I. Overview
With the rapid development of microelectronics technology, the manufacturing of PCB boards is advancing towards multi-layer construction, stack-up, functionalization, and integration. This progress has driven the design of circuit patterns to utilize tiny holes, narrow spacing, and thin wires, making printed circuit board manufacturing more challenging. Particularly in multilayer boards, the aspect ratio of through holes exceeding 5:1 and the prevalence of deep blind holes in laminates have rendered traditional vertical electroplating processes inadequate for meeting the technical demands of high-quality and high-reliability interconnection holes.
The root cause of this issue lies in the analysis of current distribution based on electroplating principles. During actual electroplating, it is observed that the current distribution in the hole forms a waist drum shape. The current gradually diminishes from the edge to the center of the hole, leading to excessive copper deposition on the hole’s surface and periphery, while insufficient copper is deposited at the center where it is needed. In some cases, the copper layer may be too thin or completely absent, potentially resulting in irreparable losses and the scrapping of numerous multilayer boards.
To address this quality issue in mass production, the problem of deep hole electroplating is currently being tackled through enhancements in current and additives. Processes for copper electroplating on high aspect ratio printed circuit boards are typically conducted at lower current densities with the aid of premium additives, moderate air agitation, and cathode movement. The effectiveness of electroplating additives relies on increasing the control area for electrode reactions within the hole. Additionally, cathode movement is crucial for enhancing the deep plating capability of the plating solution and increasing the polarization of the plated part. This balance between crystal nucleus formation and crystal grain growth facilitates the production of a robust copper layer.
However, as the aspect ratio of through-holes continues to escalate or deep blind holes become more prevalent, these measures may prove ineffective. This has led to the introduction of horizontal electroplating technology, a new approach built upon the foundation of vertical electroplating processes. Key to this technology is the establishment of a compatible horizontal electroplating system, enabling the plating solution with enhanced dispersibility to outperform vertical methods through advancements in power supply modes and collaboration with other auxiliary devices.
2. Introduction to the Principle of Horizontal Electroplating
The methods and principles of horizontal electroplating and vertical electroplating are the same, with both requiring cathode and anode electrodes. Upon electrification, an electrode reaction occurs to ionize the main components of the electrolyte, allowing charged positive ions to move towards the negative phase of the electrode reaction zone and charged negative ions to move towards the electrode. This results in a metal deposition coating and outgassing due to the positive phase shift of the reaction zone. The process of metal deposition at the cathode is divided into three steps: first, hydrated metal ions diffuse to the cathode; second, the metal hydrated ions are dehydrated and adsorbed on the cathode surface as they pass through the electric double layer; and third, the adsorbed metal ions accept electrons and enter the metal lattice. The observation of the working tank reveals an out-of-phase electron transfer reaction between the solid-phase electrode and the liquid-phase plating solution interface, explained by the principle of the electric double layer in electroplating theory. The arrangement of cations with positive charges surrounded by water molecules at the cathode forms the Helmholtz outer layer, while the diffusion layer, affected by convection and thermal motion, is less compact. The thickness of the diffusion layer is inversely proportional to the flow rate of the bath.
Convection of the plating solution is generated by mechanical stirring, pump stirring, electrode movement, and temperature differences. The flow interface layer, formed by a rate gradient, slows down the flow of the electroplating solution towards the solid electrode’s surface, creating the diffusion layer. Ion migration, influenced by the electric field and various factors, determines the ion transport rate. Increasing the temperature and convection rate can accelerate ion transport, impacting the rate of electroplating. The theory also describes the formation of a copper ion concentration gradient near the cathode and the importance of diffusion layers in electroplating.
Continued in next comment.
With the rapid development of microelectronics technology, the manufacturing of PCB boards is advancing towards multi-layer construction, stack-up, functionalization, and integration. This progress has driven the design of circuit patterns to utilize tiny holes, narrow spacing, and thin wires, making printed circuit board manufacturing more challenging. Particularly in multilayer boards, the aspect ratio of through holes exceeding 5:1 and the prevalence of deep blind holes in laminates have rendered traditional vertical electroplating processes inadequate for meeting the technical demands of high-quality and high-reliability interconnection holes.
The root cause of this issue lies in the analysis of current distribution based on electroplating principles. During actual electroplating, it is observed that the current distribution in the hole forms a waist drum shape. The current gradually diminishes from the edge to the center of the hole, leading to excessive copper deposition on the hole’s surface and periphery, while insufficient copper is deposited at the center where it is needed. In some cases, the copper layer may be too thin or completely absent, potentially resulting in irreparable losses and the scrapping of numerous multilayer boards.
To address this quality issue in mass production, the problem of deep hole electroplating is currently being tackled through enhancements in current and additives. Processes for copper electroplating on high aspect ratio printed circuit boards are typically conducted at lower current densities with the aid of premium additives, moderate air agitation, and cathode movement. The effectiveness of electroplating additives relies on increasing the control area for electrode reactions within the hole. Additionally, cathode movement is crucial for enhancing the deep plating capability of the plating solution and increasing the polarization of the plated part. This balance between crystal nucleus formation and crystal grain growth facilitates the production of a robust copper layer.
However, as the aspect ratio of through-holes continues to escalate or deep blind holes become more prevalent, these measures may prove ineffective. This has led to the introduction of horizontal electroplating technology, a new approach built upon the foundation of vertical electroplating processes. Key to this technology is the establishment of a compatible horizontal electroplating system, enabling the plating solution with enhanced dispersibility to outperform vertical methods through advancements in power supply modes and collaboration with other auxiliary devices.
2. Introduction to the Principle of Horizontal Electroplating
The methods and principles of horizontal electroplating and vertical electroplating are the same, with both requiring cathode and anode electrodes. Upon electrification, an electrode reaction occurs to ionize the main components of the electrolyte, allowing charged positive ions to move towards the negative phase of the electrode reaction zone and charged negative ions to move towards the electrode. This results in a metal deposition coating and outgassing due to the positive phase shift of the reaction zone. The process of metal deposition at the cathode is divided into three steps: first, hydrated metal ions diffuse to the cathode; second, the metal hydrated ions are dehydrated and adsorbed on the cathode surface as they pass through the electric double layer; and third, the adsorbed metal ions accept electrons and enter the metal lattice. The observation of the working tank reveals an out-of-phase electron transfer reaction between the solid-phase electrode and the liquid-phase plating solution interface, explained by the principle of the electric double layer in electroplating theory. The arrangement of cations with positive charges surrounded by water molecules at the cathode forms the Helmholtz outer layer, while the diffusion layer, affected by convection and thermal motion, is less compact. The thickness of the diffusion layer is inversely proportional to the flow rate of the bath.
Convection of the plating solution is generated by mechanical stirring, pump stirring, electrode movement, and temperature differences. The flow interface layer, formed by a rate gradient, slows down the flow of the electroplating solution towards the solid electrode’s surface, creating the diffusion layer. Ion migration, influenced by the electric field and various factors, determines the ion transport rate. Increasing the temperature and convection rate can accelerate ion transport, impacting the rate of electroplating. The theory also describes the formation of a copper ion concentration gradient near the cathode and the importance of diffusion layers in electroplating.
Continued in next comment.
