Issue link: https://iconnect007.uberflip.com/i/1541367
74 PCB007 MAGAZINE I NOVEMBER 2025 As line/space requirements shrink to single-digit microns (2–5 µm for UHDI), substrate manufactur- ing begins to look almost identical to the cutting edge of PCB manufacturing, just at smaller geom- etries and tighter tolerances. The push toward UHDI in PCBs mirrors what has already been happening in advanced package substrates. Substrates and HDI: Evolution by Necessity Early substrates could be made with conven- tional PCB-like processes. As chip I/O counts rose, substrates had to evolve, with technologists adopt- ing build-up layers, stacked vias, and laser drilling, which are hallmarks of HDI. Today's high-end substrates (for CPUs, GPUs, networking ASICs) are produced with UHDI-level features: <10 µm lines/spaces, microvias <50 µm, and highly controlled layer-to-layer registration. In many respects, substrate manufacturing has leap- frogged PCB HDI technologically, and the PCB industry is catching up. Interposers and UHDI: The Overlap Silicon interposers (with TSVs) are manufac- tured using semiconductor processes, not PCB processes. But the RDL fan-out interposers (like those used in FO-WLP) and glass/organic interpos- ers use processes very similar to HDI PCB fabrica- tion, but only at extreme resolutions. Some in the industry even call fan-out panels "PCB-like interposers" because the toolsets and design principles (vias, laminates, redistribution layers) overlap with advanced PCB manufactur- ing. UHDI technology directly supports the density needed for chiplet interconnection, which is exactly where interposers operate. Manufacturing Convergence Historically, PCB fabs, substrate fabs, and semi- conductor fabs were distinct. But with HDI/UHDI, those distinctions are blurring. PCB fabricators are investing in UHDI capability to support next-gen substrates. Substrate makers (Ajinomoto, Ibiden, Unimicron, etc.) are adopting PCB-style panel processing at finer resolutions. Fan-out packaging is being done on large panels, using equipment that overlaps with PCB manufac- turing. This convergence means the skills and processes of HDI/UHDI PCB manufacturing are directly enabling the evolution of interposers and substrates. Why Does This Matter? As the boundary between chip package and PCB blurs, the technologies for substrates and UHDI PCBs are becoming interoperable. In some road- maps, the "board" and the "package" will merge into a single integration platform. PCB manufacturers with UHDI expertise may find opportunities to enter advanced packaging markets, while OSATs and substrate makers are exploring panel-scale manufacturing. UHDI capabil- ity is expected to be essential not just for PCBs, but for all interconnect platforms, including substrates, interposers, and possibly hybrid chip/board integra- tion. Substrates have long been a specialized branch of HDI PCB manufacturing, pushing finer features to keep pace with semiconductors. Interposers, espe- cially organic and fan-out types, are deeply tied to UHDI processes and often use PCB-like manu- facturing methods. As HDI evolves into UHDI, the distinction between PCB, substrate, and interposer manufacturing is narrowing, pointing toward a future where they form a continuum of high-density interconnect platforms. Still Confused? A Handy Analogy • Interposer = adapter/translator: It takes the ultra-fine signals from the die and disperses them. • Substrate = base/carrier board: It takes those adapted signals and connects the whole package to the outside world (PCB). Key Differences • Interposers connect die-to-die or die-to-sub- strate with ultra-fine pitch routing • Substrates connect package-to-PCB with larger-scale routing and mechanical support.