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PCB etching is a process of removal of unwanted copper (Cu) from the circuit board. When I say unwanted, it is nothing but the non-circuit copper that is removed from the board as per the PCB design. As a result, the desired circuit pattern is achieved.

In other words, etching is like chiseling the circuit board. If you can think like an artist, the board is a rock, and etching chisels the rock into a beautiful sculpture. During this process, the base copper or the start copper is removed from the board. Rolled and annealed copper is easy to etch off compared to electroplated copper.

Before the process of etching, a layout is prepared so that the end product is as per the designer’s requirement. The designer’s desired image of the circuit is transferred on to a PCB by a process called Photolithography. This forms the blueprint that decides which part of copper must be removed from the board.

There are two distinctive approaches for the inner layer and outer layer etching. In the outer layer etching process, the tin plating acts as the etch resist. Whereas, in the inner layer, the photoresist is the etch resist.

Methods of wet PCB etching

Wet etching is a type of etching process where the unwanted material is dissolved when immersed in a chemical solution. Two methods of wet etching are employed in common by the PCB manufacturers depending on the etchants used.

Acidic etching (Ferric chloride and Cupric chloride).

Alkaline etching (Ammoniacal)

Both these methods have their own pros and cons.

Acidic etching process

The acidic method is used to etch off the inner layers in a rigid PCB. This method involves chemical solvents like Ferric chloride (FeCl3) OR Cupric Chloride (CuCl2). The acidic method is more precise and cheaper but time-consuming, compared to the alkaline method. This method is implemented for the inner layers because the acid doesn’t react with the photoresist and doesn’t damage the required part. Also, the undercuts are minimum in this method.

Undercuts are the lateral erosion of the etched material below the protective tin/lead layer. When the solution hits the copper, it attacks the copper and leaves behind the tracks that are protected. The tracks are protected with either a plated etch resist or a photo imaged resist. At the track edge, there is always some amount of copper removed under the resist, this is known as an undercut.

Cupric chloride etching

Cupric chloride is the most widely used etchant since it accurately etches off smaller features. The cupric chloride process also provides a constant etch rate and continuous regeneration, comparatively at a lower cost.

The maximum etching rate from the cupric chloride system is obtained from a combination of cupric chloride-sodium chloride-HCl systems. This combo gives a maximum etching rate of 55s for 1oz copper at 130°F. Hence, this type of etching is used to etch fine line inner layers

Note: The use of chlorine gas requires adequate ventilation, tank and cylinder storage, and leak-detection equipment. Furthermore, it requires emergency protocols, personal protective equipment, trained operators, and approval from the fire department.

Ferric chloride etching

The ferric chloride etchant has limited usage in the industry because of the costly disposal of the copper-containing etchant. However, ferric chloride is an attractive spray etchant because of its ease of use, holding capacity for copper, and the ability to be used in infrequent batch applications. Ferric chloride can be used with screen ink, photoresist, and gold patterns, but it cannot be used with tin or tin/lead resists.

Usually, a ferric chloride solution is dissolved in water with a concentration ranging from 28 – 42% by weight. HCI (up to 5%) is also mixed with this solution to prevent the formation of insoluble precipitates of ferric hydroxide.

The specific gravity of ferric chloride usually used is 36 Be, or approximately 4.0lb/gal FeCl3. The acid (HCL) content will be within 1.5 to 2% for commercial purposes.

Alkaline etching process

The alkaline method is used to etch off the outer layers in a PCB. Here, the chemicals utilized are chloride copper (CuCl2, 2H2O) + hydrochloride (HCl) + hydrogen peroxide (H2O2) + water (H2O) composition. The alkaline method is a fast process and is a bit expensive as well. The parameters for this process must be diligently followed since the solvent can damage the board if it is in contact for a longer period. The process must be well controlled.

The whole process is implemented in a conveyorized, high-pressure spray chamber where the PCB is exposed to a refreshed spray of etchant. There are important parameters to be considered during the alkaline PCB etching process. They are the rate of the panel movement, the chemical spray, and the amount of copper to be etched off. This ensures that the etching process is uniformly done with straight sidewalls.

During the etching process, the point at which the etching of the unwanted copper is complete is called the breakpoint. This is usually achieved at the midpoint of the spray chamber. For example, consider the spray chamber length is 2 meters, the breakpoint will be achieved when the board reaches the midpoint i.e.1 meter.

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