As printed circuit boards become more complex, the impedance requirements for different traces or copper lines have increased. Therefore, you need stricter control over the width of the lines, which highlights the etching solution for PCB. PCB etching has and continues to evolve, and we will analyze the process and its types below. Take a look!
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PCB etching is a process involving the removal of unwanted copper from the circuit board to create circuit patterns or copper traces. Before the etching process, manufacturers use the design file to define the board's circuit pattern layout. After that, they transfer this image to the PCB using a process known as photolithography. This image creates a blueprint to determine the areas in the copper layer that need etching and the ones that should remain on the board to form the patterns.
A monocrystalline silicon wafer with microchips after photolithography
Usually, manufacturers use tin plating to create this image (etch resist) on outer PCB layers to secure the copper areas that should not undergo etching. On the inner layers, they use photoresist as the etch-resist material.
There are two methods to eliminate the excess copper after placing these protective layers. They are dry and wet etching processes.
The dry etching process does not use chemical solutions to dissolve copper. It creates plasma (positively charged gas) in a vacuum to shave off the excess copper.
The process is more complex than wet etching and requires dedicated etching machines to eliminate the excess copper material. Also, it does not produce any residue, is more flexible, and can achieve more precise material removal directionally.
A PCB trace wiring design
More importantly, it is cleaner and requires less training, making it the preferred option for critical and agile manufacturing processes, such as those used in manufacturing microelectronics.
The most typical dry etching technique is Reactive Ion Etching (RIE).
Wet PCB etching involves immersing the PCB in a chemical solution to remove the unwanted copper. The PCB etching process can employ two methods depending on the PCB etching solution. They include acidic and alkaline methods.
Chemical etching of a PCB
With the acidic method, fabricators use an acidic solution to etch the inner layers of rigid PCBs because it does not react with photoresist.
Compared to the alkaline etching method, acids do not create many undercuts. So the process is more precise. An undercut is a lateral erosion of the copper layer that occurs under the protective photoresist or tin material.
PCB etching using an acidic solution
Also, acidic etching is cheaper and does not corrode crucial sections on the circuit's blueprint. However, it is more time-consuming than its alkaline counterpart.
The solutions used for acidic etching include the following.
Cupric chloride is the most typical acidic etchant because it dissolves tiny features accurately. Additionally, the solution gives a constant etching rate and continuous regeneration at a lower cost.
Manufacturers can increase the etch rate of the cupric chloride solution by using a cupric chloride-HCl chloride sodium system. This combination increases the maximum etching rate of 55s for one ounce of copper at °F.
PCB tracks up close
However, you must adhere to safety and emergency protocols when handling chlorine gas and should work in a well-ventilated room with cylinder storage, a tank, and leak-detection sensors.
Also, you will need personal protective equipment, approval from your area's fire department, and trained operators.
This solution has the following benefits as a commercial etchant.
However, it is not a common PCB etchant because of the high cost of disposing of the copper-saturated etchant as a hazardous waste product from the process.
PCB etching in a ferric chloride solution
Usually, manufacturers use it with photoresist, gold patterns, and screen ink because it is highly corrosive. Therefore, you cannot use the solution on tin or tin/lead resists.
In most cases, you must dissolve the ferric chloride solution in water to a concentration of 28-42% by weight. After that, add about 5% HCl to the mix to avoid the formation of insoluble ferric hydroxide precipitates. The final acid concentration for the solution used for commercial purposes is about 1.5-2%.
The alkaline etching process uses only one chemical solution containing the following.
Chloride Copper + Hydrochloride + Water + Hydrogen Peroxide
This solution is highly corrosive, meaning it etches the board quickly and can damage it if you leave the PCB in the solution for a long time. Therefore, you must follow the parameters used in the process to the letter for precise control. And the procedure is expensive compared to acidic etching.
A photolithography slide for a semiconductor crystal
The process occurs in a high-pressure spray chamber with conveyors where the PCB gets exposed to a refreshed etchant spray. The following factors are crucial to creating uniform traces with minimal errors and straight sidewalls.
After eliminating every part of the excess copper, this alkaline etching process hits a point known as the breakpoint, where the procedure is complete. It usually occurs at the midpoint section through the conveyor spray chamber. For instance, if this chamber is one meter long, the breakpoint milestone will be at the 50 cm mark.
The following factors affect the etchant quality and smoothness of the wet etching process.
