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18 PCB007 MAGAZINE I SEPTEMBER 2018 Feature by Roger Massey ATOTECH Evolution of the HDI PCB Since their advent in the early-to-mid-1990s, high-density interconnect (HDI) PCBs have un- dergone several changes and could now be said to be entering their third evolution. Based on subtractive, or print-and-etch processes, the ear - ly HDI panels made use of traditional cores and sequential lamination steps to produce high- end boards with ~60 mm line and space (L/S) capabilities. But most importantly, they relied on microvias to enable their high interconnect density, which, at that time, could not be read - ily achieved with other technologies. As board producers improved their process- es, HDI board capability also improved; with the release of what we now accept as the smart- phone, in the early 2000s, the second genera- tion of HDI panels came into being. While the laser microvias remained, stacked vias began to replace staggered vias, and in combination with the "any layer" or "every layer" build-up technique, these new HDI boards eventually begin to exhibit 40 mm L/S. Still based on subtractive technologies, this any-layer approach remains the king of the HDI techniques, and it is true to say that the majority of the advanced HDI PCBs that are typical to mobile devices are still produced with this technique. However, in 2017, the HDI market began its next evolution by starting to move away from subtractive processes and into those based on pattern plating. While still reasonably common in Europe and the USA, in Asia HDI has been generally limited to IC sub- strate manufacturers. Semi-additive processing (SAP) uses pattern plating processes to realize features <15 mm, and while this size is needed for package sub- strates, it's not yet called for in advanced HDI boards. However, mSAP and amSAP are mod- ified and advanced modified variations which are now on track to become the next genera- tion of HDI PCBs.

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