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In the trend of miniaturization, low cost, and the performance of electronics, high density interconnect has been required for interfacing with very fine pitch BGA, CSP and SIP. This raises a great challenge to the substrate technology and related interconnect technology in electronic packaging for high density, small feature size and high performance. Miniaturization in electronics means finer lines and smaller vias in substrate technology. Very fine lines on the substrate are difficult to produce by thicker copper layer in conventional CO2 laser blind via. In this paper, ultra-fine blind via with solid Cu filled at an entry diameter of 20 mum, over the current blind via size of 50-200 mum by CO2 laser drilling, is demonstrated on polyimide (PI) based flexible substrate. A via pitch at 30 mum for the blind via has been developed for next generation of stack die packaging accompanying with dimpless design, which ameliorates the void entrapment failure caused by soldering and direct flip-chip (FC) bonding, and strengthens interfacial bond strength. In the meantime, thinner Cu conductor at top and bottom side could be achieved for high circuit density. The reliability of the ultra-fine blind vias has been assessed in daisy chain modules at substrate level, subjected to JEDEC air-to-air thermal cycle and thermal shock, and low/high temperature storage tests. Applications in direct FC bonding and their virtues including high electrical and thermal performances, and feasible of various metals surface finishing, will be discussed. In the end, the ultra-fine Cu filled blind via technology has introduced to the production in Compass for SIP, stack die CSP, 2-metal layer chip-on-flex (COF) and multi-layer buildup flex, etc.