With the explosive growth of generative AI, large language models (LLMs), and high-performance computing (HPC), data centers are undergoing an unprecedented architectural transformation. As the core of this revolution, AI servers have reached staggering levels of internal data throughput and power density. Serving as the "neural hub" connecting GPUs, CPUs, memory, and I/O modules, the design and manufacturing of server backplane PCBs face severe challenges. In an era dominated by SMT (Surface Mount Technology), a seemingly traditional technique—THT/through-hole soldering—plays an indispensable role in AI server backplanes. It not only ensures mechanical robustness but also directly impacts high-speed signal integrity, power distribution efficiency, and overall thermal performance.

From the perspective of an AI server and backplane high-speed interconnect architecture expert, this article delves into the core value, technical challenges, and advanced manufacturing processes of THT/through-hole soldering in modern AI server design. We will explore how to optimize THT processes to meet the requirements of high-speed buses like PCIe Gen5/Gen6 and CXL, while also examining how critical workflows such as Selective wave soldering, First Article Inspection (FAI), and Traceability/MES collectively ensure top-tier AI server motherboard PCB quality. With its deep expertise in complex PCB manufacturing and assembly, Highleap PCB Factory (HILPCB) is committed to providing customers with one-stop solutions that meet stringent performance demands.

Why is THT/Through-Hole Soldering Still Indispensable in AI Servers?

Despite SMT technology's dominance due to its high density and automation advantages, THT/through-hole soldering remains irreplaceable in AI servers, where power consumption can exceed thousands of watts and data rates reach up to 64 GT/s. Its core value lies in three key aspects:

1. Unmatched Mechanical Strength: AI server backplanes must support large, heavy connectors such as high-speed backplane connectors, power input terminals, and fan interfaces. These connectors endure significant mechanical stress during insertion/removal and long-term operation. THT component pins penetrate the PCB and are securely soldered along the entire hole wall, creating solder joints far stronger than SMT components attached only to surface pads. This robust connection is fundamental to ensuring the long-term reliability of data-center AI server motherboard PCBs.

2. Exceptional High-Current Carrying Capacity: AI accelerator cards (e.g., NVIDIA H100/B200) demand extremely high peak currents, placing stringent requirements on power distribution networks (PDNs). THT connectors and power terminals feature thick pins and larger solder areas, providing low-resistance, low-inductance current paths capable of handling hundreds of amps. This minimizes voltage drops and power losses, ensuring stable power delivery to GPU clusters.

3. Optimized Thermal Pathways: THT component pins serve not only as electrical conduits but also as efficient heat dissipation paths. For heat-generating components like power inductors and MOSFETs in VRMs (Voltage Regulator Modules), THT pins rapidly transfer heat to PCB-internal heavy copper layers and thermal planes, aiding system thermal management and preventing performance degradation or hardware damage due to localized overheating.

High-Speed Signal Integrity: SI Challenges and Optimization for THT Connectors

In the era of PCIe Gen5/Gen6 and CXL 3.0, the doubling of signal rates has made THT connection points a critical bottleneck in signal integrity (SI) design. Unoptimized THT vias and pin stubs can introduce severe impedance discontinuities, signal reflections, and inter-symbol interference (ISI), leading to a sharp increase in link bit error rate (BER).

To address these challenges, precise optimization strategies must be adopted:

• Back-drilling: This is one of the most effective optimization techniques. By drilling out the excess, unused copper stubs (i.e., stubs) from the back of the PCB, the resonance caused by stubs can be eliminated, significantly improving S-parameters (insertion loss and return loss) and ensuring smooth transmission of high-speed signals.
• Optimized Anti-pad Design: Designing appropriately sized isolation areas (Anti-pads) around the vias in the reference planes (GND/VCC) allows precise control of the via's characteristic impedance, matching it to the 100-ohm or 90-ohm impedance of differential pairs, thereby reducing impedance discontinuities.
• Teardrop Design: Adding smooth copper transitions (teardrops) at the junctions between pads and traces enhances mechanical strength and improves impedance matching for high-frequency signals, which is crucial for overall AI server motherboard PCB quality.

At HILPCB, advanced EDA tools are used for precise 3D electromagnetic field simulations in high-speed PCB (high-speed pcb) manufacturing, optimizing THT connector via structures for customers to ensure exceptional signal integrity even at rates up to 112 Gbps/PAM4.

### Selective Wave Soldering: A Precision Solution for Modern AI Server PCBs

For complex data-center AI server motherboard PCBs that contain a large number of SMT components alongside a few THT components, traditional wave soldering processes expose the already mounted SMT components to repeated high-temperature shocks, posing a risk of thermal damage. To address this challenge, Selective Wave Soldering has emerged as an innovative solution.

Selective Wave Soldering is a highly automated THT/through-hole soldering process that utilizes a miniature, precisely controlled solder nozzle to target only designated THT solder joints. Its core advantages include:

• Precise Heat Control: Heat is applied accurately to the target area, avoiding thermal shock to nearby sensitive SMT components (such as BGAs and optical modules), significantly enhancing product reliability.
• High Flexibility: By programming, unique soldering parameters (preheating time, soldering temperature, contact time) can be set for different types of THT components, achieving optimal soldering results.
• Exceptional Soldering Quality: The automated process ensures consistency and repeatability of solder joints, with full solder fill, effectively eliminating manual soldering defects. It is a key technology for ensuring AI server motherboard PCB quality.

HILPCB's Through-Hole Assembly production line is equipped with advanced selective wave soldering equipment, enabling high-precision, high-reliability mixed-technology PCBA assembly services for customers.

Power Integrity (PI) and Thermal Management: The Dual Advantages of THT

Power Integrity (PI) and thermal management in AI servers are interconnected systemic challenges. THT components play a dual role in addressing these issues.

