Thick Copper Coil + HDI PCB: Power Supply PCB Manufacturing by SunKey

Driven by both artificial intelligence (AI) computing power and new energy vehicles, the design of power modules is facing unprecedented challenges. For example, AI servers require higher computing power and typically adopt a 48V architecture, while new energy vehicles are pursuing higher efficiency and have successively upgraded to 800V platforms.

Traditional magnetic-core wire-wound transformers can no longer meet the design requirements for high power densities above 5 kW/L due to their large size, poor heat dissipation, and difficulty in ensuring consistency. As a result, “wireless” thick copper coil PCB have become an industry trend. Hardware engineers are now using spiral interlayer routing in PCB instead of enameled wire to create planar transformers, but this has brought significant challenges to PCB manufacturing.

• High current + resistance to side etching: Conductive layers need to reach 4oz–10oz, making etching precision difficult to control.

• High density + multi-interconnection: Routing space is compressed, so HDI and Any-layer (any-layer interconnection) technologies must be introduced to break through the limitations of through-holes.

• High heat dissipation + strong insulation: Dielectric layers must be thin to facilitate heat dissipation, yet still withstand breakdown voltage tests of over 1500V at very close distances.

For PCB manufacturing, this means that two major challenges must be addressed simultaneously on the same board: “fine-line interconnection” and “high-current transmission with thick copper.” Ensuring both fine-line precision and high connection density, while also enabling the board to carry large currents and dissipate heat through thick copper, places extremely high demands on PCB manufacturing processes.

SunKey Electronics has many years of experience in the manufacturing of specialized power supply PCB. In the following, we will discuss how process improvements can be used to solve these practical engineering challenges. We hope that this experience will help engineers better address the difficulties involved in power module design.

High-Density Interconnection: From Traditional Drilling to 7-Layer HDI

In traditional power supply board design, through-hole interconnection technology is commonly used. Simply put, holes are drilled in the PCB, and then metalization inside the holes is used to achieve electrical connections between layers. This method was mainstream for a long time. However, as AI computing modules continue to demand higher levels of integration, traditional mechanical drilling has reached its limits. Because both the diameter and pitch of mechanically drilled holes are subject to certain constraints, it is difficult to meet the requirements of high-density interconnection. In other words, if too many holes are drilled too closely together on a PCB, the routing space and electrical performance of the board will be severely compromised.

To overcome the limitations of mechanical drilling, SunKey has adopted a new approach: we have introduced world‑class laser drilling equipment and upgraded our HDI (High-Density Interconnection) capabilities to support 1–7 layer HDI with Any-layer interconnection technology, achieving a minimum laser-drilled hole size of 3 mil (0.075 mm). This technology allows for more complex power-plane partitioning and signal routing on the inner layers of the PCB, no longer constrained by through‑holes as in the past.

Real-World Case: Industrial Control Power Main Board

This project used a 16-layer board requiring 6 rounds of laser drilling to achieve high‑density signal interconnection, with extremely tight layer‑to‑layer alignment requirements. We abandoned traditional mechanical blind vias and adopted a “laser microvia stacked-via” process. Through high‑precision alignment with LDI, we successfully achieved vertical stacking of micro‑blind vias.

II. Current Carrying and Heat Dissipation: 6oz Hybrid Stack-up and Copper Paste Filled Via Technology

In power modules, thick copper PCB are almost indispensable. Why is that? Because thick copper can carry higher currents and offers better thermal conductivity. However, manufacturing thick copper boards also presents several challenges, the most critical of which are controlling etching precision and achieving high‑quality resin filling.

Etching Control: As copper thickness increases, the difficulty of etching also rises. For example, when copper thickness reaches 6oz (approximately 210 microns), the undercut effect becomes very pronounced. Undercutting can cause irregularities along the edges of the traces, affecting both precision and electrical performance. To address this issue, we adjusted compensation coefficients and etching factors, ultimately controlling the line‑width tolerance of thick copper traces within ±8%. In addition, we support copper paste filled via technology, which provides a more effective solution for dissipating heat in areas with high local current density.

Deep‑Hole Plating: In applications involving thick boards (we can process prototypes up to 8.55mm thick), deep‑hole plating is a critical process step. The goal of deep‑hole plating is to deposit a uniform copper layer on the inner walls of the holes to ensure reliable electrical connections between layers. However, as board thickness increases, deep‑hole plating becomes significantly more challenging. To ensure hole‑wall smoothness and consistent copper thickness, we use top‑grade diamond tools and imported electroplating chemicals. This means that even for very deep holes, we can still guarantee that the copper thickness meets specifications. By doing so, we effectively avoid the risk of hole‑wall cracking under high‑current stress, thereby enhancing product reliability and stability.

Real‑World Case: High‑Power Power Supply Board

This project had a very unique design requirement: layers L6 and L7 needed to achieve a copper thickness of 4oz, while the remaining inner layers were required to reach 6oz…

Heat Dissipation Challenge: Copper Paste Filled Via Technology

Real‑World Case: Special Thick Copper Board, 7mm Thickness, 5 Press Cycles

To address localized hotspots in this project, we introduced the “copper paste filled via” process, which offers a thermal conductivity of 8–10 W/mK—approximately 20 times higher than conventional resin.

III. Insulation and Withstand Voltage Testing: Specialized Controls for Planar Transformer PCB Coil Boards

In certain applications, planar transformer PCB (also known as coil boards) are gradually replacing traditional wound magnetic-core transformers and are being adopted in large volumes. Although these boards may appear structurally simple, they actually impose extremely high requirements on insulation and voltage‑withstand performance.

Manufacturing Challenges: In multilayer coil boards, the dielectric layers between coil layers serve as insulation. If these dielectric layers contain even tiny air bubbles or impurities, they can easily lead to breakdown under high voltage, posing a serious threat to product safety and reliability.

The image above shows the failure site of a coil board manufactured using a standard process during high‑voltage testing: internal bubble discharge caused an instantaneous breakdown, and the resulting high temperature burned through and carbonized the board material, creating a permanent short circuit.

To address this issue, we implemented the following measures:

Material Optimization

We selected PP sheets with high resin content to ensure full resin filling during the lamination process. High‑resin PP sheets better fill the gaps between coils, reducing the formation of air bubbles.

Rigorous Testing

We introduced stricter high‑voltage testing standards, requiring that a 1‑mil dielectric layer must withstand 500V DC. To support this, we deployed an integrated high‑voltage inductance testing system that can measure inductance and voltage at any time. In addition, we implemented a dedicated AOI (Automated Optical Inspection) cross‑scanning strategy for coil patterns to ensure there are no gaps, shorts, or other hidden defects. Through these stringent tests, we can maximize product quality and reliability.

How to Access Our Services

Process capability improvements are always driven by our ultimate goal of serving customers. SunKey Electronics has successfully provided R&D prototyping and small‑to‑medium batch PCB manufacturing services to many leading companies in the power supply industry. Our power module PCB products cover a wide range of applications, including power semiconductors, airborne power supplies, server power supplies, special‑purpose power supplies, and communication power supplies.

With years of engineering experience in end‑use power supply products, SunKey has developed unique process methodologies for manufacturing high‑difficulty power module PCB. We help power supply companies avoid costly detours during new product development and small‑batch trial production, improve product yield, and shorten time‑to‑market.

If your new project involves conflicts between HDI design and thick copper manufacturing processes, we welcome you to contact us. We offer free DFM (Design for Manufacturability) evaluation services to support your design and production needs.