What Are PCB Etching Techniques?
Etching is a technique to remove unwanted materials from the surface. As part of the PCB fabrication process, etching takes off the excess copper from the copper layer, leaving the desired circuit on it. There are two types of etching techniques: wet etching and dry etching.
1. Wet Etching
Wet etching refers to chemical etching, uses two types of chemicals: acidic chemicals and alkaline chemicals.
1.1 Acidic Etching
In acidic etching, the used etchants are Ferric Chloride (FeCl3) and Cupric Chloride (CuCl2). The acidic etching is often for inner layer etching in a rigid PCB. The reason is that the acidic etching is more accurate and cheaper than the alkaline one. The acidic solvent does not react with the photoresist as well as not damage the desired part. Furthermore, this method has the smallest undercuts, which are the portion of the copper removed under the photoresist. However, acidic etching is much more time-consuming than alkaline etching.
Cupric chloride (CuCl2) is a more common etchant than ferric chloride (FeCl3) as it offers a constant etching rate and is less costly. Also, it can correctly etch small features such as fine line inner layers and continuously recycle and regenerate. Cupric chloride in combination with sodium chloride (NaCl) is one of the methods to obtain the maximum etching rate. The chemical reactions include:
Cu + CuCl2 2CuCl
2CuCl + 4Cl– 2(CuCl3)2-
2CuCl + 2HCl + H2O2 2CuCl2 + 2H2O
Unlike cupric chloride, ferric chloride (FeCl3) is an etchant with limited usage due to its costly disposal after etching. But it is a favored spray etchant on account of its ability to retain copper.
1.2 Alkaline Etching
Alkaline etching is chemically removing the undesired copper layer in the circuit pattern under alkaline conditions. Alkaline etching is suitable for the etching of the outer layer circuit patterns with lead/tin plating, nickel plating, gold plating, etc. The alkaline etchant is the cupric chloride + ammonia composition. The [Cu(NH3)4]2+Cl2 in the etching solution is a strong oxidizing agent, which reacts with and dissolves the metal copper. The whole process carries out in a high-pressure spray chamber, therefore some key parameters such as temperature, pH, and concentration of Cu2+ and NH4Cl in this process need to be considered. The reacting temperature is generally between 48 to 52 . The chemical reactions include:
CuCl2 + 4NH3 [Cu(NH3)4]2+Cl2
Cu + [Cu(NH3)4]2+Cl2 2[Cu(NH3)2]+Cl
2[Cu(NH3)2]+Cl + 2NH4Cl + 2NH3 H2O + O2 2[Cu(NH3)4]2+Cl2 + 3H2O
2. Dry Etching
Dry etching utilizes gas or plasma as an etchant to remove undesired substrate materials. Unlike wet etching, dry etching avoids the use of chemicals and generating enormous hazardous chemical waste. At the same time, it reduces the risk of water contamination.
Laser etching is one of the dry etching methods, which utilizes computer-controlled hardware to produce high-quality PCB. The trace lines on the PCB substrate are engraved by a high power contained in the laser beam. Then, the computer etches away the undesired copper traces. Compared with other wet etching methods, laser etching greatly reduces the number of steps, thereby diminishing production costs as well as production time.
Plasma etching is another dry etching method designed to help lessen liquid waste disposal in the manufacturing process and realize selectivity that is hard to acquire with wet chemistry. It is a selective etching reacting with chemically active free radicals. It also includes directing the high-speed stream of plasma of a proper gas mixture at the etched material. Compared with wet etching methods, plasma etching is clean. Also, the entire process can be simplified, and dimensional tolerances can be improved. Plasma etching can perform controlled and precise etching on a very small scale. This special process also reduces the occurrence of polluted vias and solvent absorption.