When a UAV avionics suite must fit into a 100 mm diameter fuselage tube or a conformal wing bay, traditional planar PCBs connected by cables reach their volumetric limits. The HDI Interconnect PCB solves this through any-layer microvia technology combined with rigid-flex construction, allowing a single continuous circuit to fold into three dimensions — replacing multiple boards, connectors, and cable harnesses with one monolithic interconnect system. This blog examines the design and fabrication of these space-saving avionics interconnects.
Any-Layer Microvia (ALIVH) Technology
Any-layer HDI — also called ALIVH (Any Layer Interstitial Via Hole) or every-layer interconnect — places microvias on every layer pair in the stackup rather than limiting them to outer layers. In a 10-layer ALIVH board, a signal can transition from layer 1 to layer 10 through a chain of stacked microvias without using a single through-hole. This liberates routing channels on every layer, as through-hole anti-pads no longer consume real estate.
For a UAV interconnect board connecting a 484-ball BGA processor to three 200-pin mezzanine connectors, ALIVH enables 60–80% higher breakout density compared to through-hole via technology. The microvias are laser-drilled (UV YAG or CO₂) at 75 µm diameter with 200 µm capture pads, and copper-plated to 15 µm minimum thickness per IPC-6012 Class 3.
Rigid-Flex Construction
The HDI interconnect board typically adopts a rigid-flex construction where rigid PCB sections (containing components and connectors) are joined by flexible polyimide sections that bend to navigate the 3D enclosure geometry. Key rigid-flex design rules include:
Flex layer count: 2–4 layers in the flex region, using adhesiveless polyimide with rolled-annealed copper (RA, higher ductility than electrodeposited) to survive 10,000+ flex cycles.
Bend radius: Minimum 10× the total flex thickness (typically 1.0–1.5 mm) for dynamic applications, and 6× for static "bend-to-install" applications.
Transition zone: The rigid-to-flex transition includes a 1–2 mm stiffener (polyimide or FR-4 bead) to prevent stress concentration at the rigid edge, which is the most common failure point in rigid-flex designs.
Trace routing in flex: All traces are routed perpendicular to the bend axis, with curved corners (radius ≥ 1.5× trace width) and staggered on adjacent layers to prevent I-beam cracking.
Board-to-Board Connector Arrays
Within a 3D-folded assembly, multiple rigid sections connect through high-density mezzanine connectors. A 200-pin 0.5 mm-pitch connector array occupies only 25 × 8 mm, but its PCB breakout demands microvias to escape all signals from the tight pin field. The connector land pattern uses non-solder-mask-defined (NSMD) pads with 0.25 mm diameter, and the breakout vias are laser-drilled microvias to layers 2 and 3, with subsequent conventional vias dropping to lower layers.
Signal Integrity in Folded Assemblies
When a rigid-flex PCB is folded, traces in the bend region experience a change in dielectric environment — the bend opens the solder mask and changes the effective εr around the traces. High-speed signals (USB 3.0, PCIe) should not be routed through bend regions when possible. When a bend must carry high-speed signals, a 3D EM simulation of the folded state verifies that the impedance remains within ±10% of the flat-state value.
Conclusion
HDI Interconnect PCBs with any-layer microvias and rigid-flex construction represent the state of the art in volumetric avionics packaging, enabling UAV designers to fit more capability into smaller airframes. Superb Tech manufactures rigid-flex HDI interconnects with laser-drilled microvias, polyimide flex layers, and 3D bend-validated signal integrity.
