RRU PCB Design: Remote Radio Unit & AAU Boards

What Is an RRU PCB?

An RRU PCB is the printed circuit board inside a Remote Radio Unit—the active RF module mounted at or near the antenna on a cellular base station tower. It converts digital baseband data from the baseband unit (BBU) into amplified RF signals and vice versa, handling frequencies from roughly 700 MHz to 3.8 GHz in 4G LTE systems and up to 7.125 GHz in 5G NR sub-6 GHz bands. In 5G massive MIMO deployments the RRU and antenna merge into a single Active Antenna Unit (AAU), where the same PCB also carries the antenna feed network and beamforming distribution. Designing these boards means solving simultaneous problems in high-frequency signal integrity, high-power thermal dissipation, and outdoor environmental reliability—all inside a sealed enclosure bolted to a tower.

RRU vs AAU: Architecture and PCB Implications

Traditional macro base stations split the system into a BBU in a ground-level cabinet, an RRU on the tower, and a passive antenna. The RRU PCB handles transceivers, power amplifiers (PAs), filters, and digital front-end (DFE) processing. A coaxial jumper cable connects the RRU output to the passive antenna panel above it.

In a 5G AAU, the PA outputs feed directly into an integrated antenna array—often 64T64R (64 transmit, 64 receive channels)—eliminating the jumper cable and its loss. This changes the PCB dramatically:

• Channel count: An AAU board may route 64 or 128 independent RF paths versus 2–8 in a legacy RRU.
• Layer count: AAU motherboards commonly reach 16–24 layers to accommodate dense RF routing, power distribution, and digital buses.
• Antenna integration: Patch or dipole antenna elements may be etched directly on the top laminate layers, requiring tight Dk tolerance (±0.02 or better) for consistent element impedance.
• Thermal density: Total PA power can exceed 320 W in a 64T AAU, concentrated in a board area under 500 × 400 mm.

Despite these differences, the core PCB disciplines—material selection, impedance control, thermal via design, and surface finish—overlap heavily, so the rest of this guide applies to both form factors.

Frequency Bands and Signal-Integrity Requirements

RRU PCBs must support one or more of the standard 3GPP bands. Sub-6 GHz NR bands like n77 (3.3–4.2 GHz) and n78 (3.3–3.8 GHz) are the primary drivers for new 5G AAU designs. At 3.5 GHz the free-space wavelength is about 86 mm; a quarter-wave microstrip stub is roughly 15 mm on a Dk ≈ 3.5 substrate, leaving little room for layout error.

Impedance and Loss Targets

Transmission lines on the PA output path are typically 50 Ω single-ended microstrip, held to ±5 % tolerance for a maximum return loss of roughly −20 dB at the operating frequency. Insertion loss through the PCB feed network must stay below 0.15 dB per centimeter of trace at 3.5 GHz—achievable on low-loss laminates but not on standard FR-4 (whose Df of ~0.020 at 3 GHz would push loss above 0.35 dB/cm on the same geometry).

Digital and CPRI/eCPRI Interfaces

The digital side of the RRU PCB carries high-speed serial links—CPRI at up to 24.3 Gbps or eCPRI over 25G Ethernet—between the DFE ASIC and the optical transceiver. These lines demand controlled differential impedance (typically 100 Ω ±7 %), low-skew routing, and clean reference planes, which influences the inner-layer stackup even though the frequencies are in the baseband domain.

Material Selection for RRU PCBs

Material choice is the single biggest design decision. RRU boards almost always use a hybrid stackup: high-frequency laminate layers where RF traces run, bonded to standard high-Tg FR-4 or mid-loss material for digital and power layers.

A common 16-layer AAU stackup places RO4350B on the outer two RF signal layers, Megtron 6 for the high-speed digital pair, and FR-4 for the remaining power and ground planes. Bonding dissimilar materials requires compatible prepreg—Rogers CuClad or Isola TerraGreen—and careful CTE matching to avoid delamination during reflow. The Rogers RO4350B datasheet specifies a z-axis CTE of 32 ppm/°C, which pairs reasonably with FR-4’s ~50 ppm/°C when the hybrid bond layer is thin (2–3 mil / 50–75 µm).

Power Amplifier Integration and Thermal Management

The PA stage dominates RRU PCB design. A single Doherty amplifier board in a 4T4R LTE RRU may dissipate 60–80 W per channel, and a 64T AAU multiplies that across dozens of GaN PA devices. Managing this heat on the PCB involves several coordinated techniques.

Thermal Via Arrays

Under each PA device footprint, a dense field of plated-through vias (typically 0.3 mm drill, 0.6 mm pitch, filled and capped with copper) conducts heat from the topside pad to an exposed copper coin or a thick internal copper plane. Thermal resistance through a well-designed via array can be kept below 2 °C/W for a 10 × 10 mm device footprint.

Embedded Copper Coins and Heat Spreaders

For the highest-power GaN PAs, the PCB fabricator presses a solid copper slug (coin) into a routed pocket in the laminate, flush with the top surface. The coin provides a direct thermal path to the aluminum housing. This is standard practice for Ericsson, Nokia, and Samsung AAU platforms and adds roughly 15–20 % to bare-board fabrication cost.

