Visitor Location Register: Tackling the High-Speed and High-Density Challenges of Data Center Server PCBs

In the grand blueprint of 5G and future communication networks, every millisecond of latency and every bit of transmission is critical. As a core component of the mobile communication network's nervous system, the Visitor Location Register (VLR) and its evolved form in the 5G era—the Access and Mobility Management Function (AMF)—are key to ensuring seamless connectivity and roaming for trillions of devices worldwide. However, the implementation of this functionality has shifted from traditional dedicated telecom equipment to virtualized software running on high-performance data center servers. This fundamental transformation has shifted the focus of challenges directly to the printed circuit boards (PCBs) that carry these complex computations—they are the physical foundation determining network performance, reliability, and scalability.

Core Functions of VLR and Its Evolution in 5G Architecture

In the 2G/3G/4G era, the Visitor Location Register was a database tightly integrated with the Mobile Switching Center (MSC), with its core responsibility being the temporary storage of roaming users' subscription information, location data, and authentication parameters. When a user entered a new MSC service area, the VLR would retrieve data from the user's Home Location Register (HLR), enabling localized call processing and mobility management and avoiding frequent remote queries to the core network database. In this process, the VLR needed to collaborate with the Authentication Center PCB to verify the legitimacy of user identities; it would also query the Equipment Identity Register (EIR) to confirm the validity of mobile devices, preventing stolen or unauthorized devices from accessing the network.

In the 5G era, as network architectures evolve toward service-based (SBA) and cloud-native designs, the standalone physical form of the VLR has disappeared. Its core functions—mobility management, registration, and reachability management—have been integrated into the AMF (Access and Mobility Management Function) within the 5G Core (5GC). This shift means that tasks once handled by dedicated hardware are now performed by highly complex software running on commercial off-the-shelf (COTS) servers. Therefore, what we refer to today as "VLR hardware" is actually the high-performance Packet Core PCB that supports the entire 5G Core network. These PCBs must handle unprecedented data throughput and signaling storms, providing a solid foundation for stable network operation.

From Dedicated Nodes to Cloud-Native: A Fundamental Transformation for PCB Hardware

The shift from dedicated telecom equipment to cloud-native servers has introduced disruptive requirements for PCB design concepts and technologies. Traditional telecom hardware typically uses customized ASICs and network processors, where PCB designs, though complex, are relatively fixed with clear optimization goals. In a cloud-native architecture, the server PCBs hosting AMF (formerly VLR functionality) must achieve extreme versatility, scalability, and computational density.

This transformation brings several key challenges:

1. Surge in Computational Density: Modern server CPUs feature hundreds of cores, supplemented by FPGAs and Smart Network Interface Cards (SmartNICs) for traffic acceleration, all integrated onto a single motherboard. This demands extremely high PCB wiring density, often exceeding 20 layers, to accommodate tens of thousands of connection points and complex power networks.
2. Explosive Growth in I/O Bandwidth: The AMF requires high-speed communication with other functions in the core network (e.g., SMF, UDM, AUSF) and massive numbers of Radio Access Network (RAN) devices. This means server PCBs must support multiple 100/200/400 Gbps Ethernet links and employ high-speed buses like PCIe 5.0/6.0 to connect internal components.
3. Unchanged Reliability Requirements: Despite the shift to general-purpose servers for hardware platforms, the telecom-grade "five nines" (99.999%) reliability requirement remains unchanged. This imposes stringent standards on PCB material selection, manufacturing processes, and long-term stability. Even if connectivity issues arise in edge Small Cell PCBs, the stability of the core network must not be affected in the slightest.

High-Speed Signal Integrity (SI): Mastering PCB Design in the 224 Gbps Era

On server motherboards carrying VLR/AMF functionality, data travels at astonishing speeds. 224 Gbps PAM4 signaling technology is already being discussed and gradually adopted in the industry, posing unprecedented challenges to PCB signal integrity (SI). Even minor design flaws, such as impedance mismatches, via stubs, or material losses, can cause severe signal distortion, leading to significant bit errors and ultimately network service disruptions.

