In the wave of artificial intelligence (AI) and high-performance computing (HPC), industry focus has largely centered on advanced packaging technologies such as CoWoS, Chiplet, and high-density interconnect (HDI) substrates. However, when we delve into the AI accelerator cards and server motherboards that handle kilowatts of power and massive data loads, a seemingly traditional yet critical technology—THT/through-hole soldering—still plays an irreplaceable role. As an engineer specializing in thermal interface design and tolerance control, I understand that system stability and reliability depend not only on cutting-edge chip interconnects but also on those foundational components silently enduring immense electrical and mechanical stress. This article will thoroughly analyze the core value, technical challenges, and integration of THT/through-hole soldering with advanced manufacturing processes in modern AI hardware.

Why Is THT/Through-Hole Soldering Still Indispensable in the AI Era?

Although surface-mount technology (SMT assembly) has become mainstream due to its high density and automation advantages, THT technology offers three core strengths unmatched by SMT in the demanding applications of AI hardware: exceptional mechanical strength, robust current-carrying capacity, and efficient thermal conduction paths.

1. Unparalleled Mechanical Robustness: AI servers and accelerator cards are often equipped with large, heavy components such as high-power connectors (e.g., PCIe card edge connectors, power input terminals), large inductors, transformers, and heatsink mounting brackets. THT component pins penetrate the PCB and are fully encapsulated by solder within the holes, forming an extremely sturdy mechanical anchor. This connection method can withstand intense vibrations, shocks, and mechanical stress from frequent plugging and unplugging, ensuring the physical integrity of the system during transportation, installation, and long-term operation—something the fragile shear-force structure of SMT solder joints cannot achieve.

2. Ultra-High Current and Power Handling: Modern AI GPUs can reach instantaneous power consumption levels in the kilowatt range, placing extreme demands on the power distribution network (PDN). THT pins provide significantly larger cross-sectional and contact areas compared to SMT pads, enabling them to carry hundreds of amps with minimal resistance. This is critical for the stability of main power inputs, voltage regulator module (VRM) output stages, and other high-current pathways, effectively reducing power loss and voltage drop to ensure stable power delivery to AI chips under extreme loads.

3. Efficient Thermal Pathways: As a thermal interface design engineer, I pay special attention to component heat dissipation paths. THT metal pins and plated through-holes (PTH) are inherently excellent thermal conductors. For high-heat components like MOSFETs in VRMs and large inductors, the THT structure not only conducts heat through the pins to the PCB's internal power and ground layers but also efficiently dissipates heat to the air or heatsinks via larger solder joint areas. This "three-dimensional" thermal path is far superior to the "planar" heat dissipation of SMT components, which rely solely on solder pads.

Thus, modern AI hardware design is not a choice between THT and SMT but a synergy of both. High-density logic and control sections employ SMT assembly, while high-power, high-stress, and high-heat sections rely on THT/through-hole soldering, together building a stable and reliable complex electronic system.

The Critical Role of THT in AI Substrate Power Distribution Networks (PDN)

AI chips are highly sensitive to power quality, characterized by high steady-state power consumption and severe transient current fluctuations. A robust PDN is the foundation for ensuring stable operation of AI SoCs, and THT technology is key to building this foundation.

First, the main power input of AI accelerator cards is typically achieved through rugged THT connectors, such as 12VHPWR or custom multi-pin power terminals. These connectors must endure significant insertion/extraction forces and continuous high currents, and only THT's mechanical anchoring capability can guarantee long-term reliability. Any loosening or increased resistance at connection points could lead to catastrophic failures. Secondly, in onboard VRM design, inductors and output capacitors on high-current paths typically use THT packaging. These components are not only large in size and weight but also generate significant electromagnetic forces and heat during operation. THT soldering ensures a robust electrical and mechanical connection between them and the PCB, preventing solder joint fatigue and failure caused by vibration or CTE (Coefficient of Thermal Expansion) mismatch under high-frequency switching and thermal cycling.

Lastly, THT through-holes themselves play the role of "vertical highways" in PDN design. In AI motherboards or carrier boards with up to 20 or more layers, it is necessary to connect power components on the top layer to deeply buried internal power and ground planes with minimal inductance and resistance. Arrays of numerous THT vias, especially those integrated with THT component pins, form low-impedance vertical interconnects, effectively suppressing voltage transients and ensuring smooth power rails. Highleap PCB Factory (HILPCB) has extensive experience in manufacturing heavy copper PCBs, capable of providing copper layers as thick as 6 ounces or more for these high-current THT components, further optimizing PDN performance.

