Wet PCB Etching Using Acidic and Alkaline Methods
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 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 the 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 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. In addition, the undercuts are minimum in this method.
To throw some light on undercuts, undercuts are the lateral erosion of the etched material below the protective layer. When the solution hits the copper, it attacks it and leaves behind the tracks that 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 undercut.
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 Castle, 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. The important parameters to be considered during the PCB etching process are the rate of the panel movement, spray of the chemicals, and the amount of copper to be etched off. This ensures that the etching process is uniformly done with straight side walls.
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, if you consider the spray chamber length is 2 meters, then the breakpoint will be achieved when the board reaches the midpoint i.e.1 meter.
The end product will have the circuitry as per the designer’s specifications. Soon after this, the board will be further processed for stripping. The stripping process removes the electroplated tin or tin/lead or the photoresist from the surface of the board.
So, this is the inside story about how the etching process happens in a PCB manufacturing unit. Hope this article scratches your itch for etching.
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