5G CU Centralized Unit Board PCBA

26-Layer Server-Grade Board — L2/L3 Protocol Stack (PDCP, RLC, MAC, RRC) with GTP-U Offload & Hardware Root-of-Trust

Product Overview

The 5G CU Centralized Unit Board PCBA is the L2/L3 processing engine of the disaggregated 5G RAN, executing the PDCP, RLC, MAC scheduler, and RRC protocol layers that manage user sessions, QoS enforcement, and radio resource allocation per 3GPP TS 38.300 series. Designed on a 26-layer high-speed PCB with Megtron 6 and ultra-low-loss dielectrics, the board hosts multiple high-core-count x86 or Arm-based server-class processors with hardware accelerators for PDCP ciphering, integrity protection, and GTP-U tunnel encapsulation. The board provides up to 8× 100GbE interfaces for F1 mid-haul connectivity to distributed units and N2/N3 backhaul links to the 5G core network (5GC). A dedicated SmartNIC or FPGA-based data-plane accelerator offloads the GTP-U processing pipeline, enabling line-rate packet forwarding at over 200 Gbps aggregate throughput with sub-50-microsecond latency — critical for URLLC services per 3GPP TS 22.261. Redundant BIOS/management controllers, hot-swappable power domains, and hardware root-of-trust with measured boot (TPM 2.0 / DICE) ensure carrier-grade reliability and security for always-on network operations. The board supports RAN network slicing, enabling operators to provision logically isolated slices with guaranteed resources for eMBB, URLLC, and mMTC services on shared infrastructure.

Key Specifications

PCBA Assembly Challenges

Assembling the CU board is a server-grade SMT challenge with carrier-class reliability requirements. Multiple large server-class CPU sockets (LGA with 4,000+ pads at 1.0 mm pitch) and associated DDR5 DIMM slots demand precision placement and coplanarity control — any LGA pad damage or solder bridging on the socket pins will cause boot failures. The SmartNIC/FPGA data-plane accelerator sits on a high-density BGA with 3,000+ balls, requiring careful thermal profiling to avoid head-in-pillow defects given the package size. The board's 8× 100GbE QSFP28 cages are press-fit types in close proximity to large CPU sockets; the press-fit insertion force can flex the PCB enough to crack previously soldered BGA joints unless proper support fixtures are used. Double-sided assembly is required due to component density, demanding staged reflow with secondary-side components rated for two thermal cycles. Redundant power delivery — over 15 independent voltage rails with multiphase VRMs — requires exhaustive pre-power verification of every VRM output to prevent CPU/core damage from over-voltage. The hardware root-of-trust chain (TPM, measured boot, secure firmware storage) mandates that all security ICs are serialized and provisioned post-assembly in a secure environment with full chain-of-custody documentation.

Test Strategy

CU board testing follows server-class validation with telecom-specific extensions. Pre-power ICT verifies all passive components, power rail resistances, and socket pin continuity using a custom bed-of-nails fixture. Boundary scan validates high-speed interconnects between CPUs, SmartNIC, DDR5 slots, and peripheral controllers. First power-on runs a comprehensive BIOS POST sequence that validates all memory channels, PCIe link training at Gen4/Gen5 speeds, and management controller bring-up. Functional testing installs a 5G CU software stack that exercises the complete L2/L3 pipeline: PDCP ciphering/integrity, RLC segmentation/reassembly, MAC scheduling over emulated UEs, and RRC state machine transitions through all states. GTP-U data-plane testing loads all 8× 100GbE ports with line-rate traffic, validating tunnel encapsulation/decapsulation throughput at >200 Gbps with latency under 50 µs. Network slicing tests verify QoS enforcement across multiple slice types simultaneously. Security testing validates measured boot sequence, TPM attestation, and secure key provisioning. Final 72-hour burn-in runs the complete CU stack under sustained load, monitoring memory ECC errors, CPU thermal throttling events, and Ethernet frame errors. NEBS Level 3 compliance testing includes extended temperature operation (0°C to 55°C) and vibration resistance.

PCB Manufacturing Difficulty

The 26-layer CU PCB pushes manufacturing limits for telco server boards. All 100GbE (4× 25 Gbps NRZ or 2× 50 Gbps PAM4) traces must maintain tight intra-pair skew below 1.5 mil across the full 26-layer stack-up — backdrilling removes unused via stubs on every high-speed signal layer, with stub length controlled to under 6 mil for PAM4 lanes operating at 26.5625 GBd. The CPU socket area contains dense break-out routing with up to 12 routing layers dedicated to escaping the LGA pad field — any misregistration over ±1.5 mil breaks escape traces. The large board format (up to 450 × 350 mm) with 26 layers creates significant Z-axis expansion during reflow; laminate selection and lamination cycles are optimized to minimize CTE mismatch and warpage below 0.5%. Impedance is modeled using 3D field solvers and verified by TDR on every panel's coupon set. High-aspect-ratio plated through-holes (>12:1) require pulse plating for uniform copper deposition. Finished boards undergo 100% AOI, automated flying-probe impedance testing on all differential pairs, and Hi-Pot testing at 1,000 VDC for power-plane-to-ground isolation before release to assembly.

More information