Introduction to Sodium Hypophosphite-Based Electroless Copper Plating Solutions

A wide variety of reducing agents can be used in electroless copper plating solutions, including formaldehyde, dimethylamine borane, sodium hypophosphite, hydrazine, borane, and sugar. Currently, formaldehyde, a highly effective and inexpensive reducing agent, is used. However, its disadvantage is the generation of harmful formaldehyde vapors during production. Meanwhile, electroless copper plating using sodium hypophosphite as a reducing agent has also gained momentum. This system boasts a wide range of process parameters, a long bath life, self-limiting coating formation, and the absence of harmful formaldehyde vapors, making it a promising development direction for electroless copper plating solutions. Electroless copper plating using sodium hypophosphite as a reducing agent typically produces a layer thickness of less than 1 μm, and the deposited copper does not catalyze the reaction. Therefore, for hole metallization in multilayer boards and additive electroless copper plating on printed circuit boards, the addition of metal ions that promote autocatalysis is necessary as a reducing agent. Other reducing agents are not currently widely used.

Electroless copper plating additives are an active research topic in electroless copper plating. The amount of additives generally ranges from tens of milligrams per liter, but they often play a critical role in the deposition rate, bath stability, and coating quality of electroless copper plating. To stabilize the plating bath, complexing and chelating agents for Cu+ are added to form Cu-S and Cu-N compounds. Therefore, cyclic compounds containing both N and S are suitable. Accelerators include ammonium salts, nickel salts, chlorides, and tungstates, while surfactants can improve the quality of the deposited film. The stability of the electroless copper plating bath is crucial to its practical application. The main causes of premature bath decomposition include:

1. Improper use of stabilizers

2. Excessively high temperatures

3. Excessive loading of parts to be plated, resulting in excessively vigorous reactions

4. Excessively high pH

5. The presence of catalyst particles in the bath. Catalyst particles can be airborne dust particles, metallic copper powder formed by side reactions during electroless copper plating, and catalyst particles shed from the parts being plated. These can all cause bath decomposition.

To address these issues, a method for stabilizing electroless copper plating baths has been proposed. A highly stable, electroless copper plating solution was obtained by simultaneously adding K3[Fe(CN)6], polyethylene glycol, and ethanol to the plating solution as stabilizers. Some have used a self-prepared CHN-881 stabilizer to achieve stable electroless copper plating at room temperature using a single tartrate complexing agent. Others have used p-toluenesulfonamide as a stabilizer with good results. However, the copper deposition rates of these processes are all less than 10 μm/h, making them unsuitable for additive thick copper plating on printed circuit boards. Adding vanadium compounds to an electroless copper plating solution containing formaldehyde as a reducing agent and EDTA salts as a complexing agent reduces the free Fe ion concentration and can lower the pH range of the electroless copper plating solution to 11.2–12.0 (at 25°C), improving the smoothness and elongation of the copper layer. This results in an electroless copper layer with excellent appearance, tensile strength, and elongation, enabling the manufacture of highly reliable, hole-metallized printed circuit boards. However, the copper plating deposition rate reported for this process is 6~7um/h, which drops to 4.5~5.5μm/h after adding vanadium compounds. Therefore, it is still difficult to realize the full additive method and partial additive method for manufacturing printed circuit boards.