To assess the reliability of the PDM, we have fabricated a high-density daisy chain structure on the glass panel consisting of 400 vias of 3 μm diameter at 15 μm pitch. We have also used the PDM as passivation layer on the surface of the test vehicle. Following the fabrication of the test microvia chain, electro-less nickel immersion gold (ENIG) process was performed for surface finish. We have performed reliability measurements of the 3 μm diameter vias after nHAST (Non-Bias Highly Accelerated Stress Tests) at 130 °C, 85 % R.H. for 100 hours followed by thermal cycling test (TCT) with a dwell time of 15 minutes at 125 °C and -55 °C. We have observed no open circuit failure occurred during the TCT. We measured the resistance of the daisy chain circuit by four-point probe method every 100 cycles up to a total of 1500 cycles. The resistance change was less than 5 % even after 1500 cycles which clearly demonstrate the superior reliability of the PDMs. In conclusion, the newly developed PDM is a suitable dielectric material for high-density RDL applications such as 2.5D interposers and fan-out packages.