HDI Interconnect Board PCBA

10-Layer 3+N+3 HDI Backplane — 500+ SEARAY Contacts at 28 Gbps per Pair

Product Overview

The HDI Interconnect Board is a specialized high-density interconnect PCBA designed to solve the physical routing challenges of complex multi-module UAV avionics systems. As UAV electronics become increasingly sophisticated — often combining flight controllers, companion computers, AI accelerators, multiple radios, and diverse payload interfaces — the need for a high-density interconnection backbone becomes critical. This board serves as the central routing fabric, providing hundreds of point-to-point connections between modules using microvia and buried via technology to achieve routing densities that would be impossible with conventional through-hole PCB fabrication.

Built on a 10-layer 3+N+3 HDI stack-up, this board employs three sequential lamination cycles to create laser-drilled microvias of 75 µm diameter connecting layers 1–2, 2–3, 8–9, and 9–10. Buried vias within the rigid core connect layers 4–7, providing escape routing for high-pin-count BGA components with 0.4 mm pitch. All via structures are fully filled with electrolytic copper and planarized, enabling via-in-pad placement that dramatically increases effective routing density. The board supports controlled impedance at 50 Ω single-ended and 100 Ω differential, with trace widths as narrow as 75 µm on the outer layers. Three Samtec SEARAY high-density board-to-board connectors provide over 500 interconnection points between modules, supporting data rates up to 28 Gbps per differential pair — sufficient for PCIe Gen4 and 100GbE signaling between adjacent boards in the avionics stack.

Key Specifications

PCBA Assembly Challenges

Assembling a 10-layer HDI interconnect board with three Samtec SEARAY connectors pushes the limits of high-density SMT assembly. The SEARAY connectors are 560-position, 0.8 mm pitch open-pin-field arrays that require exceptional placement precision — any offset exceeding 50 µm risks open contacts during mating with the daughter card. Coplanarity across the entire connector footprint must be held within 0.1 mm across its 50+ mm length, demanding a flat PCB and consistent solder paste deposition. The board undergoes multiple reflow passes due to its mixed top/bottom assembly; the SEARAY connectors are placed in the final pass to avoid secondary reflow damage to their precision-aligned contacts. Via-in-pad structures under BGA components must be fully filled and planarized to prevent solder wicking into the via barrel, which would starve the BGA ball of solder. The copper filling is verified by X-ray inspection before and after assembly, with a maximum allowable void volume of 5% per via.

Test Strategy

The assembled interconnect board undergoes a comprehensive electrical validation sequence. All 500+ SEARAY contacts are tested for continuity and isolation using a custom bed-of-nails fixture that mates simultaneously with all three connectors. The fixture uses spring-loaded pogo pins with gold-plated tips, and the test program verifies every net with a four-wire Kelvin measurement for resistances below 100 mΩ. For high-speed signal pairs, TDR measurements from the fixture verify the differential impedance on every controlled-impedance pair at 100 Ω ±10%. Selected high-speed lanes are further validated with a VNA (Vector Network Analyzer) measuring S-parameters from DC to 20 GHz; insertion loss must be below -3 dB at 14 GHz (the Nyquist frequency for 28 Gbps NRZ signaling). Cross-talk between adjacent differential pairs is measured and must remain below -30 dB. The 3D X-ray system inspects all via-in-pad structures for fill integrity and void content.

PCB Manufacturing Difficulty

Fabricating a 10-layer 3+N+3 HDI board is among the most demanding PCB manufacturing processes. Three sequential lamination cycles are required, each adding two layers of build-up material (resin-coated copper foil) with laser-drilled microvias. Registration between lamination cycles must hold within ±25 µm — a single misregistered microvia on layer 1–2 can short to an adjacent trace or pad. The laser-drilled microvias at 75 µm diameter have an aspect ratio of approximately 1:1 through a 60–80 µm dielectric layer, requiring precise laser energy control to avoid under-drilling (open via) or over-drilling (damage to the landing pad). Copper filling of all via structures uses a pulse-reverse plating process that deposits copper preferentially at the via bottom, building upward to achieve a dimple of less than 15 µm after planarization. All 10 layers are fabricated from low-CTE, high-Tg (180°C) laminate to maintain dimensional stability through three lamination cycles and multiple assembly reflow passes.

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