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38 DESIGN007 MAGAZINE I NOVEMBER 2025 T he rapid proliferation of 5G/6G communica- tions, Internet of Things (IoT), high-performance computing (HPC), AI, and medical electronics has driven the need for increasingly compact, high-per- formance circuit packaging. UHDI, defined by fea- ture sizes well below traditional HDI, addresses these demands by enabling ultra-fine lines, dense via interconnects, and embedded passive func- tionality. Understanding the materials and layering strategies in UHDI is essential for advancing both manufacturing and application design. Substrates and Dielectrics UHDI technology represents the next evolutionary step beyond traditional HDI PCBs. Characterized by sub-15 μm line/space resolution, stacked and staggered microvias, and ultra-thin dielectric lay- ers, UHDI enables miniaturization, high-frequency performance, and integration of advanced function- alities. This article provides a high-level overview of the materials and layer structures that under- pin UHDI, and highlights their properties, advan- tages, challenges, and ever-increasing applications across every market sector. Dielectric Substrates UHDI requires dielectric layers as thin as 10–15 μm, enabling short microvia depths and multiple build- up cycles (sequential lamination). The thinness sup- ports high interconnect density while maintaining mechanical reliability. Low-Dk/Low-Df Materials: • Dielectric constant (Dk): Lower Dk (≈2.7–3.2) reduces propagation delay • Dissipation factor (Df): Ultra-low Df (<0.002) minimizes high-frequency losses Materials such as Ajinomoto build-up film (ABF), liq- uid crystal polymer (LCP), and other advanced epoxy resins dominate UHDI designs due to their signal integrity advantages. Copper Foils Copper foils ≤5 μm thick enable line/space dimen- sions below 15 μm, essential for UHDI's fine-pitch routing. Surface smoothness is a major factor in ultra-thin foils, and the key takeaway here is that the smoother the surface, the faster the signal, but the tradeoff is that copper adhesion is reduced as smoothness increases. Two common types of cop- per manufacturing techniques are used based on the design performance and application: electro- deposited (ED) and rolled annealed (RA) copper. ED copper is formed by electrolytic deposition onto a slowly rotating, polished drum from a copper- sulfate solution. When an electric field is applied, U H D I F U N DA M E N TA LS by A n a ya Va rd ya , A m e r i c a n S ta n d a rd C i rc u i t s An Overview of UHDI Substrate Materials and Vias