What is the PCB copper sinking process? The process of chemical copper?

ONESEINE PCB/What is the PCB copper sinking process? The process of chemical copper?

PCB copper deposition process, also known as electroless copper plating or electroless nickel immersion gold (ENIG), is two common processes used in PCB factory production. One is the technique of forming a thin and uniform copper layer on a printed circuit board (PCB). This process is crucial for manufacturing multi-layer PCBs as it allows for the deposition of metal on non-conductive surfaces, thereby creating electrical connections.

The copper sinking process usually involves the following steps:

Cleaning: Firstly, the surface of the PCB needs to be thoroughly cleaned to remove any grease, dust, or other pollutants to ensure good adhesion of the coating.

Micro etching: Use chemical solution to perform micro etching treatment on the surface of PCB to improve its roughness and facilitate the adhesion of copper layer.

Activation: Immerse the PCB in a solution containing a catalyst, usually palladium salt, to form a thin activation layer on the surface of the insulating substrate. This activation layer facilitates the deposition of copper ions.

Copper sinking: Next, the PCB is immersed in a copper sinking solution. The solution contains copper ions and reducing agents. Copper ions are deposited on the surface of the PCB under the action of a catalyst, forming a continuous layer of metallic copper film.

Post treatment: After copper deposition, it may be necessary to rinse and dry the PCB to remove residual chemicals and ensure that the copper layer is uniform and defect free.

The copper sinking process does not require an external power source because it is carried out spontaneously through chemical reactions. Due to its ability to uniformly deposit an extremely thin copper layer on the entire surface, this method is highly suitable for PCB designs without holes or difficult to electroplate.

The deposited copper layer is usually thin, and in order to improve conductivity and durability, it is often covered by subsequent copper plating processes to thicken the copper layer. In some applications, other metals such as nickel or gold may also be plated on the deposited copper layer to provide additional protection and improve welding performance.

The PCB copper sinking process, commonly known as electroless copper plating or plated through hole (PTH), is a critical manufacturing step that deposits a thin, conductive layer of copper onto the non-conductive walls of drilled holes in a circuit board. Since the substrate materials like epoxy resin and fiberglass are insulators, this chemical process creates the necessary electrical pathway to interconnect different layers of a multilayer PCB. The term "chemical copper" refers to this autocatalytic redox reaction, which occurs without the use of an external electric current, distinguishing it from conventional electroplating. The resulting copper layer, typically 20 to 40 micro-inches thick, serves as a conductive seed layer or base, enabling a subsequent, thicker layer of copper to be built up through standard electroplating.

The full process is a multi-step chemical sequence, and the quality of each step directly determines the final reliability of the board. The most common issues, such as voids or "skip plating" in the holes, can often be traced back to problems in these preparatory stages-36. Here is a breakdown of the core stages, based on industry-standard practices.

1. Pre-Treatment and Cleaning

The goal of pre-treatment is to create a perfectly clean and receptive surface on the hole walls for the subsequent chemical reactions. The process begins with deburring to remove any copper burrs or residue left from the drilling process that could interfere with plating. Next, an alkaline degreasing step cleans the board of oils, fingerprints, and oxides. Importantly, this step also adjusts the surface charge of the epoxy hole walls from negative to positive, which is essential for the next stage. This is followed by a desmear process, which uses chemicals like potassium permanganate to etch away any melted epoxy resin ("smear") created by the heat of the drill bit, exposing the clean, rough surface of the inner copper layers. Finally, a micro-etching step roughens the surface of the board and removes any remaining oxides, ensuring a strong mechanical bond for the copper layer to come

2. Activation and Acceleration

After cleaning, the board is pre-soaked to protect the expensive activation bath from contamination and to ensure the hole walls are thoroughly wetted-1. The core of the activation step is the application of a colloidal palladium solution. The positively charged hole walls attract and adsorb the negatively charged palladium particles, which act as a catalyst for the chemical copper reaction. A subsequent acceleration or degumming step is performed to remove the protective stannous ions surrounding the palladium particles, exposing the active palladium nuclei and priming them to initiate the copper deposition

3. Electroless Copper Deposition (The "Chemical Copper" Reaction)

The board is then immersed in the electroless copper bath, which contains copper ions, a reducing agent (traditionally formaldehyde), a complexing agent (like EDTA to keep copper ions in solution), and various stabilizers-. The exposed palladium nuclei on the hole walls trigger the autocatalytic redox reaction. In simple terms, the reducing agent provides electrons to convert copper ions (Cu²⁺) into metallic copper (Cu⁰), which deposits onto the catalytic surface. The unique "autocatalytic" nature means that the newly deposited copper metal itself also becomes a catalyst for the reaction, allowing it to continue and build up a uniform layer across the entire hole wall-46. This entire process happens without any external electrical current, hence the name "electroless" or "chemical" copper.

4. Panel Plating

While the electroless copper layer is conductive, it is extremely thin and fragile. To prevent it from being oxidized or damaged in subsequent processing steps, an immediate panel plating (or "primary copper") step is performed. This is a standard electroplating process where an electric current is applied to rapidly build up the copper thickness to about 5-8 microns (200-500 micro-inches), providing a robust and reliable conductive layer for the final circuit pattern

Quality Control

Given that PTH is a source of many potential defects, rigorous quality control is essential. Backlight tests and micro-section analysis are common methods to inspect the uniformity and coverage of the chemical copper layer on the hole walls before the final electroplating stage, helping to catch issues like voids or poor adhesion early in the manufacturing process