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As devices shrink and performance demands grow, engineers are turning to High-Density Interconnect (HDI) technology to pack more function into less space. Whether you're working on a compact sensor, a multi-layer RF module, or a dense wearable board, HDI and fine-line etching open up new possibilities—if you design with the right process in mind.
At PICA Manufacturing Solutions, we support a range of HDI stackups and fine-line capabilities, including laser microvias and semi-additive copper processes. Here's what you need to know to design HDI circuits that are not only functional, but manufacturable.
What Is HDI?
HDI stands for High-Density Interconnect—a class of PCB defined by:
• Line/space below 100 μm (4 mils), often down to 50 μm (2 mils) or less
• Interlayer dielectric thickness for laser microvias, usually ≤ 80 μm
• Laser-drilled microvias, usually ≤0.15 mm diameter
• Stacked or staggered via structures
• Sequential lamination, often with multiple build-up layers
HDI boards allow more routing layers, tighter trace pitch, and denser via placement than conventional PCBs—ideal for compact, high-speed, or RF-heavy designs.
Why Go HDI?
Understanding Fine-Line Etching
Fine-line etching enables trace widths and spacing as low as 25 μm (1 mil), depending on copper thickness and process. As you approach HDI dimensions, standard subtractive etching reaches its limits—introducing trace tapering, under-etching, and etch factor variability.
Ready to Push the Limits of HDI PCB Design?
Partner with PICA Manufacturing Solutions for precision-engineered Fine Line and HDI circuits. Get expert support, cutting-edge capabilities, and unmatched quality.
How We Support Fine Lines
• Subtractive process: Reliable to ~75 μm line/space (3 mil)
• Modified Semi-Additive Process (mSAP): Down to 25–50 μm with vertical sidewalls
• Sputtered seed copper & flash plating: Enables clean, narrow traces with low distortion
• Laser ablation for microvias: Allows stacked or staggered via-in-pad routing
Tip: When designing fine lines, always align your trace width and spacing with your fab house’s specific process limits—not just EDA defaults.
HDI Design Considerations
1. Plan Your Stackup EarlyMultiple sequential build-ups (e.g., 1+N+1, 2+N+2) allow via-in-pad designs and dense routing, but every lamination cycle adds time and cost.
• Use stacked or staggered microvias to escape dense BGAs.
• Avoid excessive blind/buried layers unless routing absolutely requires it.
• Consider copper-filled vias to allow for reliable via-in-pad placement.
1+N+1
2+N+2
2. Use Microvias Strategically
Laser-drilled microvias:
• Must follow aspect ratio rules (usually ≤0.8:1)
X/Y= Microvia Aspect Ratio
• Are often copper filled for SMT pads
• Should avoid stacking more than 2–3 layers without reinforcing
Use staggered microvias to improve reliability and reduce layer count where possible.
3. Trace & Pad Design Rules
For HDI:
• Line/space: 2–3 mil typical (50–75 μm)
• Annular ring: Can be reduced with laser via registration
• Via pad size: Usually 0.25 mm or less for microvias
• Via capture pads: Use teardrops or reinforced pads to prevent drill breakout
4. Watch for Manufacturability Issues
As you shrink line/space:
• Avoid acute trace angles—they can over-etch or trap chemistry
• Ensure clean reference planes under high-speed signals
• Allow for clear solder mask windows over fine-pitch pads
We’re happy to provide DFM feedback on these elements before you commit your layout.
Flex and Rigid-Flex HDI
HDI isn't just for rigid PCBs. We offer:
• Laser-drilled microvias in rigid-flex and flex-only circuits
• Ultra-thin flex layers for 0.5 mm pitch or tighter BGA escapes
• Hybrid stackups that combine HDI cores with flex arms or shields
When working with HDI flex, careful planning of coverlay openings, trace spacing, and via transitions is essential to avoid cracking or plating issues.
Final Thoughts
HDI and fine-line designs are more achievable than ever—but only when layout and fabrication are aligned. As traces get narrower and via structures more complex, early collaboration with your PCB partner becomes essential.
At PICA Manufacturing Solutions, we guide engineers through HDI stackups, via strategy, fine-line etching limits, and material selection—so your board isn’t just compact, but reliable and ready for production.
Bring us into the design early—and let’s build high-density the right way.