While fabricating Printed Circuit Boards or PCBs at Rush PCB UK, we remove unwanted copper from laminated foils, leaving an artwork of copper tracks and planes. This is an important part of the fabricating process, and the quality of the board depends on the accuracy of the etching process. Typically, chemical and etching agents help in the etching process.
Etching a PCB
As the electronic industry evolves, electronic products are reducing in form factor and thickness. This requires the PCB to also reduce in area, increasing the component density. In turn, the size and spacing of tracks on board both reduce accordingly. This puts a lot of pressure on the etching process to deliver accurate results. Currently, the PCB manufacturing industry uses two types of etching processes:
Wet etching is the most popular etching method in the industry. The process performs controlled corrosion of copper using specialized chemicals. In the wet etching process, the laminated foil has an image of the copper pattern printed on it, before it is immersed in a bath containing the chemical etching fluid. The wet etching process can rely either on an acidic or an alkaline chemical:
Acidic Etching Process
In the acidic etching process, fabricators typically use two chemicals—Ferric Chloride and Cupric Chloride. These chemicals etch only exposed copper and do not react with the copper hidden under the photo-resist layer making up the pattern. The chemicals also do not harm any plated copper on the laminated foil.
Ferric Chloride Etching Process
Fabricators use a spray of a mixture of Ferric Chloride and Hydrochloric acid on the laminated copper foil to etch away the unwanted copper. This is a slow process, and expensive. Therefore, this is not popular.
Cupric Chloride Etching Process
As with Ferric Chloride, fabricators use a mixture of Cupric Chloride and Hydrochloric acid to etch away the unwanted copper. Being an inexpensive method, this is a more popular process. The etching rate is quite high with Cupric Chloride, and the chemical also offers comparatively constant results.
Alkaline Etching Process
In contrast to acidic etching, alkaline etching uses an alkaline mixture to remove the unwanted copper from the laminated foil. The process mainly requires high-pressure spray chambers, which spray the etchant on the laminated foil.
The exposure time is critical, as higher exposure to the reagents may damage the board. Although the process is expensive, the etch rate is relatively high and the etching is highly uniform.
Dry etching or plasma etching does not involve using any chemicals. The process uses lasers to remove the unwanted copper without leaving any residues. Dry etching is a simpler operation compared to the wet etching process. It is also a less demanding method with respect to residue disposal. The laser burns away the unwanted copper, making the process much cleaner than others.
Comparison of Wet and Dry Etching Processes
Although both processes aim to achieve the same result, the method of doing it is different, and the result is also slightly different. The comparison between the two is as follows:
- While the wet etching process uses chemicals, the dry etching process uses reactive gases or lasers.
- The wet etching process has higher selectivity, but the dry etching process is more precise (does not produce undercuts).
- The wet etching process creates a large amount of chemical waste that is difficult to dispose of, but the dry etching process produces easily disposable chemical waste in small quantities.
- An operator must constantly keep watch on the wet etching process, whereas it is possible to automate the dry etching process.
- The safety level of a wet etching process is low, as it could cause an operator hazard. On the other hand, the dry etching process has a higher safety level.
For Rush PCB UK, wet and dry etching are two necessary processes for manufacturing printed circuit boards. While both aim to achieve the same results, their modes of operation differ. They also use different chemicals for the process.