With the Printed Circuit Board industry growing each day, the impedance needs of different wires have ultimately gone up. Due to this reason, wires on a PCB now need stricter control in terms of width. As per industrial production, various manufacturers are working day in and day out to make PCBs of higher quality and more reliable. Due to these reasons, various PCBs’ designing and manufacturing process has become more diversified over time. The utilization of etching techniques during the PCB’s assembly and processing has also become more widespread.
There are two broad categories of etching: Dry etching and Wet etching. Dry etching is simply an anisotropic etching process that involves the removal of semiconductor materials using plasma rather than chemical treatments. During this process, excited plasma ions combine with semiconductor materials and removes unwanted copper without chemical solutions. On the other hand, wet etching involves using chemical etching solutions to etch out unwanted semiconductor materials. But how does wet etching occurs? And most importantly, which etching solutions are applicable in this etching technique?
PCB Etching in a Nutshell
Printed circuit board etching involves the removal of any unwanted copper elements from a PCB circuit. The “unwanted” copper elements are simply the non-used copper elements in the circuit according to the board’s design. Therefore, manufacturers must etch out this unwanted copper to achieve that perfect circuit pattern.
Before the PCB etching process begins, manufacturers must get a presentation of the circuit’s layout defining the PCB’s final circuit. PCB designers transfer this layout image to the PCB manufacturer via a process called Photolithography. Manufacturers then utilize this image as a blueprint when deciding on the copper to remove and the one that remains on the board.
Two different etching approaches exist for the outer and the inner layers of a PCB. When etching outer layers of a PCB, manufacturers utilize tin plating to secure the parts that they do not want to etch. On the other hand, when etching the inner layer, manufacturers utilize a photoresist to protect the copper they do not want to remove.
Wet Etching (Isotropic)
Wet PCB etching is a simple technique that involves immersing the etching materials in a chemical solution to remove the excess copper. During this PCB etching process, three distinct factors affect the PCB’s etching rate:
- Etching chemical concentration
- Etching solution temperature
- Stirring the etching solution
Qualitatively speaking, if you wish to boost your PCB’s etching rate, you should increase the temperature of the etching solutions and include stirring during the process.
We carry out wet etching in two distinct methods, namely:
- Alkaline etching
- Acidic etching
Each of these methods bears its advantages and disadvantages.
Alkaline Etching Process and the Solution
Manufacturers utilize the alkaline etching process to remove (etch off) the PCB’s outer layer.
Solutions utilized in the Alkaline Etching Process
During this process, manufacturers utilize the following chemical solution to etch off copper elements from the outer part of the PCB:
- Chloride Copper + Hydrochloride + water + Hydrogen Peroxide
When undertaking this PCB etching process, manufacturers must diligently follow all the parameters that make up this etching solution. That is because if they leave the board inside the solution for a long period, then the solution can easily damage the PCB. Therefore, there is need to control the process to avoid any bad occurrences.
Alkaline etching is pretty fast. However, it is also expensive.
How does the Alkaline Etching Process occur?
The entire process happens in a high-pressure spray chamber. In this chamber, there is thorough exposure of the PCB to an etchant spray.
During the alkaline etching process, manufacturers have to consider some crucial parameters, which include:
- The movement rate of the panels
- The etching spray
- How much copper to remove from the PCB
When there is an optimum balance of these parameters, this etching process tends to be uniformly successful, bearing straight side walls and minimal errors.
When you remove every bit of unwanted copper from the PCB, the alkaline process hits a point known as the breakpoint. At this point, no more etching is supposed to take place. The breakpoint is, in most cases, reached when the PCB is at the spray chamber’s midpoint. For example, if the spay chamber has a total length of 2 meters, then the process will hit breakpoint when the PCB reaches the chamber’s midpoint, that is 1 meter.
The Acidic Etching Process
Manufacturers utilize the Acidic method to etch off unwanted copper elements from a rigid PCB’s inner layer.
