In high-frequency PCB manufacturing, plating is a critical process for forming electrical connections between inner layers and creating conductive surface layers. Plating is essential for metallizing the hole walls to ensure proper electrical connections in multilayer PCBs, and it also increases the surface copper thickness to enhance both electrical conductivity and mechanical strength. Since signal transmission in high-frequency PCBs demands strict electrical performance, any defects in the plating process can lead to signal loss, reflection, or even short circuits. Therefore, every step of the plating process must be carefully controlled. Below are the main steps and considerations for plating in high-frequency PCB production.

Main Steps in the Plating Process
1. Cleaning and Pre-Treatment
Before plating, the PCB must undergo thorough cleaning to remove oxides, oils, dust, and other contaminants from the surface. For hole wall plating, cleaning inside the holes is particularly important because residues can affect the uniformity and adhesion of the metal plating. Common cleaning methods include:

Acid Cleaning: Using a diluted acid solution to remove oxides from the copper surface and hole walls.
Degreasing: Using an alkaline solution to remove oils and greases, ensuring that the plating adheres uniformly across the surface.
After cleaning, the PCB surface must be free of contaminants and oxidation to ensure that the metal plating can adhere properly during the process.

2. Electroless Copper Plating
Electroless copper plating is the process of depositing a thin layer of copper onto the hole walls to create an initial conductive path. Since this is a chemical deposition rather than electrochemical deposition, electroless copper can coat the insulating surfaces of the hole walls, ensuring that a conductive layer is formed for subsequent electroplating. This step involves:

Activation Treatment: Using a palladium catalyst to activate the surface of the hole walls, promoting copper deposition.
Copper Deposition: A chemical reaction deposits a thin layer of copper onto the activated hole walls. This copper layer is usually 1–2 microns thick, which is sufficient to provide conductivity for the next electroplating step.
3. Electrolytic Copper Plating
After electroless copper plating, electrolytic copper plating is performed to thicken the copper layer and ensure reliable electrical connections and mechanical strength. In this step, the PCB is immersed in a copper electrolyte solution, and an electric current is applied to deposit copper onto the existing copper surface and hole walls, forming a thicker layer.

Electrolyte Composition: The concentration of copper ions, temperature, and additives in the electrolyte solution must be carefully controlled to ensure uniform deposition and high-quality plating.
Current Density Control: Current density is crucial during electroplating. Excessive current can cause uneven copper deposition on the hole walls, while too little current can prolong the plating time and reduce efficiency. The current density must be adjusted according to the PCB surface area and design requirements.
4. Surface Treatment and Stress Relief
After electrolytic copper plating, further surface treatments such as anti-oxidation or stress relief may be performed to improve the stability and conductivity of the plated layer. Anti-oxidation treatments prevent the copper surface from oxidizing when exposed to air, and stress relief treatments reduce internal stresses caused by the plating process, improving the long-term reliability of the copper layer.

Anti-Oxidation Treatment: A protective film is applied to the copper surface to prevent oxidation, especially on surfaces critical for high-frequency signal transmission.
Stress Relief: Heat treatment or chemical processes can be used to relieve internal stresses in the copper layer, ensuring structural stability and preventing cracks or delamination in the plated layer.
5. Post-Plating Cleaning and Inspection
Once plating is completed, the PCB must be cleaned to remove any residual electrolyte or chemicals that may contaminate subsequent processes. Additionally, the plating layer should be inspected to ensure that the thickness, smoothness, and electrical performance meet the design specifications.

Thickness Inspection: Microscopes or X-ray inspection equipment are used to verify that the plating thickness, particularly on the hole walls, meets the required specifications.
Electrical Performance Testing: Conductivity tests are performed to ensure that the plated layer provides a reliable electrical connection, especially in the plated through-holes.
Key Considerations for Plating
1. Hole Wall Quality
In high-frequency PCB, the quality of the hole walls directly affects signal transmission integrity. Roughness or defects such as cracks, voids, or burrs on the hole walls can cause signal reflection or attenuation during high-frequency transmission. Therefore, before plating, the hole walls must be smooth and clean. During electroless copper plating, the copper layer on the hole walls must be uniform and free of voids.

2. Plating Thickness Control
Plating thickness is a critical factor that affects the conductivity and mechanical strength of the PCB. For high-frequency PCBs, both excessively thick and thin copper layers can impair signal transmission. A thick copper layer increases parasitic effects during signal transmission, while a thin copper layer may result in weak mechanical strength or poor electrical connections. Therefore, plating thickness must be carefully controlled to meet design requirements, particularly in the hole walls.

3. Uniformity of Electroless Copper Plating
The uniformity of the electroless copper plating directly affects the success of subsequent electroplating. If the copper layer deposited during electroless plating is uneven, the subsequent electroplating may result in poor coverage or inconsistent thickness, leading to electrical performance issues. Therefore, it is essential to ensure uniform copper deposition on the hole walls during the electroless plating process.

4. Current Density Control During Electroplating
Current density plays a key role in determining the rate and uniformity of copper deposition during electroplating. In high-frequency PCBs, excessive current density can result in uneven plating within the holes, affecting the transmission performance of high-frequency signals. The current density must be carefully adjusted according to the surface area and hole dimensions to ensure consistent plating quality.

5. Avoiding Air Bubbles in Holes
Air bubbles trapped in the holes during electroless copper plating or electroplating can lead to incomplete plating on the hole walls or even voids, compromising the reliability of electrical connections. In high-frequency circuits, even minor defects in the hole walls can result in signal loss or reflection. Measures such as vibration or high-pressure cleaning can help eliminate air bubbles during plating, ensuring uniform plating inside the holes.

6. Environmental Control
The environmental conditions (such as temperature, humidity, and air quality) during the plating process can significantly affect the quality of the plating. The plating process for high-frequency PCBs should be conducted in a temperature-controlled environment to prevent extreme temperatures from affecting the activity of the electrolyte and copper adhesion. Additionally, the air should be free of contaminants, and the humidity should be controlled to prevent surface contamination or uneven plating.

By following these steps and considerations, the plating process in high-frequency PCB production can be optimized to ensure high-quality signal transmission, electrical connectivity, and reliable performance in high-frequency applications.