In the demanding world of automotive electronics, particularly in Advanced Driver Assistance Systems (ADAS) and Electric Vehicle (EV) power systems, every design decision directly impacts safety, performance, and long-term reliability. This is a battlefield defined by vibrations, extreme temperature fluctuations, and the risks of high-voltage arcing. Although Surface Mount Technology (SMT) has become mainstream due to its high density and automation advantages, THT/through-hole soldering technology, with its unparalleled mechanical strength, superior current-carrying capacity, and excellent thermal management characteristics, still plays an indispensable core role in high-voltage, high-power applications such as onboard chargers (OBC), DC-DC converters, and SiC/GaN-based power modules. From the perspective of an engineer with years of experience in EV powertrains, this article delves into how to harness THT technology to meet the multiple challenges of automotive-grade requirements.

Mastering the High-Frequency Challenges of Wide-Bandgap Semiconductors: The Electrical Stability Foundation of THT

The application of wide-bandgap semiconductors (SiC/GaN) is reshaping EV power systems in unprecedented ways, significantly improving power density and efficiency. However, their extremely high switching speeds (dv/dt, up to tens of kV/μs) are a double-edged sword, posing severe electromagnetic interference (EMI) and common-mode noise challenges. In these high-speed switching drive circuits, even the slightest parasitic inductance can be amplified by rapidly changing currents, leading to fatal voltage overshoots and oscillations that directly threaten expensive power devices.

This is where the value of THT components-such as isolated pulse transformers, high-capacity film filtering capacitors, and high-voltage connectors-becomes evident. Their robust physical connections, formed by sturdy pins penetrating the PCB and enveloped in ample solder, create extremely low-impedance electrical nodes. Compared to SMT components, which rely solely on surface pad connections, THT effectively reduces parasitic inductance and resistance in critical loops, providing a "quiet" and stable platform for controlling high-speed switching.

A successful SiC/GaN drive design must begin with a comprehensive and rigorous DFM/DFT/DFA review (Design for Manufacturability/Testability/Assembly). At the early design stage, we must adopt a systems-thinking approach to evaluate the layout of THT components:

• Minimize Drive Loop Area: The current loop formed by the primary-side pins of the isolation transformer, the output pins of the gate drive IC, and the gate and source pins of the SiC/GaN devices must be compressed to the absolute minimum. Although THT transformers are bulkier, their flexible pin arrangement allows engineers to place them as close as possible to the power devices, thereby minimizing the risk of high-frequency noise coupling.
• Optimize Through-Hole Design: This is more than just drilling a hole. The fit between the hole diameter and the pin diameter (typically recommended to be 0.25-0.5mm larger than the pin diameter), the size of the annular ring, and whether thermal relief pads are used to connect to large copper areas all directly impact soldering quality and the impedance of the current path. In high-speed drive circuits, we may even avoid thermal relief pads, opting for direct connections to achieve the lowest electrical impedance at the cost of some soldering convenience.
• Ground Path Planning: The ground pins of THT components provide an excellent low-impedance grounding anchor. By tightly stitching these pins to the PCB's ground plane through multiple vias, a broad and continuous return path for high-frequency currents can be established, which is crucial for suppressing common-mode noise.

HILPCB has accumulated extensive practical experience in handling such high-frequency PCBs, assisting customers in foreseeing and mitigating potential electrical performance and reliability risks caused by physical layout during the schematic design phase.

Building High-Voltage Safety Barriers: The Physical Advantages of THT in Electrical Isolation Design

As EV platforms advance toward 800V and even higher voltages, electrical isolation has become a lifeline for ensuring the safety of passengers, maintenance technicians, and the entire vehicle. In safety standards (such as IEC 60664-1), Creepage (the shortest distance between two conductors measured along the surface of insulation) and Clearance (the shortest straight-line distance between two conductors measured through air) are two core metrics. Insufficient design can lead to insulation breakdown or arcing under high voltage, resulting in catastrophic consequences.

THT/through-hole soldering demonstrates its inherent structural advantages in this field. The pins of THT-packaged isolation devices (such as optocouplers and isolation transformers) and Y-capacitors used for EMI suppression inherently feature larger physical spacing, providing a solid foundation for meeting stringent safety standards. More importantly, the THT structure allows engineers to adopt more thorough isolation enhancement measures:

• PCB Slotting: Between high-voltage THT pins, isolation slots can be easily milled into the PCB. This effectively cuts off surface creepage paths, forcing current to traverse longer air gaps, thereby significantly increasing creepage distance at minimal cost. Achieving the same effect with SMT components is far more complex.
• Integration of Physical Barriers: The size of THT components allows for better integration with physical barriers such as plastic housings and insulating partitions, forming multiple layers of protection.