Surface Finish Considerations

ENIG (Electroless Nickel Immersion Gold) is the default surface finish for RRU PCBs because it gives flat, solderable pads for fine-pitch BGA and QFN packages. However, the nickel barrier layer adds magnetic loss; on RF traces operating above 2 GHz, immersion silver or OSP is preferred for the RF signal layers while ENIG is kept for the digital/connector side. Selective surface finish—different finishes on different areas of the same board—is a standard capability for RRU-class fabrication.

Filtering and Signal Conditioning on the RRU PCB

Every RRU includes bandpass or lowpass filtering between the PA and the antenna port to suppress harmonics and out-of-band emissions per 3GPP TS 38.104. In legacy RRUs these were often discrete cavity filter assemblies bolted alongside the PCB. Modern AAUs increasingly move toward PCB-integrated coupled-line or edge-coupled bandpass filters, trading some Q factor for size and weight savings—critical when the unit hangs on a tower.

RF switching between transmit and receive paths, or between multiple bands, uses high-isolation RF switch circuits that demand careful grounding and via fencing on the PCB to maintain 40 dB+ isolation across the operating band.

Stackup Design: A Practical 16-Layer Example

Below is a representative stackup for a sub-6 GHz 5G AAU motherboard. Layer thicknesses are in mils (1 mil = 0.0254 mm).

This symmetric stackup keeps RF layers on the outside for easy tuning access and short via stubs, places high-speed digital near dedicated ground references, and uses inexpensive FR-4 for the bulk inner layers. Total board thickness lands near 100 mil (2.5 mm), compatible with standard connector and housing designs.

Manufacturing and Reliability Requirements

RRU PCBs are built to IPC-6012 Class 3 or Class 3/A for high-reliability telecom applications. Key fabrication parameters include:

• Minimum trace/space: 3.5/3.5 mil (89/89 µm) on RF layers; 3/3 mil on high-speed digital layers.
• Drill aspect ratio: Up to 12:1 for through-hole vias on 100 mil boards; blind and buried vias with sequential lamination for the hybrid stack.
• Impedance tolerance: ±5 % on 50 Ω microstrip, verified by TDR coupon on every panel.
• Copper weight: 2 oz on power planes for current handling; 1 oz on signal layers.
• Environmental rating: The finished assembly must survive −40 °C to +55 °C ambient (IEC 60721-3-4 class 4K4H), with the PA junction temperature reaching 150 °C during full-power operation.

Boards undergo microsection analysis, ionic contamination testing per IPC J-STD-001, and thermal cycling qualification (500+ cycles, −40 to +125 °C) before volume production release.

Connections to the Wider Base Station Ecosystem

The RRU PCB does not exist in isolation. Its backhaul interface connects—via fiber—to the BBU, while the RF output connects to the antenna or, in split architectures, to a microwave backhaul board for point-to-point links in the 6–80 GHz range. Testing during development relies on specialized ATE load boards that replicate the RRU’s impedance environment for PA and transceiver characterization.

Design Checklist for RRU and AAU PCBs

1. Lock down the operating band(s) and confirm Dk/Df targets at the actual operating frequency, not at 1 MHz catalog values.
2. Define the hybrid stackup early—material transitions dictate lamination sequence and cost.
3. Place PA devices near board edges closest to the heat-sink interface; minimize the thermal path length.
4. Use via fencing (ground via rows at ≤ λ/20 spacing) around every RF trace to suppress parallel-plate modes in the dielectric.
5. Simulate the full TX chain—PA, filter, feed network—in a 3D EM solver before layout release.
6. Specify selective surface finish: immersion silver for RF, ENIG for digital BGA and connectors.
7. Include TDR coupons for every controlled-impedance class on the panel.
8. Plan for conformal coating or potting of the finished assembly for outdoor IP65/IP67 protection.

FAQ

Can FR-4 be used for RRU PCBs?

FR-4 is used for the inner power and ground layers of hybrid RRU stackups, but its high loss tangent (Df ≈ 0.020 at 3 GHz) makes it unsuitable for RF signal layers above about 1 GHz. RF and antenna layers require low-loss laminates such as RO4350B or Megtron 6.

How many layers does a typical 5G AAU PCB have?

Most 64T64R massive MIMO AAU motherboards use 16 to 24 layers, depending on channel count, power distribution complexity, and whether the antenna elements are integrated on-board or on a separate panel.

What is the difference between an RRU and an AAU?

An RRU (Remote Radio Unit) is a standalone transceiver module connected to a separate passive antenna via coaxial cables. An AAU (Active Antenna Unit) integrates the radio and antenna into a single enclosure, eliminating cable loss and enabling massive MIMO beamforming. The AAU PCB carries both the transceiver circuitry and the antenna feed or radiating elements.

What surface finish is best for RRU RF traces?

Immersion silver offers the lowest insertion loss on microstrip traces above 2 GHz because it avoids the lossy nickel barrier present in ENIG. OSP is a lower-cost alternative but has a shorter shelf life and limited reflow cycles, making it less common in telecom-grade production.

How is thermal management handled in high-power AAU boards?

A combination of thermal via arrays under PA devices, embedded copper coins for direct heat conduction, and thick internal copper planes spreads heat to the aluminum enclosure. Thermal simulation targeting junction temperatures below 150 °C is performed before layout finalization.