I have always been a very visually-oriented thinker. When I was much younger, I used to look up maps to video game levels online and put a sheet of tracing paper up against the screen to copy them down. The Zelda games specifically would always leave me with a notebook full of maps and, on the more challenging dungeons, color-coded warnings. If I sneezed once in my tracing process or the screen moved or I was called away mid-tracing, my map would be restarted.
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One time, I spent hours in a dungeon only to realize that I’d done it all wrong because of a mistake in my process and had to backtrack to understand exactly where I messed up. Now, we don’t have to worry about etching mistakes so much with more intuitive dungeon-layout logic in games and, more generally, the availability of and computer screens.
It does make sense, though, that I’ve worked in positions that make me recognize the PCB etching process. When I was just getting started in design, I had very little idea about etching and how it worked. But as my experience grew, I was put into positions which required me to have more of a direct understanding of production and manufacturing processes. Having this knowledge will allow you to have a smoother transition to production.
Etching your PCB is a mandatory step for any sort of PCB production. Whether it is a simple prototype or a full-blown manufacturing batch, your board’s connection paths will need to be precisely defined and laid out so that your components will be able to successfully communicate with one another.
There are various styles of PCB etching that can occur in order to achieve similar results. The style that you choose, however, could depend on many factors involved. Is this simply a one and done PCB? Is time a factor in your decision? What about environmental impacts to larger production runs? How tightly packed and complex is your design?
There are two main schools of application when it comes to PCB etching. The first and most widely understood method of etching is wet etching. This involves the use of chemicals for your etching solution. The board essentially bathes in which chemically removes a specified area of the original copper plate leaving behind your predefined copper paths (once cleaned).
The other, slightly newer method of etching is known as dry, or plasma etching. This process involves the use of a chamber filled with a positively charged gaseous solution that essentially eats away the unwanted copper material leaving behind a similar looking path to that created by the aforementioned wet method. Of course, knowing which is right depends on your application.
If you perform a quick search of the term ‘PCB wet etching’, you’ll find a plethora of DIY videos and instructions on how to etch your own circuit board with simple, easy to attain items. The materials involved can range from simple household items such as the use of hydrogen peroxide, vinegar, and salt. Other, more robust applications involve the use of more ‘potent’ chemicals such as cupric chloride (copper chloride) or ferric chloride (iron chloride).
One advantage of using the wet method is that it is relatively simple to perform and is ideal for one-off DIY designs. If you are in a crunch for time and can’t wait for the fab house to get you a prototype board out, you will certainly be able to procure your own etched circuit board within a day or so. This method, of course, is rather messy and does require some knowledge of the chemicals and process involved as you will literally be soaking your circuit board for a short amount of time.
Using the household materials as listed above is obviously a cleaner set up, however, using the industry standard cupric chloride will involve some forethought on proper handling and disposal of the etching solution as it is terribly harmful to the environment (especially with all that dissolved copper floating around in it).
If you are in need of a higher quantity production, further considerations should be made accordingly. The amount of material alone in a production level wet etch will surpass that of the dry method, along with floor space required for bathing, proper training and education of your workers, as well as governmental permits required for disposal.
On the other side of the coin, the dry etching method is a far more complex operation requiring a dedicated machine for its operation; however, the process is a slightly cleaner way of etching in my opinion. The amount of training that is required for operation is far less than any wet method making it a better solution for a more agile operation.
The use of positively charged gas, or plasma, that is used for the physical etching of the copper leaves no residue for any further cleaning stage making it more suitable for higher quantity production runs. The disposal of the residue will be released straight to the atmosphere which can be viewed in a negative context, but when comparing to the wet method, it appears that less waste will impact the environment as a whole.
Drawbacks to this method of etching are limited if you intend to reproduce a printed circuit board in multiple quantities, but if you are simply looking to develop a handful of one-off prototypes, the investment in these machines may outweigh the return.
