Printed circuit boards require a process known as etching to ensure that only the desired copper tracks remain on the board. Etching is an essential part of the PCB manufacturing process, and you must perform it carefully to ensure that the final board is of the highest quality. This guide will take a closer look at what PCB etching is, why it’s essential, and the many ways to carry out the process effectively.
The PCB Etching Process
PCB etching includes several key steps that ensure that only the desired tracks remain on the board. While some of these steps may vary slightly depending on the specific application, the overall process is generally the same.
Photolithography is the process of creating a mask that you will use to etch the board and is performed about halfway through the PCB manufacturing process. By using a photosensitive laminate, PCB specialists can produce a high-quality, precise mask that will protect the areas of the board that should not be etched. The type of photolithography used will depend on the application and can range from simple excimer lasers to more complex processes like laser ablation.
Tin Plating Process
After you coat the PCB with a photoresist, it is time to apply a thin layer of tin to the entire board. This step is vital because it helps to protect the copper tracks from being etched away during the next step. Tin is also a good conductor, so it will not adversely affect the electrical properties of the circuit board.
You typically perform the tin plating process using a chemical bath. You may use it in conjunction with other methods like electroplating to ensure that the entire board is evenly coated. Once the tin plating process is complete, the board is ready for etching.
Once the printed circuit board is sufficiently protected and coated, it is time to begin the etching process that selectively removes the exposed copper while leaving the tin-plated tracks intact.
You can execute the etching process in many ways depending on the application and performance needs of the PCB. Ultimately, the result is a highly accurate printed circuit board with the desired tracks.
Dry etching is an anisotropic process, meaning that it etches in specific directions that lend to better geometric control when creating flat surfaces, sharp corners, and deep holes. Dry etching processes for printed circuit boards are typically used when:
- High levels of precision are required
- Timing is an issue
- Environmental and safety concerns are a factor
There are many types of dry etching processes, with the two most commonly used being plasma etching and laser etching.
Plasma etching is a simple process created in the 1960s and involves using a high-energy plasma to remove material from the surface of a substrate. Plasma is a gas that has been ionized to create positively and negatively charged particles. This process is often used for printed circuit boards because of its:
- High precision
- Faster etching rates
- RF controllable process
- Reduced waste and environmental impact
Because it requires minimal operator training and is less expensive than other methods, plasma etching has become one of the most popular methods for PCB etching when creating complex designs.
Laser etching is a process that uses a high-powered laser to remove material from the surface of a substrate. The vaporization leaves behind a clean, smooth surface removing the need for any additional acids, inks, or other harmful chemicals to create the desired tracks. This process is often used for printed circuit boards because it is:
- Highly precise
- Able to create very intricate designs
- Capable of etching a variety of materials
- Relatively fast
While laser etching is more expensive than other dry etching methods, it is often the preferred choice when working with small PCB boards. Additionally, when timing is an issue, laser etching is often the best choice because it reduces the number of process steps.
Wet PCB etching processes use chemical relations to dissolve and remove material from the surface of a substrate. Being an isotropic process that removes material in all directions, this method is typically used for printed circuit boards when:
- Inexpensive methods are necessary
- Ease of use is a priority
- The board does not require high levels of precision
Wet etching processes can be classified as either acid or base etches. The most common wet etching processes are acidic etching and alkaline etching.
You can etch the rigid inner layers of a printed circuit board using an acidic solution that is either cupric or ferric chloride. This process is time-consuming yet is still used because:
- It is less expensive than other methods
- The process is simple and does not require specialized equipment
- It reduces undercuts (the removal of material beneath tin plating being etched)
Acidic etching is preferred when continuous regeneration, small features, and a constant etch rate are necessary.
When you need to do the etching process on the outer layers of a printed circuit board, use alkaline solutions such as sodium or potassium hydroxide. This process requires a specialized spray chamber continuously flushed with a fresh alkaline solution to prevent the solution from becoming saturated and losing its etching power. The benefits of alkaline etching include:
- A higher etch rate than acidic solutions
- The ability to remove a variety of materials
- The ability to etch in high-density patterns
- Low undercuts
Alkaline etching is often used when large areas need to be etched quickly. Additionally, because it can remove a variety of materials, it is often used for multilayer printed circuit boards.
Etching Your PCB
The PCB etching process is among the final and most important stages of PCB manufacturing. It is important to consider the different methods available when considering PCB etching, as each has its advantages and disadvantages. Knowing when to use each method is critical to ensuring that your printed circuit board meets your specifications. Be sure to consult a reputable and experienced PCB manufacturer to ensure that your final product is of the highest quality.