PCB Copper Plating Process Explained

The copper plating process is one of the most critical PCB fabrication stages. Here, we take you through the plating steps. We will also explore problems that often arise when plating the boards and how to prevent or fix them.

What is Copper Plating in PCB Manufacturing?

Copper plating in PCB manufacturing means the process of coating a circuit board’s surface and drilled holes with a layer of solid copper. The layer forms the conductive surfaces that transmit power and signals.

The layer also produces component attachment surfaces called pads, test points, and thermal dissipation planes. Plated holes electrically connect the different board layers, while also conducting heat.

Copper makes a good coating material for printed circuit boards. Reasons include its excellent electrical and thermal conductivity, low cost compared to other conductive materials, and high ductility.

More importantly, it forms a highly solderable surface. This quality is desirable when mounting components to circuit boards. It produces reliable boards with lasting conductive paths and component joints.

Plating Equipment and Materials

Various materials and equipment types are necessary when depositing copper on a PCB substrate. They include cleaning and activation chemicals, electroless and electrolytic solutions, and etching agents.

The plating solutions are typically copper sulfate in distilled water and sulfuric acid. After coating, the copper areas usually require protective finishes. There are various options, as we will see later.

Essential equipment ranges from electroplating machines, deposition tanks or plating baths, and a power supply. There are also filtration systems and temperature controls for maintaining the solutions and regulating the process.

PCB Copper Plating Process

The process of copper-plating printed circuit boards comprises several stages. There are also two plating methods involved: electroless and electrolytic. Below is a step-by-step look at these steps and how PCB manufacturers conduct them.

Step 1 – Board Preparation

The board undergoes a preparation process. The procedure includes deburring the drilled holes and cleaning the board’s surfaces. It typically requires dipping the board in various chemicals that strip contaminants and water-rinsing.

Step 2 – Surface Activation

The next step is where the board is micro-etched and coated with a catalyst to activate the surface for coating. Micro-etching serves to improve adhesion, making the layer firmly attach to the board.

Surface activation involves applying a catalyst (typically palladium) that allows copper to bond to the non-conductive resin inside the drilled holes. This is immediately followed by electroless copper plating, which creates a thin, conductive seed layer.

Step 3 – Electroplating

The electroplating process can now begin. It requires electricity and involves inserting the board in an electrolytic bath and then subjecting the solution to an electric current. The solution contains copper ions.

The electric current draws positively charged copper ions from the solution to the cathode (the PCB), depositing them on the conductive surface. The copper deposition goes on until the desired thickness is achieved.

Step 4 – Pattern Plating

In this step, a photoresist material is applied and developed to expose only the circuitry pattern and holes. The board then goes back into a plating bath where copper is built up only on these exposed areas (drilled holes and pads).

Often, a thin layer of Tin is plated over the copper to act as an ‘etch resist.’ The photoresist is then stripped, and the underlying thin base copper is etched away, leaving the thick copper pattern.

Step 5 – Post Treatment

Post treatment involves coating the layer with a surface finish material. Finish options include Electroless Nickel Immersion Gold (ENIG) and Organic Solderability Preservative (OSP).

The finishes make the exposed copper surfaces corrosion-resistant. They also improve the solderability of the surfaces to aid component assembly.

Step 6 – Inspection and Testing

The coated board goes through an inspection and testing stage. Manual and automated optical inspection checks the board for surface defects, such as pitting and incorrect spacing.

There are also electrical tests to verify conductivity and identify connection issues, such as open circuits. Other tests include the peel test to ensure proper adhesion and micro-sectioning for structural integrity.

Thickness Control When Plating PCB Boards

The coating thickness determines various PCB properties. For instance, it defines the current carrying capacity of the traces, thermal performance, and impedance. Its control is, therefore, extremely important.

There are various methods to achieve that, depending on whether it’s electroless or electrolytic. This is primarily achieved by regulating the current density and plating time.

Thickness control is also achieved by altering the chemical composition and strength of an electroless plating solution.

PCB manufacturers use various technologies to measure the coating thickness. They are X-ray fluorescence, cross-sectional analysis, and with the help of eddy currents.

Copper Plating Process Defects and Solutions

Like any other industrial process, a few problems may arise when copper-plating PCB substrates. These can affect the layer’s appearance, mechanical strength, and electrical performance. They include the following issues.

• Uneven Thickness: the layer is inconsistent, which may cause some sections to be mechanically less robust.
  
• Pitting: small holes usually caused by trapped air bubbles or organic contaminants and other reasons that prevent some areas from coming into contact with the solution.
  
• Protrusions: small bumps that often result from excessive current in some areas.
  
• Roughness: Nodules or a rough texture, often caused by particulate matter in the solution (requiring better filtration), making the surface less smooth and affecting its quality.
  

Proper board cleaning, using the correct current, and servicing the plating solution help prevent these problems. There should also be measures to monitor the coating process closely.

Conclusion

The copper plating process is a rigorous procedure consisting of various phases and steps. It begins with board preparation and ends with inspection and testing. A proper control of the process is essential for a quality layer, and this calls for a deeper understanding of the requirements.