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40 DESIGN007 MAGAZINE I NOVEMBER 2025 copper is deposited on the drum as it rotates at a very slow pace; the slower the pace, the thicker the copper. The side against the drum provides the smoother finish. ED copper has extremely smooth profiles (Ra < 1 μm), which improves high-frequency performance. RA copper foils are created by successively pass- ing an ingot of solid copper through a rolling mill, and then applying high temperature to anneal the copper. RA copper is preferred for UHDI flex appli- cations due to superior ductility. Figure 1 shows a comparison of the two processes. Via Structures and Fill Materials Typical technologies are iterations of two types of microvias: blind and buried. Blind vias connect an outer layer to inner layers (with access to only one external layer), while buried vias connect inner lay- ers preferably on the same core or multiple cores (with no access to external layers). The preference for producing microvias is laser-drill (≤50 μm depth), but mechanically drilling can be used but is more limiting. UHDI employs both stacked and staggered microvia strategies to balance density with reliabil- ity. Figure 2 shows a microvia comparison. E l e ct ro d e p o s i te d P ro c e s s Ro l l e d A n n e a l e d P ro c e s s F i g u re 1 : E D vs . R A c o p p e r p ro c e s s e s . ▼