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.
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.
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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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:
- Oxidation-reduction potential (ORP)
- Chemical additives (Free acid)
Baumé is the concentration of molarity of etchant which depends on the specific gravity of the 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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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.
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|
|Specific gravity||1.170||1.34 +/-0.02||1.4
|Etch rate μm/min||30-60||25-50||25-50|
|Dissolved copper capacity||140-170||120-140||40-60|
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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.
What is acid trap?
The acid trap is a critical factor that hampers the performance of a PCB. Precisely, it is caused by the chemical solution that can not get washed away from the board during the etching process. The solution stays in that area and corrodes the copper traces and other parts of the board, causing open or short circuits and faulty connections.
Acid traps can not get detected easily. Prior checking can reduce the risk only. Many experienced designers fall victims to this. If it is not detected at the correct time, the circuit board might become non-functional.
Effects of acid trap
- Traces: The copper traces are immensely affected by the acid traps. The acid seeps into the traces, forms an isolated zone, and travels to the rest of the circuitry. Gradually, the board losses connectivity.
- Vias: If the acid finds its way to the vias, it can result in erosion. The open vias cause the acid to travel to the traces and hampers the circuit connectivity. Acid traps can attack even the vias closely connected with minimal spacing.
- Connectors: Connectors are soldered on the pads which are basically copper patches. If the acid flows to the pads, the component connectors also get affected.
- Solder masks: This is something that doesn’t happen frequently, but the solder mask gets eroded by exposure to acid traps.
What are the causes of acid traps?
Some major reasons are analyzed in this section to help you prevent damage due to acid traps:
- Traces connect at acute angles: This is the most common reason for acid traps. If the traces meet at an acute angle that is below 90°, an acid trap can occur. Even at 90°, it can be formed at the sharp corners. To prevent this, join traces at 45°.
- Merest space between traces, vias, and pads: Tiniest gap between traces or vias and edges of the board can make the corrosive solution seep into the traces and cause a breakdown. It is a major concern when the traces are thin, because the copper gets corroded easily in that case. You can use a design tool to calculate the right value of spacing and make changes accordingly to mitigate the possibility of an acid trap. Via tenting and plugging are also probable solutions to minimize the damage.
- Isolated copper zone: The abandoned copper areas are known as the copper island or dead copper. Often, this island is left unetched knowingly or unknowingly. It becomes an easy target for the acid to get trapped in. Advanced design tools can help you remove these dead areas. It is important to take care of this fact to eliminate the risk of the acid trap.
- DRC error: You can rely on automated design tools to detect the acid trap or the scope of it. But sometimes small gaps and traces connecting angle to acute angle can go undetected in these tools as well. Therefore, it is essential to set up the right settings and double-check the design rule checking (DRC) programs.
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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