Cu Fully Filled Ultra-Fine Blind Via on Flexible Substrate for High Density Interconnect

The demand for portable, multi-functional and compact electronic products raises a great challenge to the substrate technology and related interconnection technology in electronic packaging for high density, small feature size and high performance. The requirement on the circuit density of substrate, especially, is intensive for chip scale packages (CSP) and system-in-package (SIP), where the chip size is reduced and its pin-count number is increased. Thus, the formation of small and dense via is becoming more and more important for increasing the circuit density of substrate. In this paper, ultra-fine blind via with solid Cu filled at an entry diameter of 20μm, over the current blind via size of 50-200μm by CO₂ laser drilling, is demonstrated on polyimide based flexible substrate. Its advantages of the present innovation compared to the PTH in conventional substrate technology and micro-blind via structure in advanced high density interconnect (HDI) technology will be explored. In conclusions, the reliability of the ultra-fine blind vias has been assessed in 2-metal tape ball grid array (TBGA) and daisy chain modules at substrate level, subjected to JEDEC MST L3 at 260°C, Air to Air thermal cycle and thermal shock, T/H bias, pressure cooker test and low/high temperature storage tests, etc. In addition, the Cu fully filled ultra-fine blind via makes "landless" structure feasible on the substrate, further increasing the circuit density of substrate. In the end, the ultra-fine Cu filled blind via technology has introduced to the production in Compass for SIP, 2-metal layer chip-on-flex (COF) and multi-layer buildup flex, etc.