High-tech components fabrication is inextricably linked to printed circuit board (PCBs) production. Such pieces of electronics are the core of most computerized systems.
There is not much information available online about some specific aspects of PCBs production, one of which is etching. Manufacturers, technicians, and tech-savvy persons should know about this process to avoid manufacturing mistakes and difficulty constructing and repairing electronics.
This post will describe wet PCB etching in great detail. So you will know exactly how the process works, what are the necessary materials and tools to conduct it, and its advantages and drawbacks.
What is PCB Etching?
You may have heard of PCB etching as copper etching or simply the copper removal from PCBs. Basically, the name of the process is self-explanatory, and it is the controlled elimination of excessive copper traces using one of the following three techniques available:
- Wet etching. Or the controlled corrosion of copper is performed using specialized chemicals.
- Plasma etching. Or removal of copper (and technically, any other non-metallic material) using superheated, ionized material – plasma.
- Laser etching. Or removal of copper (and some other metallic and non-metallic materials) using a high-power laser.
Wet etching is the technique you are most likely to work with, so let’s focus on it further in the text.
You may wonder, why do you need to conduct PCB etching at all? Well, the simple answer is that your electronics are not functional unless the substrate metal (copper) is removed from traces that form circuit patterns.
Okay, but how do you not remove all the copper and completely destroy a PCB? A small amount of tin covers parts of PCB that must have been left untouched by chemicals. See the simplified representation of the etching process in the image below.
Methods of Wet PCB Etching
So let’s get down to business. Two methods of wet PCB etching you can rely on are Acidic and Alkaline ones. They have substantial differences that you can learn from the below sections.
Acidic Etching Process
Also known as Ferric chloride and Cupric chloride (due to the use of FeCl3 and CuCl2 acids, respectively). This method is intended for the inner layers of PCBs. The acids do not react with a photo-resistant layer of a PCB and do not damage protected with tin copper. The corrosion even does not touch plated copper and leaves only minor undercuts. See what’s understood by undercuts in the image below.
- Cupric Chloride etching
In essence, CuCl2 is the most widely used etchant overall. It is favored by most manufacturers for accurately etching off smaller parts of PCBs. It also provides comparatively constant results and is considered an affordable reactive.
The process of corrosion takes time naturally. That’s why such a definition as “etching rate” implies how fast copper deteriorates. A combination of CuCl2 and Hydrochloric acid (HCl) systems result in incredible 1 oz copper per 55 seconds. Just for a reference – 1 oz equals 1.37 thousandths of an inch. It does not seem like a high-speed material removal method, but you should be aware that the layer of copper over a PCB surface is also very thin.
- Ferric Chloride etching
As for the second acidic etching method, it employs a simple but expensive spray etchant. The use of FeCl3 is limited because it is not cost-effective as cupric chloride, but in contrast, it can be used with screen ink, photoresist, and gold patterns of PCBs. The drawback – it cannot be used on PCBs with tin or tin/lead resists applied over copper.
Ferric chloride is dissolved in water (28-425 by weight) with the addition of HCl (up to 5%) prior to use. Then it can easily be applied onto PCBs (or any other component/metal workpiece as you wish). The process is a bit slower than the previously described one.
Alkaline Etching Process
In contrast to acidic etching, alkaline etching is used to etch off the outer layers of PCBs. For this purpose, take Chloride (170 grams per liter), Copper (150 grams per liter), Ammonium Hydroxide (NH4OH), 5.5 mol., isopropyl alcohol, and distilled water. It is important not to exceed the recommended exposure time, or the reagents may damage PCB itself.
The process is considered expensive, but it offers a relatively high etch rate (1.1-1.3 oz copper per minute). It is performed in high-pressure spray chambers, where PCBs are exposed to spray etchant.
The interesting thing about high-pressure spray chambers is that they move panels with PCBs on them down during the process. The rate of the panel movement correlates with the rate of etching, and typically, the breakpoint – a moment in time when unwanted copper is removed, is achieved when the panel is in the middle of the chamber. Such a process ensures that etching is performed uniformly.
Parameters that Define the Quality of Etchants
Here is the list of parameters that define the properties of etchants you use, and therefore, the quality of the process overall:
- Baumé (Bé)
It is the molar concentration – the amount of a solute in one unit of solution that depends on the specific gravity of the etchant.
The basic formula to calculate Bé is: Bé = 145 x ( (specific gravity of the etchant -1) / specific gravity); As a rule of thumb, the higher the Bé value – the higher the molarity of the solution. And the higher the molarity – the higher the etching rate of the solution.