High temperatures generally increase an etchant's etch rate, but the etching machine will constrain the achievable temperature. Most of these machines have plastic parts to avoid corrosion from the etchant chemical solutions. Therefore, the temperatures usually don't exceed 55°C to avoid melting these components.
Baume (Be) defines the molarity concentration in an etchant, and it depends on the solution's specific gravity. A high Be implies the solution has a high etchant molarity and dissolving rate. High Baume values also reduce undercuts.
Chemical additives increase the etch rate in commercial etching solutions. HCl is the most typical additive in acidic solutions because it acts as the source of chlorine to create metal chlorides instead of hydroxides.
The extra chlorine increases the solution's ability to hold the dissolved copper. You can introduce the additive to the acidic solution before first use or during regeneration, and you must measure the pH to check the solution's acidity before use.
The etching machine restricts the additives you can introduce to the solution because chlorine increases corrosiveness.
The oxidation-reduction potential of an etchant is a measure of its relative conductivity in millivolts. This potential brings out the relationship between the following.
As the copper gets etched, the chemical solution changes from cupric or ferric to cuprous or ferrous. The higher the ORP value, the more efficient and faster the etchant operates.
If you want to learn more, please visit our website PCB Wet Process Solutions.
Adding additives (free acids) and oxidizers to the solutions increases chlorine levels. The chlorine converts the cuprous and ferrous ions back to cupric and ferric ions, making the etchant more potent.
The pH variable is crucial for alkaline etching because it should fall between 7.9 and 8.1 for reliable etching. Low pH levels below 8 are usually due to low ammonia, excessive ventilation, or heating.
And a high pH that exceeds 8.8 can be due to under ventilation, water in the etchant solution, or high copper content. Both these conditions reduce the etch rate.
With acid etching, high pH levels create incorrect copper colorimeter readings due to solution turbidity.
What is Polar Contamination?
Ionic contaminants are remains of flux that are left behind during the assembly process. Ionic compounds are held together by electrostatic forces and the compound itself has a zero net charge. These materials will disassociate when exposed to water. These are composed of positively charged cations and negatively charged anions. A simple example is table salt (sodium chloride), composed of a single positive sodium cation which lacks one electron, and a negatively charged chloride anion (Cl), which has an extra electron. Polar compounds, on the other hand, can have a positive charge on one side of the molecule and a negative charge on the other side of the molecule; these molecules never split apart. Water and Isopropanol (or IPA) are examples of polar molecules.
When populating the board with components, the components themselves can carry various ionic/conductive contaminants to your assembly including cutting oils/fluids, biocides, and corrosion preventatives. Be aware of common nonionic materials that can also affect the assembly steps – process oils, mold releases, etc. can be detrimental down the line.
Common Moisture Trapped in the “Layering” Process
Water is a polar contaminant. It is conducive for dissociating other ionic materials which then lays the foundation for conductive mishaps (dendritic growth, ECM, etc). It is common practice to “bake” the boards to remove extraneous moisture.
Corrosion from PCB fabrication contamination (photo courtesy of Foresite)
Etching Chemicals
Etching chemicals are highly conductive and can be corrosive as well. They must be chemically neutralized and removed/rinsed and are well-known as sources for current leakage.
Flux Residues from Soldering
Heavy no-clean flux residue with visible copper corrosion (photo courtesy of Foresite)
Everyone is familiar with flux residues. Fluxes, whether in liquid, cored wire, or compounded as a paste, can leave residues that can cause serious reliability defects if not removed. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. All of these must be cleaned from the substrate, whether by strict solvent cleaning such as vapor degreasing or by aqueous chemistries in the common batch or inline cleaners seen on the manufacturing floor.
Inter-Layer Residues from Drilling and Via Plating Processes
Dendritic grown between solder pads, caused by ionic contamination (photo courtesy of Foresite)
In addition, residues from the cleaning process chemistry itself must also be removed. This is noticed more in the aqueous cleaning systems. Many use saponifiers to neutralize and emulsify the flux residues and make them easier to rinse and remove from the substrate. These components themselves are highly polar and ionic and can also enhance the dendrite and/or ECM mechanism if not removed. In addition, corrosion preventatives and surfactants are commonly employed in these products. This is not a bad thing in itself, but care must be taken to ensure they are removed along with the soils during the cleaning process.