If you are in a time crunch with a need for a specific PCB layout, you can easily get away with a quick and dirty DIY wet etching operation. If you are in need of higher quantities of boards, or if you are to the point in your production where timing, accuracy, and environmental concerns arise in your decisions, then utilizing dry etching methods with the use of dedicated etching machines will likely be for you.
Take careful review of your manufacturers and ask for an overview of their process in all aspects like preparation, operation, and disposal; ensure their standards align with your needs and concerns before jumping in. To get you to a point where you can consider your manufacturing and production options for your designs though, you should be using PCB design software which can contend with your design demands, and prepare you for next steps.
A strong design software will allow you to move seamlessly between schematic, footprints, layout, and manufacturing outputs without painful transferring and translating processes. Through a unified design environment, you won’t need to rely on a horde of different softwares or tools to find the treasure at the end of the dungeon. With the idea of a smart, intuitive design approach in mind, Altium Designer® would be the right fit for any designer.
If you want to further discuss etching concerns or progressing your designs through all phases of the process, talk to an expert at Altium Designer today.
Wet PCB etching is a process of removing unwanted copper by immersing a circuit board in a chemical solution (acidic or alkaline based). Since the entire board is submerged in the chemical solution, the corrosion is uniform in all directions.
PCB etching is a process of removing unwanted copper (Cu) from the circuit board to achieve the desired circuit pattern.
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 circuit board 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 onto a PCB by a process called Photolithography. This forms the blueprint that decides which part of the 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.
PCB etching can be done using two methods: wet etching and dry etching.
The direction and rate of etching have a significant impact on the process. Focus on the following points before starting the etching process:
To have a controlled rate of etching, you need to choose the appropriate direction of the etching. There are two categories:
Anisotropic: This process removes material along a particular or designated direction. This method is employed in the dry etching process, and the etching rate can be controlled.
Isotropic: In this process, the etching rate is constant in all directions. Wet etching is an isotropic method. But, in this process controlling the rate of etching is challenging.
Selectivity can be defined as the ratio of etch rates between materials. It is essential to understand etch selectivity to accurately calculate the mask thickness and control the etching process.
Wet or chemical etching is the most fundamental type of etching, which is widely employed in microelectronic applications. Here, the substance is removed using a liquid reactant, usually, acid or alkaline based. It can either remove the material instantly or it can first oxidize the substance and then dissolve it. The quality of the etching process mainly depends on:
Chemical reactions during the wet etch process include:
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 PCB manufacturers, depending on the etchants used.
Both these methods have their own pros and cons.
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 minimal in this method.
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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 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 lines 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.
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 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.
The alkaline method of wet PCB etching is used to etch the outer layers of 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, considering the spray chamber length is 2 meters, the breakpoint will be achieved when the board reaches the midpoint i.e., 1 meter.
Dip technique: The substrate is immersed in the chemical solution and it is retained until all the unwanted layers are removed.
Spinning and spraying technique: In this procedure, the substrate will rotate and the etchant is sprayed, in parallelly. The spinning of the substrate provides a more even etch surface. The effectiveness of the process depends on:
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 is the concentration of molarity of the 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,n and a higher etchant would produce a slow etch rate. Undercut also decreases with the increase in Baumé value.
In general, at high temperatures, higher will be the etch rate. 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 to etchants. Hence, the etching temperature cannot be too high. The maximum etching temperature range allowed is 50-55°C.
The measurement of ORP indicates the activity of the etchant. It is the measure of the relative conductivity of the etchant and is 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 the etchant will be, while low values of ORP indicate a slow and inefficient etchant.
Manufacturers 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.
Chemical additives are used in commercial etchants to increase the etch rate. HCI is the commonly used additive for CuCl2 and FeCl3 etchants. HCI is the source of chlorine, forming 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 additives increases the etch rate, but the concentration of additives depends on the machine used. The high HCl addition causes the acid to react with the 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, the 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:
ParametersAlkaline etchContinuous 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 the specific gravity or density of the 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 the 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 etchants 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.
See the future of PCB etching by Laurent Nicolet for expert insights into PCB etching.
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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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