You can see how solutions with different values of Bé (32 °Bé, 36 °Bé, 40 °Bé, and 44 °Bé) differ in the depth of etching over time in the image below.
- pH value
While performing PCB etching, it is crucial to maintain a stable pH value. Alkaline solutions are especially demanding to pH value. Typically, the recommended pH is 7.9-8.8, with an optimal range equal to 7.9-8.1 pH. A higher value results in a low etch rate, while a lower value causes excessive heating and overall deterioration of the process.
Another crucial parameter is the temperature of the etching solution maintained during the process. The temperature may vary depending on numerous factors such as the type of solution, type of substrate metal, equipment unit used to perform the process, etc. In general, 50-55°C is considered a reliable range for high-quality PCB wet etching.
- Oxidation-reduction potential (ORP)
ORP is basically the measure of the activity of the etchant. Or, to be exact, the measure of relative conductivity of the etchant expressed in millivolts. As a rule of thumb – the high value of ORP equals the high efficiency of the etching solution.
It is not essential if the value is as high as possible, but rather the main aspect is that you must be constant. If this value is not controlled, the same time of exposure may result in different outcomes for every batch of PCBs.
- Chemical additives (Free acid)
It is always up to a manufacturer whether to use chemical additives or not. But typically, some commercial etchants are enhanced by free acids to increase the etch rate. For example, HCl is commonly added to Cucl2 and FeCl3 etchants.
Besides the higher speed of etching, chemical additives increase dissolving capacity. It means that the etchant can dissolve more copper – and process more PCBs respectively before it has to be regenerated or disposed of.
As a bonus, here’s several parameters that define the quality of chemical etching overall.
- Exposure time. It is known that etching is a time-dependent process. There are two consequences of not adhering to optimal exposure time – over-etching and under-etching. These concepts are self-explanatory and imply that your PCBs were either not fully etched or over etched. In both cases, their functionality may have deteriorated significantly.
- The volume of etchant applied. This concerns Ferric Chloride etching mostly. In case you sprayed more/less volume of etchant if necessary, it also leads to over-etching and under-etching.
- Bath volume/number of components. This concerns Cupric Chloride etching mostly. Basically, if you place too many components in your bath, it may happen that the corrosion process will not be held evenly, and some PCBs may be under-etched eventually.
- Copper content. This concerns Alkaline Etching mostly. A certain amount of copper in the etchant helps in preventing undercuts as it serves as a banking agent. The recommended copper range for most alkaline etch baths is between 140 gm/liter Cu and 165 gm/liter Cu.
Advantages of Wet PCB Etching
Technically, wet PCB etching can be compared to “dry” etching only, methods of which are detailed above. Yet, both strengths of web PCB etching over laser and plasma ones and benefits of the method overall are listed further:
- High safety of labor if precautions are taken.
- High etching rate.
- Mostly an affordable method.
- Widespread method.
- Comparatively simple in equipment maintenance method.
- Low operation costs.
- It can be conducted in a normal atmospheric environment.
- High selectivity.
- A wide range of materials can be processed.
Disadvantages of Wet PCB Etching
The same concerns weaknesses of wet PCB etching compared to other methods, and generall cons of the process.
- The use of hazardous chemicals is required.
- A risk of wafer defects/contamination.
- It is not an environmentally-friendly process.
- It may be challenging to find an appropriate composition of the etching solution.
- It may be challenging to maintain the terms of the process constant.
Tools and Materials Needed For PCB Etching
Basically, you require the following set of components, tools, materials, environmental conditions, and equipment units to perform wet PCB etching. They are applicable to both industrial and DIY production.
- Rubber gloves and other personal protective equipment.
- Plastic bowl/metal bath/tank for the etching solution.
- Leak detection equipment.
- Pre-prepared solution of Ferric Chloride or Cupric Chloride.
- PCBs with vital copper traces masked.
- Adequate ventilation to not inhale chlorine gas.
- Manufacturers are also obligated to have previously trained personnel, emergency protocols, and approval from the fire department.
In essence, PCB etching is an integral part of crafting PCBs. It is required to remove excess copper from a PCB so it does not prevent electronics from functioning well.
Wet PCB etching is a highly widespread and cost-effective method, based on the use of either CuCl2 and FeCl3 acids or alkaline NH4OH. You need to carefully choose parameters of etching and keep them constant to achieve the highest quality. Now you can consider yourself a pro in wet PCB etching.