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The trend toward miniaturization of electronic systems demands reliable small fine-pitch through vias in organic packages and high-density interconnect system boards. Fine-pitch through vias in glass fiber-reinforced organic packages suffer from electrical insulation failures due to the formation of conductive anodic filaments (CAFs) in the presence of temperature, humidity, and voltage. In addition to the requirement for fine-pitch through vias, restriction in use of halogens as well as the move toward green electronic systems has driven the development of halogen-free thermally stable novel resin formulations for next-generation glass fiber-reinforced substrates. The introduction of new resin chemistries can also affect the reliability, as CAF failures are known to depend on the substrate material properties. In this paper, CAF reliability of small and fine-pitch through vias in a halogen-free glass fiber-reinforced organic substrate is investigated. Test structures with through via diameter of 100 μm with two different pitches, i.e., 250 and 500 μm , were fabricated and tested using 100 V direct current (dc) bias at 85 °C and 85% relative humidity for 1000 h. Insulation failures were observed in test structures with a pitch of 250 μm, while the test structures with a pitch of 500 μm exhibited stable insulation resistance during the test. Failures were identified using optical microscopy and characterized using scanning electron microscopy. The results indicate that CAF failures are a concern with fine-pitch through vias in packages, and, therefore, careful selection of materials and processes is required for achieving reliable fine-pitch through vias in high-density packages.