The Acidic process is cheaper, and it also achieves more precise results. However, this process also takes much time to achieve the desired circuit pattern compared to utilizing the alkaline etching method. The acidic process handles inner layer PCB etching because the acids do not react with photoresist. The acids do not damage crucial parts appearing on the PCB circuit blueprint. Moreover, this process does not breed many undercuts compared to the alkaline method.
An Undercut is a simple lateral erosion that occur below the etched material’s protective layer. When the etching solution hits the unwanted copper, it attacks it, leaving behind the copper layers that were protected using either:
- Photo imaged resist or,
- Plate etch-resist
At the edges of the tracks left behind, there are almost always some copper elements that one has removed. These elements leave structures known as undercuts.
Solutions utilized to undertake Acidic Etching
For this process to take place, manufacturers utilize the following chemical solvents:
- Cupric Chloride etchant
- Ferric Chloride etchant
Cupric Chloride Etching
PCB manufacturers mostly utilize Cupric Chloride to undertake acidic etching because it tends to etch off tiny features accurately. Moreover, this solution also provides continuous regeneration and constant etching, all at a pretty affordable price.
To maximize the etching rate of the Cupric Chloride process, manufacturers utilize a combination of cupric chloride-HCI chloride-sodium systems. Using this combination, manufacturers can achieve a max etching rate of about 55s for 10z when the copper is at 1300F. Due to these features, utilize this process to undertake fine line etching of inner layers.
Points to note when working with chlorine gas
When working with chlorine gas, ensure you do so in a well-ventilated room and also ensure you utilize the following equipment:
- A tank
- A cylinder storage
- Leak detection equipment
Additionally, when utilizing chlorine, you will need emergency protocols, trained operators, approval to utilize the chemical from the area’s fire department, and personal protective equipment. All this features ensure that you stay safe when working with this chemical. Lagging in any of this precautions might lead to serious consequences.
Ferric Chloride PCB Etching process
Ferric Chloride is not utilized as much as Cupric Chloride in the PCB industry due to the cost of disposing of the hazardous waste that comes from the process. However, this solution is attractive in that:
- It is super easy to use
- Manufacturers can utilize it to handle infrequent batches
- It has a great copper holding capacity
PCB manufacturers utilize Ferric Chloride solution with gold patterns, photoresist, and screen ink. However, don’t use this chemical with lead/tin resist or tin.
In most cases, you dissolve Ferric Chloride in water with a concentration range of 42% – 28% by weight. After making this solution, manufacturers add about 5% of HCI into the mix to avoid insoluble precipitate formation of the now formed ferric hydroxide.
Finally, the acid content of this solution is usually between 1.5% – 2% for commercial usage.
Which parameters define an etchant’s quality?
Ideally, etching rates depend on the time it takes to etch a PCB, assuming that the etching solution remains constant. However, this is not true. That is because the composition of an etchant changes continuously. Therefore, manufacturers must control certain parameters to ensure that the PCB etching process breeds a high-quality PCB. Here are some parameters that manufacturers use to assess an etchant’s quality to ensure that the etching process flows smoothly:
- pH
- Chemical additives
- Baume
- Oxidation-reduction potential
- Temperature
Baume
Baume, or simply Be, is the molarity concentration of an etchant, and it depends upon the gravity of the solution.
A high Be usually coincides with high etchant molarity and ultimately a high etching rate. However, if the Baume of a solution is low, then that means that the solution’s molarity is low; hence the chemical will have a low etching rate.
A high Baume also reduces the number of undercuts formed after the etching process. Therefore, Baume is a crucial parameter to consider when working with any etchant.
Temperature
Generally, high temperatures improve the etch rate of an etchant. However, etching temperatures are constrained to the type of etching machine in use. Most etching machines utilize plastic parts since all metals tend to react with the etchant. Due to this reason, manufacturers tend to utilize low etching temperatures to avoid melting the plastic. The maximum etching temp range permitted falls between the range of 500 and 550C.