In terms of PI, the backplane serves as the central power distribution hub, requiring efficient and stable delivery of kilowatts of power to various computing modules. THT power connectors, high-current terminals, and Press-fit connectors provide extremely low DC resistance, minimizing I²R losses. Additionally, their robust structure can withstand electromagnetic forces generated by high currents, ensuring long-term connection stability.

For thermal management, THT component pins act as critical heat dissipation channels. For example, on a Backplane PCB, the power inductors and MOSFETs of VRM modules transfer heat through THT pins to the PCB's embedded thick copper layers, which then evenly dissipate the heat across large GND/VCC planes or transfer it to chassis heat sinks. This design effectively reduces component operating temperatures, improving the efficiency and lifespan of power modules.

First Article Inspection (FAI): The First Line of Defense for Ensuring THT Soldering Quality

For high-value, high-reliability AI server motherboards, even the slightest manufacturing deviation can lead to catastrophic consequences. First Article Inspection (FAI) is a comprehensive and rigorous verification process conducted on the first piece or batch of products before mass production, serving as a critical step in the quality control workflow.

For THT/through-hole soldering, the FAI process focuses on the following aspects:

• Component Accuracy: Verify that the part numbers and specifications of THT components match the BOM (Bill of Materials) exactly.
• Installation Orientation: Check whether polarized components (e.g., electrolytic capacitors, diodes) are installed in the correct direction.
• Solder Quality: Use high-magnification microscopes, X-Ray equipment, and other tools to inspect solder joints for appearance, gloss, wettability, pin protrusion length, and most importantly—hole fill rate according to IPC-A-610 Class 3 standards. For high-reliability products like AI servers, the hole fill rate typically must exceed 75%.
• Process Parameter Validation: Confirm whether selective wave soldering parameters—such as temperature profiles, conveyor speed, and nozzle type—align with the engineering documentation (ECN).

Through stringent First Article Inspection (FAI), HILPCB can identify and rectify potential process issues early in mass production, ensuring that every data-center AI server motherboard PCB delivered meets the most rigorous quality standards.

How Does the Traceability/MES System Ensure Full-Process Traceability for THT Assembly?

In complex PCBA manufacturing, traceability is the cornerstone of exceptional quality control and rapid issue resolution. The Traceability/MES (Manufacturing Execution System) assigns a unique serial number to each PCB, enabling end-to-end data recording and monitoring from raw materials to finished products.

For THT/through-hole soldering, the Traceability/MES system plays a pivotal role:

1. Material Traceability: The system logs batch numbers, supplier details for each THT component, and the batches of solder and flux used. If material-related issues arise, all affected products can be quickly identified.
2. Process Parameter Traceability: Key parameters during selective wave soldering—such as preheat temperature, soldering temperature, and soldering time—are automatically recorded. When soldering defects occur, engineers can review this data to swiftly diagnose the root cause.
3. Personnel and Equipment Traceability: Operator IDs and equipment serial numbers are documented. This helps correlate issues with specific personnel or machines for targeted training or maintenance.
4. Quality Data Integration: FAI, AOI, X-Ray, and functional test results are linked to the PCB serial number, creating a comprehensive quality record. This provides robust data support for continuous process improvement and customer quality reports.

HILPCB’s smart manufacturing facilities are fully equipped with advanced Traceability/MES systems, offering customers end-to-end traceability from bare PCBs to finished PCBA products. This ensures unparalleled transparency and reliability throughout the product lifecycle.

Analysis of THT Soldering Defects and IPC Standards

Despite being a mature process, THT/through-hole soldering can still exhibit typical defects that impact product performance and reliability. Manufacturers must conduct rigorous inspections and process controls in accordance with the IPC-A-610 standard ("Acceptability of Electronic Assemblies"). For demanding applications like AI servers, the highest-grade Class 3 standards are typically followed.

Common defects and their impacts:

• Cold Solder Joints: Dull, rough solder surfaces caused by insufficient temperature or duration. These joints have poor mechanical strength and unreliable electrical connections, potentially leading to intermittent failures.
• Solder Bridging (Short Circuits): Excess solder connecting adjacent pins, directly causing circuit shorts.
• Poor Wetting: Solder fails to form a proper intermetallic layer with pins or hole walls, reducing connection strength and conductivity.
• Insufficient Hole Fill: One of the most critical THT soldering metrics. IPC Class 3 typically requires ≥75% vertical through-hole fill. Inadequate filling weakens mechanical strength and current-carrying capacity.

To systematically prevent these defects, HILPCB implements comprehensive process controls, including incoming PCB solderability inspections, precise flux spraying, strict monitoring of preheating and soldering temperature profiles, and multi-layered automated/manual inspections to ensure every THT joint meets Class 3 standards.

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Conclusion: Masterful THT Process is the Foundation of High-Performance AI Servers

In summary, THT/through-hole soldering is far from obsolete. In the pursuit of extreme performance and reliability for AI servers, it remains an indispensable process due to its unique advantages in mechanical strength, high-current handling, and thermal management. However, successful application in cutting-edge data-center AI server motherboard PCBs carrying PCIe Gen6 and other high-speed signals requires advanced design optimizations (e.g., back drilling), precision manufacturing techniques (e.g., Selective wave soldering), and stringent quality assurance systems (e.g., First Article Inspection (FAI) and Traceability/MES systems). Selecting a partner who understands both high-speed PCB design principles and possesses exquisite manufacturing and assembly capabilities is crucial. HILPCB is committed to providing one-stop services from PCB design optimization, high-quality bare board manufacturing to highly reliable PCBA assembly, helping customers address the formidable challenges brought by the AI era and ensuring their products stand out in the fiercely competitive market. Our commitment to AI server motherboard PCB quality excellence permeates every stage of production.

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