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8 The PCB Magazine • April 2015 Abstract High-density interconnection (HDI) PCB technology is advancing to enable increased miniaturization and functionality of products such as smartphones, tablet computers and wearable devices. This dictates continual reduc- tion in feature size for conductor line width and spacing, micro-via pad diameter and pitch, and conductor and insulating layer thickness to accommodate more components and layers without increasing size, weight or volume of the PCB assembly. Furthermore, as wireless data transmission bandwidth and processing speed increase, electrical performance of the PCBs be- comes ever more critical. by Tarja Rapala-Virtanen, Erkko Helminen and Timo Jokela TTM TeCHnoloGIeS InC. Next-Generation Ultra-Thin HDI PCB Manufacturing Challenges Just as the IC industry met serious road- blocks in feature scalability to keep pace with Moore's law of doubling performance improve- ment, so the PCB industry now faces challenges in basic process capability and material prop- erties to continue delivering improvements in interconnection density and electrical perfor- mance on their forecast trajectories. Even in the state-of the-art process node for all-layer-via (ALV) HDI design, limitations in process scal- ability and escalating factory cost raise ques- tions about diminishing returns in packaging density and performance. In particular, the industry faces significant challenges increasing PCB layer count and re- ducing thickness as individual insulation layers cross the 50 micron threshold, where degrada- tion of dimensional stability and electrical per- formance (particularly signal impedance and FeAture