To address these challenges, PCB design must employ a range of advanced techniques:

• Ultra-Low-Loss Materials: Traditional FR-4 materials exhibit excessive loss at high frequencies and can no longer meet requirements. Designers must turn to ultra-low-loss (Very Low Loss) or extremely low-loss (Extremely Low Loss) laminate materials like Megtron 7 and Tachyon 100G. These materials significantly reduce signal attenuation during transmission. For high-speed PCBs pursuing ultimate performance, material selection is the first step to success.
• Precision Routing and Simulation: The length, spacing, bends of differential pairs, and via design must be precisely modeled and optimized using professional SI simulation software (e.g., Ansys HFSS, Cadence Clarity). Back-drilling is widely used to remove excess stubs in high-speed signal vias, eliminating signal reflections.
• High-Density Interconnect (HDI) Technology: To shorten critical signal path lengths and increase routing density, HDI PCB technologies like microvias and stacked vias have become standard. This enables compact and efficient layouts around CPUs and high-speed interfaces.

Power Integrity (PI): The Art of Powering Thousands of Cores

Modern server processors and AI accelerators consume hundreds of watts, with peak currents reaching hundreds of amps, and current demands fluctuating dramatically within nanoseconds. Providing stable, clean power to these "power-hungry beasts" is the core task of power integrity (PI) design. A poorly designed power distribution network (PDN) can cause voltage droop (Vdroop) and power noise, affecting system performance at best and causing crashes at worst.

The key to PI design lies in constructing an ultra-low-impedance path from the VRM (Voltage Regulator Module) to the chip pins:

• Multi-Layer Power/Ground Planes: Utilize multilayer PCB designs with multiple complete copper planes dedicated to power and ground. These planes act like giant capacitors, providing low-impedance paths for high-speed current transients.
• Precision Decoupling Capacitor Networks: Hundreds to thousands of decoupling capacitors with varying capacitance values must be carefully placed around the chip and across the PCB. These capacitors serve as local energy reservoirs at different frequency ranges, quickly responding to the chip's transient current demands.
• Co-Simulation and Optimization: PI and SI are no longer isolated issues; they influence each other. High-speed signal switching can cause power supply noise (i.e., Simultaneous Switching Noise, SSN), which in turn affects the signal reference level and increases jitter. Therefore, SI/PI co-simulation is essential to ensure the robustness of the entire system. This complexity is also reflected in the migration of traditional Mobile Switching Centers to virtualized platforms, which imposes higher requirements on the power stability of underlying hardware.

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Thermal Management Challenges: Staying Cool Under kW-Level Power Consumption

Power consumption and heat generation are two sides of the same coin. A fully loaded high-performance server rack can consume several kilowatts of power, with almost all of this energy ultimately converted into heat. If the heat cannot be effectively dissipated, chip temperatures will rise sharply, leading to throttling or even permanent damage. The PCB plays a critical role in the entire thermal management chain.

PCB-level thermal management strategies include:

• High thermal conductivity materials and thick copper layers: Using high Tg PCB materials with higher glass transition temperatures ensures mechanical and electrical stability under high temperatures. Embedding thicker copper planes (e.g., 3-4 oz copper) or using heavy copper PCB technology can effectively spread heat laterally from under high-power components.
• Thermal vias: Dense arrays of thermal vias under high-power chips like CPUs and FPGAs quickly transfer heat to the opposite side of the PCB, where it can be dissipated by heatsinks.
• Embedded cooling technologies: More advanced techniques include embedding copper coins or heat pipes directly in contact with heat-generating components to provide ultra-low thermal resistance paths.
• Layout optimization: During PCB layout, airflow design within the server chassis must be considered, placing high-heat components in areas with maximum airflow and avoiding concentrated "hot spots."

Evolution of PCB Materials and Manufacturing Processes

To meet the simultaneous demands of high speed, high power, and high reliability, server PCBs hosting VLR/AMF functions are pushing the limits of materials and manufacturing processes.