Poor quality control in fabrication, poor soldering or component population, and even final cleaning stages are all potential sources of contamination. Many of these can be found by ionic contamination testing and analysis such as ROSE testing, ion extraction, and chromatography. Initial high humidity validation testing at the beginning of the project can also identify potential issues.
Strict quality control and standard operating procedures during the PCB assembly, manufacturing stages, and validation testing can go a long way in preventing a reliability nightmare. Just think – simple mishandling of a part by an operator not using gloves can transfer salts and oils from skin to the substrate that could potentially be catastrophic for your item!
White residue is generally a symptom of ineffective PCB cleaning. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, and succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. When a cleaner does not fully dissolve all the constituents, or the cleaner is not allowed to flow off the PCB, the remaining solvent can evaporate off and leave behind residue that is either white or like water spots.
White flux residue with visible copper corrosion (photo courtesy of Foresite)
White residues can generally be cleaned by a flux remover. If the residues are the result of insufficient solvency of the original cleaning process, a stronger solvent cleaner may be required. Often agitation is required to remove the residues, which may include a wipe, swab, brush, or an aerosol with a brush attachment. Follow these steps to remove white residue:
Flux-Off® Rosin with a brush attachment
PCB flux removal can either happen at the benchtop, which generally requires a manual cleaning method, or in automatic or semi-automatic processes. This is common for low volume electronic PCB assembly, rework, and repair. Manual cleaning methods are generally more laborious and less repeatable, so results may vary from operator to operator. For higher volume assembly or reduced variability, more automated cleaning methods are used.
Manual Flux Removal Methods
Automated or Semi-Automated Flux Removal Methods
The type of flux can have a big impact on the cleaning process. R, RA and RMA fluxes are generally easier to remove with standard flux removers and isopropyl alcohol. No-clean fluxes are intended to stay on the PCB, so can be more difficult to remove. They may require a more aggressive solvent flux remover, additional agitation like brushing, or a heated solvent. Aqueous fluxes are generally designed to be removed in a batch or inline cleaning system with straight deionized water or water with a saponifier. Alcohol-based or specially formulated solvents can also be used to clean aqueous fluxes, but the same cleaners may have mixed results on other types of fluxes.
The short answer is to match the flux remover with the flux type. However, this can be challenging for an EMS supplier that may have to use a variety of fluxes as required by their various customers. Flux removers are available that can break down a large variety of fluxes while changing the variables, like cleaning time, agitation, or additional heat, can treat distinctive needs adroitly.
For water-based cleaners in batch or inline cleaning systems, cleaner concentration can be adjusted, cycle time increased, and temperature increased to improve performance against various flux types.
Any process engineer will tell you that the key to designing a repeatable process is to control the variables. When removing flux from electronic circuit boards, there are a number of variables that can drastically change the cleaning performance of a cleaner and process:
If you are suddenly surprised by white residues or some other clear evidence of a cleaning problem that didn’t exist before, step back and look at your process before calling for help. Has anything changed? That will be the first question a technician will ask, and necessary to know before you can identify and solve the problem.
How do you remove solder flux?
The most common way to clean flux residues from a repair area is to saturate a cotton or foam swab with isopropyl alcohol or another cleaning solvent, and rub it around the repair area. While this may be adequate for no-clean flux, where the goal is a visually clean PCB, this may not be clean enough when more heavily activated fluxes are involved, like RA or aqueous. The dirty little secret is that flux residues will not evaporate along with the solvent. You may dissolve the flux, and some of the residues will soak into the swab, but most of the residues will settle back onto the board surface. Many times these white residues are more difficult to remove than the original flux.
Flux residues don't evaporate along with the solvent.
One quick and easy improvement to this process is to rinse the board after swabbing around the repair area. While the solvent is still wet, spray over the entire board with an aerosol flux cleaner. Hold the PCB at an angle to allow the solvent to flow over the board and run off, along with any residues that are picked up.
The straw attachment that comes with aerosol flux removers is a good way to increase the spray force and penetrate under the components.
Aerosol with straw good for cleaning under components
Chemtronics offers the BrushClean™ system with many of their flux removers. The cleaning solvent sprays through the brush, so agitation can be increased by scrubbing while spraying. To absorb the flux residues, a lint-free poly-cellulose wiper can be placed over the repair area, and the spraying and scrubbing can occur over the material. Then remove the wipe and brush attachment, and spray over the board for the final rinse.
Aerosol brush attachment over a wiper dissolves and absorbs flux residues at the same time.
For more information, please visit Stencil Cleaning Machine.