Managing Signal Integrity Challenges Introduced by THT in High-Speed Circuits

While THT offers clear advantages in power delivery and mechanical robustness, it can become a signal integrity (SI) nightmare in high-speed digital circuits. When high-speed signals (such as PCIe 5.0/6.0) must pass through THT connectors or components, their physical structure introduces impedance discontinuities, potentially causing severe signal reflections and attenuation.

The primary challenge stems from "via stubs." THT component pins typically penetrate the entire PCB, but signals may only travel between specific layers. The unused portion of the pin below the signal layers forms a stub, which acts like an antenna, resonating at certain frequencies and significantly degrading signal quality.

To address this challenge, advanced design and manufacturing techniques must be employed:

• Back Drilling: The most effective solution. After THT soldering, the excess metallized barrel (i.e., the stub) is precisely drilled out from the opposite side of the PCB. This requires high-precision drilling equipment and strict depth control. HILPCB's high-speed PCB manufacturing services include back drilling as a mature critical process, effectively eliminating stub-related signal degradation.
• Optimized Pad and Anti-pad Design: By precisely calculating the dimensions of pads and anti-pads around THT pins, the characteristic impedance of the area can be controlled to closely match the transmission line impedance (typically 50 or 100 ohms), minimizing impedance discontinuities.
• Signal Path Planning: During layout, high-speed differential pairs should avoid THT component areas whenever possible. If unavoidable, ensure the shortest path and perform precise 3D electromagnetic field simulations to predict and compensate for the impact.

Thermal Management and Mechanical Reliability Design for THT Components

From a thermal design perspective, THT components serve as heat sources, heat dissipation paths, and mechanical stress concentration points. A reliable THT solder joint must excel electrically, thermally, and mechanically.

Solder joint quality is paramount. The IPC-A-610 standard specifies clear requirements for THT solder joint appearance and fill percentage. An ideal solder joint should form a smooth, concave fillet with at least 75% hole fill (for Class 3 high-reliability products). Insufficient fill weakens mechanical strength and thermal conductivity, while excessive solder may cause shorts or stress concentration. Advanced SPI/AOI/X-Ray inspection (Solder Paste Inspection/Automatic Optical Inspection/X-Ray) technologies are critical here. Particularly, X-ray inspection can penetrate the PCB to clearly visualize internal solder fill, making it the only reliable method for assessing THT joint internal quality.

Regarding thermal cycling, significant CTE mismatch exists between large THT components and their host PCBs. Temperature fluctuations during power cycling or load changes subject solder joints to repeated shear stress, eventually causing fatigue cracks. Design countermeasures include:

• Selecting Appropriate Solder Alloys: For example, SAC (tin-silver-copper) alloys with trace additives offer better fatigue resistance.
• Optimizing Pin Design: Some components feature stress-relief bends in their pins to absorb thermal stress.
• PCB Material Selection: Choosing high-Tg PCB materials with lower Z-axis CTE reduces via deformation during temperature changes, protecting solder joint integrity.

As an experienced PCB manufacturer, HILPCB deeply understands the relationship between material science and structural reliability. We help clients select the most suitable materials and design solutions for their applications, ensuring THT component reliability throughout the product lifecycle.

THT Manufacturing Process Flow and Key Quality Control Points

A high-quality THT solder joint is the product of perfect integration of design, materials, and process. Its manufacturing process is typically performed after the SMT assembly process to prevent thermal shock to small SMT components during wave soldering.

1. Component Preparation and Insertion: THT component leads need pre-forming according to PCB hole diameters. Insertion can be done by Auto-Insertion (AI) machines, but for irregularly shaped or large components, manual operation is often required. The operator's proficiency and correct judgment of component polarity are crucial.
2. Soldering Process Selection:
   • Wave Soldering: Suitable for cases where THT components are densely distributed on one side of the PCB. The PCB passes through a molten solder wave to complete all solder joints simultaneously. Precise control of process parameters (preheat temperature, soldering temperature, conveyor speed) is key.
   • Selective Soldering: When THT components are close to SMT components, a micro-nozzle is used to perform independent, programmable soldering for each THT joint. It offers high precision and a small heat-affected zone, making it the preferred choice for modern high-density mixed circuit boards.
   • Manual Soldering: For rework or very few components, certified technicians perform manual soldering.
3. Quality Inspection: Comprehensive inspection is essential after soldering. In addition to visual checks, SPI/AOI/X-Ray inspection equipment provides objective, repeatable detection capabilities. AOI can quickly verify whether solder joints meet IPC standards, while X-rays can detect internal defects such as voids, insufficient fill, or cold solder joints. During new product introduction, the First Article Inspection (FAI) process is particularly important. By thoroughly measuring and analyzing all THT solder joints on the first article, the entire manufacturing process parameters can be validated and solidified to ensure consistency in subsequent mass production.