However, in harsh environments like moisture, dust, and condensation that vehicles may encounter during their lifecycle, relying solely on physical distance is insufficient. Contaminants can form conductive paths on insulating surfaces, gradually eroding the designed safety margin. To address this, the Potting/encapsulation process offers the ultimate solution. It involves completely encasing the entire PCBA module with insulating materials like epoxy or silicone, forming a robust and dense protective layer.

Potting not only completely isolates moisture, dust, and corrosive chemicals but also significantly enhances the module's mechanical strength, effectively resisting continuous vibrations and shocks during vehicle operation. However, successful potting is no easy task, and meticulous DFM/DFT/DFA review is equally critical at this stage:

• Avoiding Voids: Bubbles or voids formed during the curing of potting materials can be fatal. Under high voltage, these voids can trigger partial discharge, gradually degrading the insulation material over time and eventually leading to breakdown. During the DFM phase, it is essential to ensure that the layout of THT components does not create "dead zones" that hinder the flow and degassing of the potting compound.
• Stress Management: Different materials have different coefficients of thermal expansion (CTE). Under extreme temperature cycling (-40°C to 125°C or higher), CTE mismatches between potting materials and THT components (especially ceramic capacitors and magnetic cores) can generate significant mechanical stress, potentially causing component cracking or solder joint fatigue. Selecting low-modulus, flexible potting materials and avoiding sharp edges in the design are effective ways to mitigate stress.

Addressing Power Density Challenges: THT Thermal Management Strategies in OBC/DC-DC Applications

In on-board chargers (OBC) and DC-DC converters, high-efficiency soft-switching topologies such as LLC and PSFB are mainstream choices. The core components in these circuits-power transformers, resonant inductors, and output filter capacitors-are all "power giants" characterized by large size, heavy weight, and high heat generation. For these components, adopting THT/through-hole soldering is not only a consideration for mechanical reliability but also a critical aspect of thermal management strategies.

The uniqueness of THT connections lies in the fact that their pins, which penetrate the PCB, serve not only as electrical pathways but also as highly efficient thermal conduits. Heat can transfer from the component's interior through the pins directly to the inner copper layers of the PCB or even to heat sinks on the opposite side. To maximize this effect, we often employ heavy copper PCB designs, where the inner or outer copper thickness can reach 3oz (105μm) or higher. The plated through-holes (PTH) of THT components can seamlessly connect with multi-layer thick copper planes, forming a three-dimensional, highly efficient heat dissipation network that rapidly spreads localized heat sources across the entire board. A well-designed THT solder joint can have significantly lower thermal resistance than an SMT solder joint, which is crucial for improving system power density and extending component lifespan.

When assembling these complex mixed-technology (SMT+THT) circuit boards, traditional wave soldering is no longer sufficient, as it indiscriminately heats the entire PCB and may damage temperature-sensitive SMT components (such as microcontrollers and sensors) that are already mounted. In this context, Selective wave soldering technology becomes the key to ensuring quality. It employs a programmable, precise micro-solder nozzle to perform point-to-point soldering on designated THT pins. The entire process is highly controllable:

1. Precise Flux Application: Flux is sprayed or dispensed only on the areas to be soldered, avoiding contamination of other regions.
2. Localized Preheating: The soldering area is preheated from the bottom to activate the flux and reduce thermal shock during soldering.
3. Controlled Soldering: Parameters such as solder wave height, movement path, and contact time can be independently programmed for each solder joint, ensuring full solder wetting, fillet formation, and shiny, robust solder joints while minimizing thermal impact on adjacent components.

This precise process eliminates the inconsistencies and potential human errors associated with manual soldering, serving as the cornerstone for achieving automotive-grade THT soldering quality.

From Manufacturing to Validation: Building a Closed-Loop Process for Automotive-Grade Quality

Reliable THT/through-hole soldering is not achieved overnight; it relies on precise manufacturing processes and rigorous testing and validation throughout the entire workflow.

