The global electroplating PCB industry represents a rapidly growing segment, accounting for an increasing share of the total output value within the electronic component sector. It holds the largest proportion in this industry, occupying a unique and vital position. As electronic products become lighter and thinner, the design approach shifts towards denser interconnection. To ensure effective via stacking, the base of the hole must remain flat. There are various methods to achieve a typical flat hole surface, with the electroplating hole filling process being one of the most representative solutions.

In addition to minimizing the need for additional process development, electroplating and filling are also compatible with existing process equipment, thereby supporting improved reliability.

Electroplating hole filling offers several key advantages:

(1) Facilitates the design of stacked holes (Stacked) and via-on-pad configurations (Via.on.Pad);

(2) Enhances electrical performance, aiding high-frequency designs;

(3) Contributes to efficient heat dissipation;

(4) Completes both hole filling and electrical interconnection in a single step;

(5) Electroplated copper used in blind hole filling provides higher reliability and better conductivity compared to conductive adhesives.

Physical parameters that need to be considered include: anode type, anode-cathode spacing, current density, agitation, temperature, rectifier settings, and waveform, among others.

(1) **Anode Type.** When discussing anode types, they can generally be categorized as soluble or insoluble. A soluble anode is typically a phosphorous copper ball, which is prone to generating anode mud, contaminating the plating solution, and negatively impacting its performance. On the other hand, insoluble anodes, also known as inert anodes, are usually made from titanium mesh coated with mixed oxides of tantalum and zirconium. Insoluble anodes offer excellent stability, require no maintenance, do not generate anode mud, and can be used for both pulse and DC electroplating. However, they tend to consume a higher amount of additives.

(2) **Cathode-Anode Distance.** The spacing between the cathode and anode in the electroplating hole-filling process is crucial and varies depending on the design of the equipment. However, it’s essential to note that no matter the design, it must always comply with Faraday’s first law.

(3) **Stirring.** There are various types of stirring methods, including mechanical shaking, electric shaking, air shaking, air stirring, and jet (Eductor) stirring. For electroplating and hole filling, there is a trend toward incorporating jet designs alongside traditional copper cylinder configurations. Key considerations include whether to use bottom or side jets, how to arrange the jet and air stirring tubes within the cylinder, the flow rate of the jets, the distance between the jet tube and the cathode, and the placement of the jets relative to the anode. If using bottom jets, careful attention must be paid to avoid uneven mixing, as this could result in weak stirring at the top and stronger stirring at the bottom of the plating solution. The number, spacing, and angle of the jets must all be considered in the design process, which requires extensive experimentation. Ideally, each jet tube should be connected to a flow meter to monitor the flow rate. Because high jet flows can heat up the solution, temperature control becomes a critical factor.

(4) **Current Density and Temperature.** Low current density and low temperature reduce the copper deposition rate on the surface while allowing sufficient Cu²⁺ and brightener to enter the holes. This improves hole filling ability but also lowers plating efficiency.

(5) **Rectifier.** The rectifier plays a critical role in the electroplating process. Current research on electroplating hole filling is mostly focused on full-plate electroplating. However, when considering patterned electroplating hole filling, the cathode area becomes much smaller, which places higher demands on the rectifier’s output accuracy. The rectifier’s output accuracy should be selected based on the product line and via size. For finer lines and smaller holes, higher accuracy is required, typically a rectifier with an output accuracy of less than 5%. While a high-precision rectifier increases equipment costs, it’s necessary for optimal performance. For the rectifier’s output cables, it’s advisable to position the rectifier close to the plating tank to minimize cable length and reduce pulse current rise time. The output cable specification should ensure that the voltage drop across the cable is within 0.6V when the maximum output current is at 80%. Cable cross-sectional area is generally calculated based on a current-carrying capacity of 2.5A/mm². If the cable is too small or too long, excessive voltage drop could prevent the required current from reaching the workpiece. For plating tanks with a groove width greater than 1.6m, a double-sided power supply method should be considered to ensure current is evenly distributed and the current error is minimized. A rectifier should be connected to each side of the plating tank to allow independent current adjustment on both sides.

(6) **Waveform.** Currently, there are two main types of electroplating hole filling methods: pulse electroplating and DC electroplating. Both methods have been well studied. DC electroplating hole filling uses a traditional rectifier and is easy to operate, but it may be insufficient for thicker boards. Pulse electroplating hole filling uses a PPR rectifier, which involves more complex procedures but offers better performance when processing thicker boards.

**Influence of the Substrate**

The substrate material has a significant impact on the electroplating hole filling process. Factors such as the dielectric layer material, hole shape, aspect ratio, and chemical copper plating all play a role.

(1) **Dielectric Layer Material.** The dielectric material influences hole filling. Non-glass fiber reinforced materials tend to fill holes more easily than glass fiber reinforced ones. It’s worth noting that glass fiber protrusions in the hole can negatively affect chemical copper plating. In such cases, improving the adhesion of the seed layer for electroless plating is more critical than the hole filling process itself. In practice, hole filling on glass fiber reinforced substrates has been successfully implemented.

(2) **Aspect Ratio.** Both manufacturers and developers place great importance on hole filling technology for various hole shapes and sizes. The ability to fill holes is significantly affected by the thickness-to-diameter ratio. Typically, DC systems are used for smaller holes with diameters ranging from 80µm to 120µm, depths of 40µm to 80µm, and an aspect ratio not exceeding 1:1.

(3) **PCB Layout and Electroless Copper Plating.** The uniformity and thickness of the electroless copper plating layer, as well as the time elapsed after electroless copper plating, all affect hole filling performance. If the electroless copper layer is too thin or uneven, the hole filling performance will be poor. Generally, a thickness of around 0.3µm for electroless copper plating is recommended for optimal hole filling. Additionally, oxidation of the electroless copper layer can negatively impact hole filling.
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