How to Optimize THT Design and Verification During NPI Phase?

The success of New Product Introduction (NPI) directly impacts the product's time-to-market and final quality. Systematic optimization of THT (Through-Hole Technology) design and processes during each phase of NPI EVT/DVT/PVT (Engineering/Design/Production Validation Testing) is critical to ensuring project success.

• EVT Phase: The focus is on Design for Manufacturability (DFM) validation. At this stage, the PCB design team should collaborate closely with manufacturing partners like HILPCB. We provide professional recommendations on THT hole-to-pin diameter ratios, pad design, component spacing, etc., to ensure the design is physically manufacturable and reliable. For example, excessively large holes may lead to insufficient solder, while overly small holes can affect insertion efficiency and solder penetration.
• DVT Phase: The core lies in process validation and reliability testing. During this phase, we conduct small-batch trial production and perform rigorous First Article Inspection (FAI). Through thermal shock, vibration testing, and cross-section analysis of trial boards, we verify the long-term reliability of THT solder joints under simulated real-world operating conditions. Based on test results, we fine-tune soldering parameters (e.g., temperature profiles) to achieve optimal soldering quality.
• PVT Phase: The goal is to validate mass production capability and process stability. By this stage, production line configurations, tooling fixtures, and Standard Operating Procedures (SOPs) are finalized. We continuously monitor production data and employ Statistical Process Control (SPC) methods to ensure THT soldering quality fluctuations remain within acceptable limits, preparing for a smooth transition to Mass Production (MP).

The entire NPI EVT/DVT/PVT process is an iterative and optimization-driven cycle, aiming to identify and resolve all THT/through-hole soldering-related issues early in the design phase, thereby reducing risks and costs in later production stages.

Achieving Full-Process Traceability for THT with Traceability/MES Systems

For high-value AI hardware, even the slightest defect can lead to significant financial losses. Therefore, full-process traceability becomes crucial. A robust Manufacturing Execution System (Traceability/MES) can record and correlate every step in the THT production process, providing a solid data foundation for quality control and failure analysis.

The applications of Traceability/MES systems in THT processes include:

• Material Traceability: The system records the batch numbers and supplier information of each THT component and binds them to the final assembled board's serial number. If issues are found with a specific batch of components, all affected products can be quickly identified.
• Process Parameter Traceability: For selective soldering or wave soldering, the MES system records key parameters such as temperature profiles, solder batch numbers, and flux types used during the soldering of each board in real-time.
• Personnel and Equipment Traceability: For workstations requiring manual insertion or soldering, the system records the operator ID, tools used, and operation time. This facilitates targeted training when human errors occur.
• Quality Data Integration: All SPI/AOI/X-Ray inspection results and images are uploaded to the MES system and linked to the corresponding board serial number. This creates a comprehensive quality archive, accessible anytime to analyze defect trends or respond to customer inquiries.

By implementing a comprehensive Traceability/MES system, HILPCB not only enhances production transparency and controllability but also provides customers with the highest level of quality assurance, which is indispensable in the AI and data center sectors where reliability demands are extremely stringent.

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Conclusion: Mastering THT is the Cornerstone of Navigating AI Hardware Complexity

In summary, THT/through-hole soldering is far from obsolete. As AI hardware advances toward higher power density, integration, and reliability, its role as a robust bridge connecting the physical and digital worlds grows even more critical. From providing solid mechanical support and high-current pathways to serving as key thermal dissipation channels, THT technology addresses fundamental challenges that SMT cannot overcome.

However, to fully leverage its advantages while mitigating potential risks (e.g., in high-speed signals), deep design expertise, precise manufacturing processes, and stringent quality control systems are essential. This includes DFM optimization during NPI, SPI/AOI/X-Ray inspection in production, and end-to-end Traceability/MES systems. Partnering with experts like HILPCB, who excel in advanced PCB manufacturing and complex mixed-technology assembly, is key to ensuring your AI products succeed in a competitive market. We are committed to perfecting every foundational process, delivering the most reliable physical foundation for your innovative visions.

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