Introduction

Printed circuit boards (PCBs) are essential components of most modern electronic devices. They provide the pathways for electrical signals and act as a base for mounting electronic components. A key step in manufacturing PCBs is plating the board with copper to create the conductive traces that connect the components. This article will provide a comprehensive overview of the copper plating process for PCBs, including preparations, plating steps, quality control, and safety considerations.

Preparing the Board

Before copper can be plated onto a PCB, the board must be prepared properly. This involves several steps:

Cleaning

Any oil, grease, or other contaminants must be removed from the raw PCB substrate. This is done using cleaners and solvents in a series of cleaning baths. Any residual organic films left on the board surface can interfere with adhesion of the copper layer.

Surface Roughening

The board surface is roughened lightly to promote mechanical adhesion of the plated copper. This is often done by etching the surface slightly with acid or abrasive blasting.

Activation

Catalytic seeds are deposited onto the board surface to initiate the copper plating reaction. This typically involves sensitizing with tin chloride, followed by activation with palladium chloride. This leaves a layer of palladium metal sites that will catalyze copper deposition.

Photoresist Application

A layer of photoresist is then applied over the entire board surface. This photoresist layer is a polymer that will protect areas of the board from getting plated with copper. The photoresist is exposed to UV light through a mask, then developed to remove the exposed areas. This leaves a pattern matching the desired copper traces protected by photoresist.

Copper Plating Steps

With the board prepared, it is ready for electroplating of the copper traces. This involves submerging the board in a series of plating baths with controlled current flowing through. There are several stages to the plating process:

Cleaner

The board first goes through a cleaner bath to remove any residual organic contamination. This ensures good adhesion and uniform copper deposition.

Acid Copper Plating

The board is plated in an acidic copper sulfate bath. Copper ions in the solution are attracted to the activated board surface when current flows through. The copper deposits on the seeded areas not protected by photoresist. Plating continues until the desired copper thickness is achieved.

Diffusion Barrier

A thin layer of nickel or other metal may be plated over the copper to act as a diffusion barrier. This prevents migration of the copper over time which can lead to shorts and reliability issues.

Tin-Lead or Gold Plating

The copper traces are often coated with an etch-resistant metal for oxidation protection and solderability. Thin layers of tin-lead or gold are commonly used as a finish over the copper.

Quality Control Checks

Throughout the plating process, quality control checks are performed to ensure the copper layer meets specifications:

• Visual inspection of plating uniformity and expected resist clearing.
• Microsectioning to measure copper thickness at multiple locations.
• Adhesion testing using tape peel or pull tests.
• Resistivity measurements to confirm electrical properties.
• Etch back testing by selectively dissolving the plating to inspect grain structure and internal quality.

Boards that fail any specification are rejected or reworked to avoid quality issues in finished PCBs.

Stripping and Etching

Once plating is complete, the photoresist is chemically stripped away, leaving only the desired copper circuit pattern on the board. Any areas of unwanted copper are removed using chemical etching, which dissolves away the excess copper not protected by the plated resist.

The plated boards then undergo a final cleaning and drying process to remove all processing chemicals. They are now ready for electronic assembly.

Safety Considerations

Working with electroplating baths and the associated chemicals requires careful safety precautions:

• Plating tanks are enclosed to contain chemical fumes and splashing.
• Operators wear appropriate PPE such as rubber aprons, gloves, and eye protection.
• Proper ventilation and fume extraction is provided around tanks.
• Facilities have emergency wash stations and eyewash fountains available.
• MSDS (material safety data sheets) are followed for proper chemical handling.
• Tanks are periodically analyzed to maintain proper chemical balance.
• Hazardous wastes are disposed of according to local environmental regulations.

Following standard safety protocols ensures that PCB copper plating can be performed efficiently while protecting worker health.

Conclusion

Plating high-quality copper traces on printed circuit boards involves many preparatory, plating, and finishing steps. Careful process control and testing ensures that the copper layer meets stringent electrical and reliability needs for PCBs in electronics manufacturing. This plating technology allows intricate circuit patterns to be deposited, enabling ongoing miniaturization and performance gains of modern electronic devices.

Frequently Asked Questions

Here are some common questions about the PCB copper plating process:

What are some key factors that affect copper plating quality?

Some of the key factors are:

• Surface cleanliness and prep – Ensures good adhesion and plating uniformity
• Bath chemistry – Proper levels of copper, acid, brighteners ensure even deposition
• Current density – Affects growth rate and material properties
• Impurities – Can lead to poor adhesion or physical defects
• Temperature – Higher temperatures increase plating rate but can reduce ductility
• Agitation – Keeps concentration uniform and prevents depletion

What thickness of copper is typically plated for PCBs?

For standard PCBs, copper thickness is usually between 1-3 oz/ft2 or about 35-105 microns. High current or thermal boards may use thicker plating up to 6 oz/ft2.

What causes poor adhesion of electroplated copper?

Insufficient surface prep is the most common cause of poor adhesion. Other factors include contamination from organic films, excessive etching, high internal stresses, hydrogen embrittlement from the plating process, and improper rinsing steps.

How are different thicknesses of copper plated across a PCB panel?

Areas that need thicker copper are plated for a longer duration compared to thinner areas. Photoresist patterns determine which areas plate longer or are covered earlier. Complex pulse plating waveforms also allow differential copper buildup.

Why is a diffusion barrier like nickel plating used over copper traces?

Nickel prevents migration of copper over time, which can cause electrical shorts. It also serves as a solder diffusion barrier and protects against corrosion during assembly or harsh operating environments in some electronics.