Performance Comparison of Server-Grade PCB Materials

Parameter	Standard FR-4	Mid-loss materials	Ultra-Low Loss Material
Dielectric Constant (Dk @10GHz)	~4.5	~3.8	~3.2
Loss Tangent (Df @10GHz)	~0.020	~0.008	<0.003
Glass Transition Temperature (Tg)	130-140 °C	170-180 °C	>200 °C
Application Scenarios	Low-speed control boards	PCIe 3.0/4.0, 10GbE	5G core networks, 112G+ SerDes

In terms of manufacturing processes, for complex PCBs with over 20 layers, interlayer alignment accuracy is one of the biggest challenges. Any minor deviation may cause via drilling misalignment, leading to open or short circuits. Additionally, through-hole plating with high aspect ratios (board thickness/hole diameter), flatness control of BGA pads (Via-in-Pad Planarization), and other factors require top-tier equipment and strict process control. The reliability of these manufacturing processes directly impacts the stability of critical security functions such as the **Authentication Center PCB** and **Equipment Identity Register**.

The Role of VLR/AMF in Network Slicing and Edge Computing

One of 5G's revolutionary capabilities is network slicing—creating multiple virtual, end-to-end networks on the same physical infrastructure to meet diverse application needs (e.g., eMBB, URLLC, mMTC). The AMF (successor to VLR) plays a pivotal role in user access and slice selection. It must identify the user's slice and ensure mobility and session continuity across slices.

Moreover, to support ultra-low-latency applications like URLLC, network functions are shifting from centralized data centers to the network edge (MEC). This means some AMF functionalities may also be deployed distributively. This architecture introduces new PCB design requirements:

• Diverse Hardware Form Factors: Edge-deployed servers may no longer follow standard 19-inch rack designs but adopt smaller, ruggedized forms for varied environments.
• Environmental Adaptability: Edge node PCBs may need to withstand wider temperature ranges, humidity, and vibration, demanding higher reliability and durability.
• Synchronization and Coordination: Distributed AMF nodes require precise time synchronization and state coordination, posing new challenges for clock circuit design and high-speed interconnects on PCBs. This complexity of distributed core networks far exceeds the traditional centralized Mobile Switching Center architecture, profoundly impacting the entire Packet Core PCB ecosystem.

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Vision for 6G: AI-Native Core Networks and the Future of PCBs

Looking ahead to the 6G era, networks will become more intelligent, endogenous, and converged. The core network is expected to be "AI-native," capable of predictive resource scheduling, intelligent fault self-healing, and context-aware mobility management. The future form of VLR/AMF will be a highly intelligent cognitive mobility management function.

This will have profound implications for underlying PCB technologies:

• Deep Integration of Computing and Interconnect: AI/ML accelerators (e.g., TPU, NPU) will be more tightly integrated with CPUs and network interfaces on the same substrate, even adopting chiplet and co-packaged optics (CPO) technologies. PCBs will evolve into highly integrated system-in-package (SiP) substrates.
• Co-Packaged Optics: As data rates approach Tbps levels, traditional electrical interconnects will face bottlenecks. Optical interconnect technologies will be introduced at the PCB level, enabling ultra-high-speed, low-power data transmission between chips, boards, and even racks.
• Intelligent Thermal and Power Management: PCBs will integrate more sensors to monitor temperature and voltage in real-time, dynamically adjusting fan speeds and VRM outputs through AI algorithms for precise intelligent power and thermal management.
• Application of New Materials: To support THz-band communication and higher-speed digital signals, new PCB substrate materials such as ceramics, glass, and liquid crystal polymers (LCP) will need to be explored.

Conclusion

The evolution of the Visitor Location Register is a microcosm of the entire mobile communication network's transition from dedicated hardware to open, cloud-native, and intelligent systems. Today, this critical function is no longer supported by isolated circuit boards, but by high-performance server PCBs located in global data centers and network edges, which are highly technologically advanced. Mastering the three major challenges of high-speed signal integrity, power integrity, and thermal management has become key to determining the performance of 5G networks and the feasibility of future 6G networks. For PCB manufacturers, network equipment providers, and operators, continuous innovation and investment in materials science, design methodologies, and manufacturing processes are not only necessary to address current challenges but also the strategic cornerstone for winning the communication technology race in the next decade.

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