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22 The PCB Magazine • April 2015 leading edge semiconductor component pack- aging densities. Background Beginning in the 1980s, the electronics in- dustry began the early days of transition from pin-in-hole wave soldering to surface mounting of increasingly complex semiconduc- tor packages. Strategic technical analysts at companies such as IBM, Bell Labs (Western Elec- tric), Digital Equipment, Hita- chi, NEC, and Hewlett Pack- ard realized the existing PCB processes and materials faced an impending industry-wide capability bottleneck with ur- gent implications in limiting copper trace and solder pad featuring. From this exigency came the industry-wide in- vention of a continuing va- riety of microvia processes and production systems that continues to this day. Today, the copper filled, stacked la- ser drilled microvia multilay- er process is the dominant in global production. In a parallel but separate roadmap, the flexible circuit has evolved since its inception as an early alternative to cabling and power distribu- tion. Flexible printed circuits evolved quickly in the early 1980s to become the dominant form factor for very fine-pitch semiconductor pack- aging substrates and liquid crystal display inter- connection. With reel-to-reel mass production lines adapted for wet chemical and fine-line lithographic techniques, FPCs continue to be an essential element in product design solutions today, especially for touch screen and large area LCD and LED based displays. The HDI-FPC hybrid platform, known as rig- id-flex or RFPCs, has been adapted to a number of different stackups to solve designers' inter- connect-product structuring challenges, result- ing in an extensive patent literature of innova- tion. In general, one or more fine line single or double sided FPCs are applied as the conformal connection planes in a stackup combining HDI rigid PCB inner and outer layers, and appropri- ate adhesives and coverlays to both combine and protect the core for laser via creation, as well as final plated through-hole (PTH) and fi- nal metal finish plating steps. HDI layers and sub-composites with laser drilled vias combined with stacked, copper filled vias up to 16 layers has become the state of the art in circuit design for the newest generation of mobile phones, wearable electronics, and IoT modules. High-Density Interconnect Stackup Basics The increasing need for greater functionality in a small form factor drives the HDI PCB stackup designs. The interconnect struc- tures in HDI PCB include bur- ied vias and microvias. In stan- dard HDI, such as a 1+N+1 and an i+N+i stack up, both buried vias and microvias are used. Ev- ery layer interconnect connec- tion (ELIC) uses only stacked copper filled microvias. Each of these stackup structures en- able designs for smart, connected devices and the choice of HDI struc- tures is dependent upon several factors includ- ing functionality, connectivity, product dimen- sions, weight, reliability, assembly requirements and user experience. Table 1 provides a high-level definition and comparison of the HDI Stackup structures. Buried vias may be drilled into just one in- ternal core that connects the top and bottom layer or into a multi-layer subpart. These buried vias connect multiple layers together internally using standard through-hole requirements for annular ring, aspect ratio and drill to copper re- quirements. Stacked microvias are just as it states. The vias are laser drilled one on top of the other in adjacent layers creating a stack of microvias in the PCB. For best reliability, it is recommend- ed that only copper filled microvias be used HIGH-DENSITy INTERCONNECTS: ENABLING THE INTELLIGENCE OF THINGS continues HDI layers and sub-composites with laser drilled vias combined with stacked, copper filled vias up to 16 layers has become the state of the art in circuit design for the newest generation of mobile phones, wearable electronics, and Iot modules. " " FeAture