As everything keeps getting smaller this has become one of the most important steps. A speck of dirt is enough to cause an open circuit on fine lines. The slightest misalignment on artwork top to bottom has the potential to shift dimensions outside the tolerances.
Boyang are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.
There are many different exposure units on the market now. Most do a very good job, but there are a few things that are important to have to make sure you can etch fine features.
Those features are collimated light (or LDI) and features that help create near perfect alignment. You will also need a yellow room. Keeping the yellow room and material clean will help you significantly.
The last tip we have is to hold the panel for 30 minutes after exposing it so the resist can lock into the metal more before developing it.
Approximate Cost = $200,000
The developer is used to dissolve the photoresist that was not exposed to the UV light in the above step. Removing the unpolymerized resist exposes the metal you would like to etch away. This provides you with the design you want to etch into your material.
Developing is normally looked at as less critical than etching. This is true, BUT it doesn’t mean you can overlook the developing step. The photoresist can be underdeveloped or overdeveloped.
Underdeveloped means all the unpolymerized resist was not completely removed. So, you did not fully expose the metal that you want to etch away. This results in smaller holes or no breakthrough for chemical milling and shorts for circuits.
Overdeveloping will cause the developer solution to start undercutting the resist. This can cause your resist to lift while etching and reduce your etch quality. With resist lifting, you may not meet the specifications you need or you may receive open circuits.
Approximate Cost = $92,000
The etcher is where the metal removal happens and is the most critical piece of wet processing equipment. Like photoresists, the chemistry used in the etcher needs to be matched with the metal you want to etch.
What is so critical about the etcher?
The etch uniformity is first and foremost. To have great etch uniformity you need to have the solution moving. If the solution sits on one spot for too long the solution becomes less or non-reactive.
This causes poor etch uniformity. This is known as the puddle effect.
Secondly, transport is important. Most material thickness has no issue in the developer because all the metal is there, and the photoresist adds to the thickness. However, during etching, the metal is being removed.
That means the panel is becoming less rigid and prone to wrapping around a roller. (Most transportation issues only apply to thin material processing)
Buying an etcher that has great etch uniformity and transportation is key to building a successful etching shop.
Approximate Cost = $97,000
Stripping the photoresist from the metal is the next step. As a whole stripping is less technical than the other wet processes. Basically, the resist needs to be in the chemistry long enough to strip.
There are a couple of important factors to consider if you want an efficient stripping line.
First, you will need to check with your chemical and photoresist supplier to ensure that they are compatible with each other. If they are not compatible you will see your resist stripping off in sheets, as goo, as large pieces, or very fine pieces. All of which can create problems for your equipment and process.
You want your resist to strip in small chips. The reason you want small chips is that strippers are designed to filter out small chips. If you are getting any other size or goo, this means your resist and chemistry are a mismatch, and the stripper will have a hard time filtering out particles.
This will result in plugged nozzles.
Secondly, you will want to decide what design is best for your shop. Are you running high volume or just a couple of boards a day? This will help you determine how the filtering is set up.
You will also want to understand how the filtering works. If the machine you select does not filter the resist well, you will experience lots of downtime due to nozzle clogging.
No one wants down time, so make sure your stripper has an adequate filtering system.
Approximate Cost = $103,000
Many etching shops put all three of these processes in one line. This allows shops to use less operators because the panel will go through all three processes before it needs to be unloaded.
There are some negatives to putting all three together. The line is much larger, so you need a large room.
The processes do not run at the same speed so some chambers may need to be larger than a stand-alone machine. If you move forward with opening a shop a Chemcut employee can help you through this decision process.
Approximate Cost = $360,000
There are multiple options for wastewater treatment, but the most popular is a batch wastewater treatment. A batch wastewater treatment system will consist of a collection tank, a treatment tank, filter press, clear well tank, ion exchange, and final tank.
The collection tank will collect all the runoff from your wet processing equipment. All your machines will have some liquid leaving the system. Mostly from your rinses.
The wastewater that is collected can be transferred to the treatment tank (precipitation tank). The wastewater will be treated here, and all the metals will precipitate to the bottom of the tank.
The clear water can be decanted into the clear well tank and if all the metals are removed the water can be transferred to the final tank. From the final tank, the water can be pH corrected and sent down the drain.
The metals (sludge) that settles at the bottom of the treatment tank will be pumped through the filter press. The filter press will remove all the sludge and this sludge can then be shipped off your facility as toxic waste.
The last piece of equipment you will need is a scrubber for the fumes that are being pulled out of your wet processing line.
Approximate Cost = $350,000
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:
Are you interested in learning more about Cu Etching Liquid? Contact us today to secure an expert consultation!
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.
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.
Pros:
Cons:
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.
For more information, please visit Semiconductor Chip Manufacturing.