Surface treatment process
To meet the requirements for the production of PCB-grade electro-deposited copper foils, surface treatment is essential for the raw copper foil. Here we primarily discuss the surface treatment process for PCB copper foils. In the case of lithium-ion battery copper foils, the surface treatment process is more simplified. Surface treatment is crucial in ensuring sufficient bond strength between the copper foil and the insulating substrate in the absence of adhesives. Hence, an appropriate surface treatment process is imperative for electro-deposited copper foils.
The surface treatment process for the electro-deposited copper foil in this project involves an “electro-deposition” technique. Electro-deposition is a process where a metal or alloy layer is deposited onto a metallic or non-metallic substrate through electrochemical reactions, forming a functional metal material.
In this particular process, zinc, among other metals, is part of the alloy deposition layer, acting as a barrier layer. It serves to prevent etching marks, corrosion, and undercutting while being soluble in all etchants, allowing standard etching solutions to form circuitry. These deposition layers are typically passivated in a chromate solution. This serves two purposes: preventing oxidation and enhancing the bond strength between the copper foil and the resin.
The fundamental steps in the electrochemical treatment of copper foil involve a three-step process. Firstly, the “coarsening treatment” roughens the surface by depositing granular or dendritic copper on the foil. Secondly, the “sealing” process deposits a smooth copper layer on the coarsened surface, enhancing its attachment to the foil substrate. Lastly, the “passivation treatment” is applied. Each of these treatments occurs in different solution compositions to prevent contamination. Subsequently, a rinsing process with pure water is generally employed to ensure cleanliness.
According to the surface treatment procedures, the produced copper foil undergoes acid etching, coarsening, antioxidation, and passivation treatments.
1. Acid Etching
This step involves removing the oxide layer from the copper foil’s surface in an acidic copper sulfate solution (2-10g/L) at a temperature of approximately 30-60°C. The solution is not replaced but periodically replenished with materials and water. The primary contaminant in this process is sulfuric acid mist.
2.Coarsening and Sealing
Coarsening treatment typically employs an acidic electroplating method. It roughens and seals the copper foil surface by depositing a loose structure of nodules, which are then surrounded and reinforced by a normal copper electro-deposition layer. This reinforcement ensures a stronger bond with the copper foil substrate, creating the final coarsened layer.
The basic electrochemical coarsening reaction process involves:
- a. Migration of hydrated metal ions Cu2+ to the cathodic copper foil surface.
- b. Transformation reaction of Cu2+ near the cathodic surface, reducing and rearranging the hydration degree.
- c. Reduction of Cu2+ to adsorbed atoms at the cathode.
- d. Diffusion of adsorbed atoms along the electrode surface, embedding into the lattice structure, completing the coarsening process.
Sealing treatment applies a well-dispersing electro-deposition layer uniformly onto the coarsened surface, providing reinforcement without significantly altering the coarsened surface shape.
In a copper sulfate electrolyte (20-70g/L), the copper foil surface roughness is increased through electrochemical action, at a temperature of about 30-60°C. The solution is not replaced but periodically replenished with materials and water. The primary contaminant in this process is sulfuric acid mist.
3. Antioxidation
During the copper foil production process, it is necessary to conduct antioxidation and passivation treatments on the surface of the copper foil (both shiny and matte sides). By electroplating, a complex structure primarily composed of zinc and chromium is deposited onto the copper foil surface to form an antioxidation film. This prevents direct contact of the copper foil with air, avoiding oxidation discoloration during storage, transportation, and during the PCB pressing process. This preservation of the copper surface ensures its solderability and affinity with inks are not compromised. The purpose of antioxidation is to provide a protective oxide layer on both sides of the copper foil. This oxide layer is formed during the cathodic treatment process. During transfer and drying processes, the copper foil may be susceptible to oxidation corrosion, and the oxide film on the shiny side of the copper foil is intended to prevent such corrosion.
The antioxidation treatment involves immersing the copper foil in a zinc sulfate electrolyte solution (1-7g/L) adjusted to pH 7-13 by adding potassium pyrophosphate. Through electrochemical action, this process results in the formation of an antioxidation film on the surface of the copper foil. The temperature of the antioxidation tank solution is approximately 30-60°C. The solution is not replaced but periodically replenished with materials and water. The primary contaminant in this process is sulfuric acid mist.
4.Passivation
When metals or alloys experience significantly enhanced chemical stability due to certain factors, it’s referred to as passivation. The passivation of metals induced by certain passivating agents is termed chemical passivation. Presently used antioxidation treatments are akin to metal passivation. A layer of non-copper metal, such as tin, zinc, or a tin-zinc deposit with trace amounts of other metals, is deposited on the surface of the copper foil. Chemical passivation occurs either in a chromic acid solution or by directly generating a mixed layer of zinc-chromium or tin-chromium.
Potassium dichromate is utilized for metal passivation. After metal passivation, the electrode potential of the metal shifts towards the positive direction, causing it to lose its original properties. In a potassium dichromate solution (0.05-0.5g/L), a trace amount of NaOH is added to adjust the pH to 11-14. Through electrochemical action, a very thin, dense, well-covering, and firmly adhered passivation film is generated on the surface of the copper foil treated for oxidation resistance. The tank solution temperature is approximately 30-60°C. The solution is not replaced but periodically replenished with materials and water. The primary contaminant in this process is chromic acid mist.
5. Rinsing
Throughout the surface treatment process, solutions used in each step, such as coarsening, sealing, antioxidation treatment, and passivation, are not entirely identical. They may have adverse effects on each other. Therefore, between adjacent processing tanks, a rinsing tank is required to eliminate the electrolyte residues from the preceding step adhering to the surface of the copper foil.
6.Drying
Drying is the essential final step in the surface treatment process. Its purpose is to completely remove moisture from the surface of the copper foil, preventing any residual moisture from harming the copper foil. The drying temperature varies based on the processing speed of the copper foil, generally not lower than 100°C. The goal is to thoroughly remove surface moisture without causing damage to the copper foil due to excessively high temperatures. In this project, an electric drying oven is used to dry the products after cleaning.
Third and fourth steps，cutting and inspection packaging.
The cutting process primarily involves copper powder cleaning and burr treatment. Based on different customer requirements for the width of copper foil products, they are cut accordingly. The Quality Measurement Department conducts quality inspections on the cut copper foil products. Upon successful inspection, the corresponding copper foil products are packaged and placed into storage.