Application of Acid Copper Plating in PCB Manufacturing:
After a series of processes, the printed circuit board (PCB) needs to be metallized and copper needs to be deposited on the hole wall surface of the through hole. This ensures interconnection between boards and facilitates strong conductivity. Copper is considered the most widely used material for deposition in the industry. Typically, the metallization process for holes involves two steps: electroless plating and electroplating.
Electroless Plating:
Electroless plating, also known as chemical copper precipitation, is a process where copper is deposited onto the PCB through a chemical reaction. This reaction is self-catalyzed, meaning it continues due to the generation of copper atoms and hydrogen atoms, facilitating the reaction towards completion.
The quality of the electroless plating copper layer is critical to achieving optimal PCB performance. Strict control over experimental conditions and operational specifications during the electroless plating process is essential, such as time, temperature, and solution quality. Inadequate solution fluidity and contamination can directly impact the copper plating quality. If the plating solution has poor fluidity, it may lead to an uneven copper layer and result in the formation of voids in the through-holes, posing challenges for subsequent plating steps. To ensure uniform plating, the PCB is typically moved from side to side, and air pumping is used to improve solution flow.
Electroplating Technology:
In the copper electroplating industry, sulfate-based systems are commonly used. The figure below illustrates the principle of copper deposition in a cupric acid electroplating system. As the DC power supply is applied, the ions in the solution move toward the cathode, where copper ions (Cu²⁺) are reduced and deposited onto the cathode surface, including the inner walls of the through-holes, forming a copper plating layer.
During this electrochemical reaction, copper ions (Cu²⁺) from the solution are reduced at the cathode, forming copper (Cu) that adheres to the cathode surface and through-hole walls. Over time, as the copper ions are consumed, the anode dissolves to replenish the copper ion concentration, maintaining the balance of the plating solution.
This cyclic process of copper consumption at the cathode and replenishment at the anode ensures that the electroplating process remains balanced and continuous.
Electroplating reactions occur in roughly two steps. As shown in the reaction equations (1-2) and (1-3), initially, the reaction produces an intermediate that then forms the desired product. This process eventually leads to the formation of Cu, which is deposited as a copper layer.
As the plating progresses, the concentration of certain ions in the solution decreases as they are consumed. At this point, insufficient oxidation may cause other reactions, leading to hydrolysis as shown in equation (1-4).
As the acid concentration decreases, the hydrolysis reaction continues, producing a compound that may eventually cause “burrs” on the cathode plate. These burrs roughen the copper plating surface and reduce the overall performance and quality of the PCB. To avoid this, it is crucial to monitor and replenish the acid concentration in the plating solution.
Additionally, the concentration and type of additives play a crucial role in the copper acid electroplating system. These additives interact under the influence of current, affecting the current density in different areas. This interaction can either inhibit or accelerate copper deposition, ultimately controlling the crystal growth rate to achieve a smooth and uniform electroplating surface.
However, as additives are consumed during the electroplating process, an irreversible reaction with the anode can lead to a significant change in the concentration of additives. This can negatively impact the quality and performance of the copper deposit. Therefore, it is essential to monitor the consumption of additives in real time and replenish them as needed to ensure successful copper deposition.
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