In modern power supply and cooling systems, the high power density and stringent thermal management requirements pose unprecedented challenges to PCB design and manufacturing. Although surface mount technology (SMT) has become mainstream due to its automation and integration capabilities, THT/through-hole soldering technology, with its unparalleled mechanical strength, high current-carrying capacity, and exceptional thermal performance, remains an indispensable core technology in fields such as power electronics, automotive, industrial automation, and renewable energy. It is not an outdated technology but rather a strategic choice to ensure system stability and reliability under extreme operating conditions. From the perspective of a seasoned VRM/PDN design expert, this article will delve into how to strategically leverage THT/through-hole soldering technology to address design, manufacturing, and validation challenges in high-current, high-transient, and complex thermal environments.

The Core Value of THT/Through-Hole Soldering in High-Power PDN Design: Deep Connections Beyond the Surface

The ultimate goal of a power distribution network (PDN) is to provide a stable and clean voltage "plane" for core chips (such as CPUs, GPUs, and FPGAs) under various static and dynamic load conditions. In high-power applications like servers, communication base stations, electric vehicle inverters, or industrial control systems, PDN performance directly determines the success or failure of the entire system. In such scenarios, THT/through-hole soldering components-such as large-capacity electrolytic capacitors, high-inductance power inductors, heavy-duty connectors, and power modules-serve as the physical foundation for building a robust PDN.

Compared to the precision of SMT assembly, THT component pins penetrate the PCB and are soldered on the opposite side, forming a three-dimensional, deeply embedded mechanical and electrical connection. This structure offers advantages that SMT cannot match:

1. Ability to Handle Extreme High Currents: This is the most intuitive advantage of THT. A standard SMT 1206 package pad with 1-ounce copper thickness may safely carry only 2-3 amps of current, whereas a well-designed THT through-hole pad, combined with heavy copper PCB processes, can easily handle tens or even hundreds of amps. The fundamental reason lies in the fact that THT solder joints not only utilize surface pads but also direct current to large-area copper planes on inner and bottom layers through plated through-holes (PTH), creating a three-dimensional current path. The "solder column" formed after the solder completely fills the through-hole tightly bonds with the pin and hole wall, offering a contact area far larger than SMT pads, thereby significantly reducing contact resistance and Joule heating (I²R losses).

2. Exceptional Resistance to Mechanical Stress: In applications such as automotive, industrial robotics, or frequent plugging and unplugging, vibration, shock, and mechanical stress are commonplace. SMT solder joints are inherently two-dimensional connections, and their reliability heavily depends on the intermetallic compound (IMC) layer between the solder and the pad. Under continuous mechanical stress, the IMC layer is prone to micro-cracks, eventually leading to solder joint fatigue failure or pad cratering. In contrast, THT component pins penetrate the substrate, anchoring the component firmly like a "ship's anchor." Their tensile and shear resistance is an order of magnitude higher than SMT, effectively preventing connection failures caused by mechanical stress, which is critical for ensuring long-term system reliability.

3. Efficient Vertical Heat Dissipation Channel: The "lifeline" of power devices lies in heat dissipation. The metal pins of THT components are inherently excellent thermal conductors. They act like miniature "heat pipes," rapidly transferring heat generated by MOSFETs, IGBTs, or power inductor cores from the device interior to the PCB. Once heat reaches the PCB, it can spread laterally through large-area power or ground planes (typically thick copper layers) or be conducted vertically to the PCB backside via dense thermal via arrays, where it is then dissipated by heatsinks. This three-dimensional heat dissipation network-from point to surface and top to bottom-exhibits significantly lower thermal resistance compared to SMT solutions that rely on PCB surface copper foil for heat dissipation. The effect is particularly pronounced when using High Thermal PCBs (such as aluminum or ceramic substrates).

PDN Impedance Targets and Frequency Band Coverage: A Collaborative Strategy for THT and SMT Components

An ideal PDN should maintain an extremely low target impedance across a broad frequency range (from DC to several GHz). No single type of capacitor can achieve this alone. Thus, a successful PDN design is an art of collaboration between THT and SMT components. We can break down the PDN's response frequency bands as follows:

• Low-Frequency Band (DC ~ hundreds of kHz): Impedance in this band is primarily determined by the response speed of the voltage regulator module (VRM) and the large-capacity electrolytic or polymer THT capacitors. When load current undergoes slow but significant changes (e.g., a server transitioning from idle to full load), the VRM control loop requires time to respond. During this period, these THT capacitors function like "pumped-storage power stations" in the grid, releasing their substantial stored energy (high capacitance) to maintain voltage stability. Their relatively high ESR (equivalent series resistance) and ESL (equivalent series inductance) render them ineffective at higher frequencies, but they remain irreplaceable "energy reservoirs" in the low-frequency band.

• Mid-Frequency Band (hundreds of kHz ~ tens of MHz): This is the core battleground for decoupling networks and where most digital circuit switching noise concentrates. Impedance control in this band relies on numerous low-ESR/ESL multilayer ceramic capacitors (MLCCs) installed on the PCB via SMT assembly. By carefully arranging MLCCs of varying capacitances (e.g., 10μF, 1μF, 0.1μF) around chip power pins, a broadband low-impedance path can be created to effectively suppress noise in this band. This strategy, known as "layered decoupling," assigns each capacitor layer to a specific frequency band, collectively "flattening" the impedance curve.

