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44 DESIGN007 MAGAZINE I DECEMBER 2025 layers are embedded or integrated into UHDI stacks to dissipate heat from high-power chips or modules, maintain electrical and mechanical reli- ability, and enable long-term stability in ultra-minia- turized electronics. Embedded heat spreaders (copper, metals) help dissipate heat in UHDI substrates for AI and HPC systems. These layers can be copper heat spread- ers, metal core or inserts, thermal vias, embedded passives, and/or hybrid thermal-flex technology. Solder Mask and Cover Layers High-resolution, ultra-thin solder masks are nec- essary to accommodate fine UHDI pads (≤30 μm). In UHDI packaging, solder mask and cover lay- ers are not just protective finishes. They are critical enablers for: • Protecting ultra-fine circuitry (<25 μm lines/ spaces) • Ensuring solder joint reliability on micro-pads • Providing insulation in stacked microvia architectures • Supporting both rigid UHDI substrates and flexible UHDI interconnects Solder mask characteristics include ultra-thin application (≤15 μm) to avoid covering fine traces, laser-defined openings with laser direct imaging (LDI), low Dk/Df, and high Tg and thermal endur- ance. Cover layer characteristics include mechan- ical protection in flex circuits, providing dielectric insulation and enhancing bendability in flex cir- cuits, and can be laser defined. Cover layer types include polyimide films (PI), liquid polyimide coat- ings, and liquid crystal polymers (LCP). Redistribution Layers (RDLs) Redistribution layers (RDLs) are ultra-fine-pat- terned metal layers used to reroute chip I/O pads to larger, more accessible interconnects on a UHDI substrate or package. They are a critical feature in UHDI and advanced packaging, enabling high- density interconnects, signal routing, and system miniaturization. UHDI achieves RDL line widths below 10 μm and is prevalent in applications such as fan-out wafer-level packaging (FOWLP), chiplets, heterogeneous integration, 5G/6G RF modules, and IoT and wearables. Hybrid Material Stacks Hybrid material stacks in UHDI (Figure 2) combine rigid and flexible substrate materials within a single multilayer structure to achieve: • High interconnect density • Mechanical flexibility where needed • Thermal and electrical performance optimization These stacks are particularly relevant for wear- able devices, implantable medical electronics, RF modules, and compact IoT applications. The com- bination of rigid ABF-based layers with flexible LCP or polyimide enables hybrid UHDI substrates, sup- porting numerous applications and market sectors. Conclusion UHDI technology builds upon the foundation of HDI by incorporating ultra-thin dielectrics, low-loss materials, ultra-thin copper, stacked microvias, and embedded functionality. Together, these enable unprecedented interconnect densities, high-fre- quency signal integrity, and compact device foot- prints. As demands for smaller, faster, and more integrated electronics accelerate, innovations in UHDI materials and layering strategies will remain central to the next wave of electronics packaging. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co- author of The Printed Circuit Design- er's Guide to… Fundamentals of RF/ Microwave PCBs and Flex and Rigid- Flex Fundamentals. He is the author of Thermal Management: A Fabrica- tor's Perspective, The Printed Circuit Designer's Guide to DFM Essentials, and The Companion Guide to Flex and Rigid-Flex Fundamentals. Figure 2: Hybrid material stack.