Wet PCB Etching Using Acidic and Alkaline Methods

by | Dec 2, 2020 | 1 comment

Copper etching is one of the important processes in PCB manufacturing. We can simply say that wet PCB etching is a process of controlled corrosion. In normal conditions, corrosion damages metals but with an effective machined process, corrosion can be controlled and this process is called etching. Let’s get into the details of the wet etching process. 

In this article, we will discuss the following:

What is PCB etching?

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.

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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.

Wet PCB etching process removal of unwanted copper

Two methods of wet etching are employed in common by the PCB manufacturers depending on the etchants used.

  1. Acidic etching (Ferric chloride and Cupric chloride).
  2. 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.

Etching undercuts formed after the wet etching process

Etch undercuts

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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Parameters that define the quality of etchants

In an ideal case, the etching rate depends upon the etching time and the etchant composition would be constant. But in real cases, the etchant composition changes continuously. Thus for quality assurance, we must control some parameters. The following are the parameters that are used to assess the quality of the etchant for the smooth running of the process:

  • Baumé 
  • Temperature
  • Oxidation-reduction potential (ORP)
  • Chemical additives (Free acid)
  • pH

Baumé (Bé)

Baumé is the concentration of molarity of etchant which depends on the specific gravity of the etchant. 

Equation for calculating Baumé (Bé) value

Equation for calculating Baumé (Bé) of etchant

The higher the Bé value, the higher will be the molarity of the etchant hence the etching rate of the solution. A lower Bé value would give a low etch rate with poor line resolution and a higher etchant would produce a slow etch rate. Undercut also decreases with the increase in Baumé value.

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Temperature

In general, at high temperatures higher will be the etch rate for any etchants. But the selection of etching temperature depends on the etching machine used. Most of the etching machines use plastic parts because all metals are reactive towards etchants. Hence the etching temperature cannot be too high. The maximum etching temperature range allowed is 50-55°C. 

Oxidation-reduction potential (ORP)

The measurement of ORP indicates the activity of the etchant. It is the measure of relative conductivity of the etchant and expressed in millivolts. The ORP indicates the relationship between cupric ions to cuprous ions or ferric ions to ferrous ions. As the copper is etched, the etchant changes from a cupric/ferric to a cuprous/ferrous state. The higher the value of ORP, the more efficient will be the etchant while low values of ORP indicate a slow and inefficient etchant.

Manufactures will maintain ORP at a high constant value to achieve a constant etch rate of the metal. ORP value is influenced by free acid content and etchant temperature. The addition of free acid and oxidizer to the etchant produces free chlorine. This makes cuprous ions back to cupric form. 

Adding chemical additives (Free acid)

Chemical additives are used in commercial etchants to increase the etch rate. HCI is the commonly used additives for CuCl2 and FeCl3 etchants. HCI is the source of chlorine, to form metal chlorides instead of hydroxides, which enhances the etchant’s ability to hold dissolved metals. Additives are very important for the continuous etching process. They are added before the first use of the etchant or at regeneration. The pH value of the solution is evaluated to check the acidity of the solution.

The addition of additives increases the complexity of etchants but provides a higher etching rate. It also increases the dissolving capacity of the etchant. The addition of addictives increase the etch rate but the concentration of additives depend on the machine used. The high HCl addition causes the acid to react with etching machine parts.

pH value

The pH value is a very important parameter in the etching process, especially for alkaline ammonia etching. It is between a range of 7.9 to 8.1 for reliable alkaline etching. Low pH below 8 can be caused by low ammonia, excessive ventilation, heating, etc. In such cases, pH can be raised by adding anhydrous ammonia. A higher pH value above 8.8 also causes a low etch rate. This can be caused by under- ventilation, higher copper content, or due to water in the etchant. 

In the case of acid etching methods, pH value is used in solution control. An increase in pH leads to incorrect readings of the copper colorimeter caused by solution turbidity. 

Parameters for different etchants are mentioned below:

Parameters Alkaline etch
Cupric etch
Ferric etch
Temperature (°C) 43-55 50–54 43–49
Specific gravity 1.170
1.34 +/-0.02 1.4
Baumé, Be° 25-27 32-33 36
Etch rate μm/min 30-60
25-50 25-50
Dissolved copper capacity 140-170 120-140
40-60
Operational cost High Low
Medium
pH
8.0-8.8

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Continuous etching and regeneration

The continuous etching is the method used for commercial etching of PCB which uses automated controlled feeding of the etchant. The parameters that will be controlled in such systems are specific gravity or density of etchant. 

As the copper is dissolved in etchants, the etchant’s density increases. To evaluate the volume of copper in a solution, the density of the etchant in the etcher is measured. When an upper limit of density is recorded in the density sensor, a switch activates a pump. The pump automatically feeds etchants into the etcher. At the same time, the system eliminates waste etchant.

For uninterrupted production in such systems, the regeneration of etchant is an important factor. Regeneration is the process of recycling the used etchant and feeding this etchant back to the etching process. When regeneration is continuous, constant conditions of etching will be achieved. 

The regeneration processes for waste etchant have been developed to solve the waste problems of the etching industry. The use of the regeneration process would also bring other benefits. Like saving equipment and operation costs and significantly reducing liabilities associated with the disposal of hazardous waste etchant. 

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Advantages of wet PCB etching

  • The most important advantage of wet PCB etching is that it can be conducted in a normal atmospheric environment.
  • High etching rate
  • Low operational cost
  • High selectivity: wet etching can be used to etch off a wide range of materials.
  • Ease of equipment maintenance

Disadvantages of wet PCB etching

  •     Inadequate power to etch traces <1μm.
  •     High use of chemicals
  •     Creating a significant amount of hazardous chemical waste
  •     The risk of wafer contamination
  •     Chemical hazards
    • Direct exposure to liquids
    • Direct and indirect exposure to fumes

 

After the etching process, the end product will have the circuitry as per the designer’s specifications. Soon after the etching is completed, 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 takes place in a PCB manufacturing unit. We also covered different parameters that a manufacturer must monitor when etching. Hope this article scratches your itch for etching.

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    Tags: circuit board etching, etching pcb, etching pcb at home, PCB, pcb etching, pcb etching process, printed circuit board etching

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