• High-Frequency Band (> tens of MHz): At these frequencies, discrete capacitors become largely ineffective, and the PCB's parasitic inductance and capacitance dominate. Here, PDN performance depends on physical design: optimized PCB stackup (e.g., tightly coupled power/ground planes), minimized current return paths, and dense grounding vias (stitching vias) are critical for controlling high-frequency impedance.

Throughout the NPI EVT/DVT/PVT (New Product Introduction) phases, PDN validation is a core task. During the EVT (Engineering Verification Test) phase, engineers use vector network analyzers (VNAs) to measure actual impedance on prototype boards and compare it with simulation results (e.g., Bode plots) to verify design compliance. In the DVT (Design Verification Test) phase, more rigorous load transient tests are conducted, using electronic loads to simulate worst-case current steps (dI/dt) while observing voltage droop (Vdroop) and overshoot to ensure the collaborative THT-SMT strategy performs effectively in real-world conditions.

Transient Load and Dynamic Response Optimization: A Nanosecond Charge Relay Race

Modern high-performance processors or FPGAs can experience drastic load current changes (high dI/dt) within nanoseconds (ns), which poses the ultimate test for a PDN's dynamic response capability. Imagine when a GPU starts rendering a complex frame, its current demand may surge from 10A to 200A within 100ns. At this moment, a charge relay race across the entire PCB begins:

1. Initial 1-10ns: The response comes from the capacitors inside the chip package and on the Die, which are the closest "first-aid kits" to the transistors.
2. 10ns - 1μs: The SMT MLCCs tightly surrounding the chip begin discharging. They are the "frontline soldiers," providing the first wave of support for transient current demands.
3. 1μs - 100μs: As time progresses and current demand persists, the charge in the proximal MLCCs gradually depletes. At this stage, the slightly more distant, higher-capacity SMT capacitors and ultimately the large THT bulk capacitors take over the relay. They act like "logistical supply lines," continuously delivering stored charge to the front.
4. >100μs: The VRM's control loop finally reacts and begins increasing output power to fundamentally meet the new steady-state current demand.

In this process, THT components play the role of "strategic reserves." Strategies to optimize this "charge delivery chain" include:

• Minimizing parasitic inductance: Inductance is the number one enemy of transient response (V = L * dI/dt). THT component pins inherently have non-negligible ESL. Designers should opt for short-pin or planar-packaged power devices. More importantly, ensure sufficient low-inductance via connections between their pads and the power/ground planes. A common mistake is assigning only one via to a high-current THT pin, creating a severe inductance bottleneck.
• Plane design: Use complete, low-impedance power and ground planes instead of thin traces to transmit high currents. This not only reduces DC resistance but, more critically, significantly lowers the overall inductance of the PDN. For Heavy Copper PCBs (e.g., using 3oz or thicker copper), their low-inductance characteristics are particularly important for improving high-frequency response.

Thermal Management and Reliability: Comprehensive Protection from Pad Design to Conformal Coating

Another major battlefield for THT/through-hole soldering lies in thermal management. Power MOSFETs, inductors, fuses, and connectors-often high-heat components-typically use THT packaging precisely to leverage their superior heat dissipation capabilities.

• The Art of Thermal Pads: Designing pads for THT components is a balancing art. On one hand, for efficient heat conduction, we want pins to connect directly to large copper areas. But on the other hand, this can cause rapid heat dissipation during soldering, creating "Heat Sink Pads," which may lead to cold solder joints or poor connections. Thus, Thermal Relief Pads were introduced. They connect the pad to the copper plane via several slender "spokes," ensuring electrical connectivity while providing thermal isolation during soldering, guaranteeing the solder joint reaches sufficient temperature.

  • Practical Tip: For pins requiring extreme heat dissipation (e.g., the drain of a MOSFET), a "direct connection + thermal vias" strategy can be adopted. Here, the pad connects directly to the copper plane, with a dense array of thermal vias placed below or around the pad to efficiently transfer heat to inner or back PCB layers. For signal pins or low-power pins, standard thermal relief pads should be used to ensure soldering quality.

• The Ultimate Protection of Conformal Coating: In harsh environments-such as humid, dusty, salty, or chemically corrosive conditions (e.g., automotive engine compartments, coastal communication equipment, or chemical plant control units)-exposed solder joints and metal pins are weak points. Moisture and contaminants can cause metal ion migration, forming dendrites (Dendritic Growth), eventually leading to short circuits. In such cases, applying Conformal Coating (protective coating) to the entire PCBA transitions from an "option" to a "necessity."

  • Process Details: A uniform layer of Conformal Coating (typically acrylic, silicone, or polyurethane) effectively isolates the board from external conditions. Before coating, the PCBA must be thoroughly cleaned and dried to ensure strong adhesion. For THT components, due to their complex 3D structures, automated spraying may create "shadow effects," leaving some areas uncoated. Thus, dip coating, selective coating, or manual touch-ups are often required to ensure full coverage of all solder joints, pin roots, and component bodies. Post-coating, UV light inspection (if the coating contains fluorescent agents) and thickness measurements are necessary to verify compliance with standards.