Chemical Additives
Most manufacturers utilize chemical additives to boost the etch rate of commercial etchants. For example, HCI is in common use as an additive for FeCL3 and CuCl2 etchants. When added to these solutions, HCI acts as chlorine’s source, generating metal chlorides rather than hydroxides. Due to this chemical reactions, these etchants can hold on to the dissolved metals.
Chemical additives are pretty crucial in the process of continuous etching. Manufacturers add them to their commercial etchant before utilizing it or during regeneration. However, they must first evaluate the solution’s pH to check its acidity before use.
Adding chemical additives to an etchant tends to improve its complexity, and it also boosts its etching rate. It also boosts the etchant’s dissolving capacity making it more reliable.
However, the amount of additive used in an etchant is constrained to the etching machine utilized. Therefore, adding a lot of Hydrochloric acid to the etchant might cause the acid in use to react with parts of the etching machine.
Oxidation-Reduction Potential
Printed Circuit Board manufacturers measure the oxidation-reduction potential of an etchant to determine its activity. Oxidation-reduction potential simply dictates an etchant’s relative conductivity, and we express it in millivolts.
Oxidation-reduction potential brings out the relationships that exist between:
- Cuprous ions and Cupric ions
- Ferrous ions and Ferric ions
As one removes the copper (etched), the chemical solution changes from a ferric/cupric to a ferrous/cuprous state.
An etchant with a high ORP is usually more efficient than an etchant with a low ORP.
Manufacturers maintain high ORP values to gain constant high etch rates. However, etching temperature and free acid contents influence the Oxidation-reduction potential values. Adding oxidizers and free acids into an etchant makes the solution breed chlorine. When this happens, the Cuprous formed earlier during the etching process revert to their initial cupric form.
PH Values
The pH of an etchant is a crucial etching parameter that you should greatly consider when working with etching solutions. It is especially crucial when you are undertaking alkaline etching. For reliable and efficient alkaline etching, the pH of the etching solution should fall between 7.9 to 8.1. A low pH that falls below 8 can be due to:
- Low ammonia
- Heating
- Excessive ventilation et cetera
However, a high pH that goes above 8.8 can be caused by:
- High copper contents
- Under ventilation
- Water found in the etchant
In acid etching, pH values are useful for solution control. A high pH causes incorrect copper colorimeter readings due to solution turbidity.
Pros of Wet Etching
The wet etching process has a couple of advantages that include:
- It is cheaper compared to Dry etching
- It breeds excellent surface uniformity
- Better adaptability compared to dry etching
- It has impeccable selectivity, hence it is mostly utilized for inner surface etching to achieve precision
- A higher etching rate when compared to dry etching
- The equipment used for wet etching is easy to maintain
- Wet PCB etching can also take place under a normal atmospheric surrounding
Cons of Wet Etching
Even though Wet PCB etching is without a doubt beneficial, it still has some cons that include:
- Pattern etching fidelity isn’t ideal
- Isotropic etching
- Tiny lines of the board’s etching pattern are challenging to control
- Toxic chemicals – might be due to indirect or direct fume exposure. It can also be due to direct liquid chemical exposure.
- Risk of contaminating water
Conclusion
The etching process is, without a doubt, a crucial process in the production process of a PCB. Over the years, manufacturing companies have mastered the art of etching. However, evolution and industrialization have seen PCB circuits become more complex daily. Due to circuit complexity, the PCB etching process has also evolved, forcing manufacturers to look for ways to make etching solutions more productive. In the quest to make etchants productive, additive chemicals have been brought into the process to optimize etching solutions. In addition, manufacturers have also found ways to manipulate the pH of various etchants to make them more productive. PCB manufacturing has without a doubt been in optimal use by the improvements made in the PCB etching process, and we expect more improvements in the near future.
