The walls of the drilled holes are made of non-conductive epoxy resin and fiberglass board. To establish electrical connections between different layers, the hole walls must be metallized through an autocatalytic redox reaction, which deposits a thin copper layer onto the non-conductive surface. This process, known as electroless copper plating, plays a crucial role in ensuring the electrical performance and reliability of the PCB.

Pre-treatment

Before the electroless copper plating process, the board needs to undergo a cleaning procedure that includes deburring, alkaline degreasing, desmear, and micro-etching. Deburring removes residues from the drilling process and improves the structure of the hole walls, enhancing their adhesion strength. The alkaline degreasing step follows, removing impurities such as oils, fingerprints, and oxides from the board surface.

The desmear process utilizes potassium permanganate to etch the epoxy resin on the hole walls, increasing their ability to absorb the activation solution.

Finally, micro-etching is performed to remove any oxides on the surface of the board and to increase surface roughness. This step is essential for ensuring strong adhesion between the copper layer and the substrate, preventing delamination during subsequent panel plating.

Activation

Before the activation process, the cleaned board is pre-impregnated to prevent any moisture on the surface from altering the pH of the activation solution, which could negatively impact the activation process and potentially cause delamination.

During activation, the board is immersed in an activation bath, where a thin layer of colloidal palladium is deposited onto both the surface of the board and the hole walls. This ensures that electroless copper can be evenly plated on all surfaces.

Electroless Copper Plating

After the activation process, the board is immersed in a copper bath, where a redox reaction occurs. This results in the formation of a 20-40 microinch thick layer of chemical copper on the surface, allowing the previously non-conductive hole walls to become electrically conductive and establish connections between the different layers of the PCB.

Panel Plating

To prevent the thin copper layer inside the holes from oxidizing or micro-etching before the pattern plating, the copper on both the surface and inside the holes is thickened to 5-8 microns through electroplating.

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