On the manufacturing side, Selective wave soldering is the core of ensuring soldering consistency. By customizing soldering parameters for solder joints with different thermal capacities (e.g., pins connected to large copper planes versus unconnected pins), common defects such as cold solder joints, insufficient solder, or excessive solder can be effectively avoided. On the verification side, especially during the prototype and small-to-medium batch production stages, Flying Probe Test offers an unparalleled, efficient, and flexible testing solution. Unlike In-Circuit Test (ICT), which requires expensive custom bed-of-nails fixtures for each PCB design, flying probe testers utilize 2 to 8 independently movable probes to directly probe the PCBA based on CAD data. For EV power modules densely populated with THT components, its value is demonstrated in:

• High-Voltage Isolation Testing: Capable of applying DC voltages as high as 1000V or more, it precisely measures insulation resistance between high-voltage and low-voltage networks, as well as between high-voltage networks and chassis ground. This verifies the effectiveness of creepage and clearance designs, ensuring resistance values remain at the GΩ level.
• Indirect Assessment of Solder Joint Quality: Through precise four-wire (Kelvin) measurements, it can detect minor resistance anomalies in THT solder joints, which may indicate early signs of cold solder or poor wetting.
• Flexibility and Rapid Iteration: When design changes occur, only the test program needs updating-no hardware fixture rework is required. This significantly accelerates product development and iteration cycles.

Most importantly, by systematically collecting Flying Probe Test failure data (e.g., which solder joint shows low insulation resistance or which network exhibits intermittent opens) and feeding it back into the DFM/DFT/DFA review process, we establish a robust closed-loop quality improvement system. For example, if testing reveals recurring soldering defects on a specific pin of a THT connector, engineers can trace back to inspect its through-hole design, thermal relief pad configuration, or selective wave soldering parameters, thereby addressing root causes and continuously enhancing product manufacturability and long-term reliability. HILPCB offers one-stop PCBA services from prototyping to mass production, ensuring every step from design review to final testing strictly complies with automotive-grade standards.

Ultimate Environmental Protection: In-Depth Practices of Conformal Coating and Potting

The operating environment of automotive electronics is incomparable to that of consumer electronics. They must endure extreme temperatures (-40°C to +125°C), high humidity, salt spray corrosion, and continuous random vibrations throughout their lifecycle. Conformal coating (protective coating/conformal film) is the first and most common line of defense for protecting PCBA.

It forms a transparent insulating protective film on the circuit board surface, typically 25-75μm thick, effectively isolating THT solder joints and sensitive circuits from environmental erosion. Material selection is critical in automotive applications:

• Silicone: Offers excellent high/low-temperature stability and flexibility, adapting well to stress from thermal cycling, making it the preferred choice for powertrain applications.
• Urethane: Provides superior chemical and abrasion resistance, suitable for areas exposed to oils or fluids.
• Acrylic: Easy to apply and rework, but with slightly weaker chemical and high-temperature resistance.

Selective automated spraying is currently the most advanced coating process, enabling precise control over coating areas while avoiding connectors, test points, and other unprotected regions, ensuring uniform and consistent film thickness. PCBA cleanliness before coating is a decisive factor for success-any residual flux or contaminants may lead to corrosion under the film or reduced adhesion.

For components exposed to the harshest environments, such as powertrain controllers, battery management systems (BMS), or inverter modules, Potting/encapsulation offers a higher level of nearly "indestructible" protection. As mentioned earlier, it encapsulates the entire module in a robust polymer, making it completely waterproof, dustproof, and highly resistant to vibration. The selection of potting materials is equally scientific, requiring a comprehensive evaluation of dielectric strength, thermal conductivity, viscosity, curing time, and mechanical properties to match specific application scenarios.

Whether Conformal coating or Potting/encapsulation, successful implementation heavily relies on precise control over THT component profiles, heights, and layouts, bringing us back to the starting point of design-a well-considered DFM process.

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Conclusion

In summary, THT/through-hole soldering technology is far from obsolete in automotive ADAS and EV power systems that demand high power, high voltage, and extreme reliability-it remains an irreplaceable engineering cornerstone. To successfully master this technology, a comprehensive and systematic strategy spanning design, manufacturing, testing, and final protection is essential. This begins with thorough DFM/DFT/DFA reviews during the early design phase to optimize electrical, thermal, and mechanical performance; employs precision processes like Selective wave soldering during manufacturing to ensure soldering quality; rigorously validates quality through advanced methods such as Flying probe testing, forming a data-driven improvement loop; and finally, applies advanced protective measures like Conformal coating or Potting/encapsulation to armor the product. Only by tightly integrating these steps as an organic whole can we develop next-generation automotive electronics that truly meet the safety and reliability standards of the next decade.

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