Picture this: you are an expert in PCB fabrication, and discussing this topic does not bother you at all. Well, it is possible, but only if your knowledge of PCBs’ components and materials is extensive.
Most likely, posts you have already read on the Internet are full of contradictory information. Do you need something complete and clear? Just stick with us here!
So let’s get down to business. Any PCB is basically a “sandwich” made out of the laminated substrate, copper, solder mask, and silkscreen layers + extra finish. Let’s split all of these into clear steps, listed in the correct order, and digest them one by one.
The primary PCB substrate is basically the glass fabric layer that can have other materials deposited on it. In its turn, the prepreg is the substrate coated with resin – or fiberglass. Basically, it is the glass fabric that holds the epoxy in place rather than the other way around. Types of substrates are FR4, polyimide, Teflon, and some others.
But we’re jumping ahead. We will deepen into PCB materials in the second part of the 2022 guide.
The last stage of substrate preparation is drying in prepreg treaters. Basically, these equipment units gave the basic composite layer its name. In case you are curious, the resin begins to melt at a temperature above 150°C. Once the liquid epoxy has covered the glass, it is cooled to a thermosetting point where it re-hardens.
As a result of the described process, manufacturers obtain a rigid, thin material with an exceptional weight to strength ratio. By the way, fiberglass is not intended for PCBs only. It is widely used in recreational craft and aircraft. In case you didn’t know, wind turbine blades are mostly fiberglass.
I will certainly not be mistaken if I say that you think of laminating as covering the substrate with another material, like plastic or so. You would also add that laminating is needed for PCB protection. And you’d be right, but it is not that simple this time.
Copper-clad laminates are the result of composing a sheet of prepreg with thin layers of copper foil. These layers are bonded together with heat and pressure, and it’s basically called laminating in the original sense of the word. Take a look at what it is supposed to be like in the image below.
Sometimes, manufacturers may need more advanced or just stronger PCBs. In this case, they may laminate several prepreg layers with copper foil. Or, even put additional dielectric and adhesive layers between those and even some more prepreg on the copper. For example, such PCBs are used in electronics for 5G communication infrastructure.
Well, there is not much to add about this foil. Copper is one of a few metals that actually occur in nature in minerals. It is known for its great conductive properties. Or, in other words, it transfers electrical signals without losing or deteriorating them.
Two types of copper foil are fully compatible with any PCBs – electrodeposited (ED) and rolled-annealed (RA) ones. 99.7% pure copper foil with 1µm to 210µm thickness is what fits PCBs the best.
What’s more interesting about the copper layer is the way it is etched from the laminate. Basically, you need to leave the tiny traces that form circuit patterns on a PCB. Specifically designed patterns of traces will connect a board’s components and help it interact with other pieces of electronics.
You may learn more about the etching process in PCB Manufacturing from our previous guide.
Do you remember that we were talking about the PCB protection in the laminate section? It was not the case back then, but the time to discuss how PCB deterioration is prevented has come.
A solder mask is simply a coating made of polymer resin and applied onto a PCB’s surfaces. Let’s have a brief look at the types available:
All of these are mostly epoxy or resin, mixed with ink of any color. Typically, a green one. Liquid solder masks can be printed or coated. Aerosol masks are spray-coated only. And dry films are applied and vacuumed. All the solder masks need post-treatment with UV light to fully solidify.
The main purpose of the solder mask is to ensure that the copper will not deteriorate shortly because of moisture and oxidation. Or, in other words, that the copper will not rust a week later after a PCB was fabricated.
Additionally, solder masks may have extra properties such as better resistance, enhanced conductivity, or others. Soldering may enhance PCB’s final finish as well.
You may learn more about the soldering process in PCB Manufacturing from our previous guide.
Silkscreening, white legend, or nomenclature. These are some of the terms used to describe the outermost layer of PCB. It looks like text, letters, or numbers printed onto a component’s surface. Let’s be honest; it is not the most functional layer. But still, hardly any PCB can go without it.
The truth is, technicians, engineers, and electronics enthusiasts greatly benefit from the presence of a silkscreen layer. It contains information about PCBs version, identification number, and warning symbols. The layer also indicates where the text point, reference designators, and what is the polarity of parts.
As you may have understood, information indicated in the form of the white legend is absolutely essential for manual assembly or testing a PCB. It is likely that any repair shop will not even attempt to fix a complex PCB that does not have a silkscreen layer.
As for methods of silk screen printing, they are the following:
The methods differ in the quality of text they support and the labor involvement.
The material of the layer is non-conductive, epoxy or acrylic ink. It is pretty much the same as the solder mask. But it does not have additives that would ensure PCB substrate protection as it is not designed for it. It is treated with heat in the oven rather than exposed to UV light to merge with the substrate and solidify.
You may learn more about the silkscreening process in PCB Manufacturing from our previous guide.
After all, many manufacturers want their electronic components to last for a long time. That is the need that the final finish fulfills.
The first way to ensure that a PCB will be operating for more than a few weeks is to protect the copper surface additionally. Solder masks are great, but they still lack certain rigidity to help electronics sustain metal contaminants and environmental fluctuations. So, the surface finish serves as a barrier against the external stresses.
As a bonus, the final finish impacts the number of thermal cycles. Or in other words, it defines the extent to which a PCB is resistant to thermal shocks like pre-heating and instant cooling. Without this feature, a PCB would collapse much faster.
The second great thing about the final finish is that it benefits both the functionality and the appearance of a PCB. For example, it helps to prevent solder bridges that connect wrong PCB components and make electronics malfunction. On the other hand, some finishes add a visually attractive, shiny look for PCBs. Such feature benefits devices in which circuit boards are cases’ parts.
Let’s take a look at some of the available PCB finishes:
The range of all the materials that are potentially used in the PCB fabrication is vast. So let’s stick to the ones that serve as core layers or substrates in modern electronics.
We have used this one as a benchmark from the early beginning of the post. It is reinforced fiberglass or epoxy resin laminate that serves the substrate for a majority of PCBs in the current period.
FR4 is flame retardant, lightweight, moderately rigid, water-resistant, and has good mechanical strength, even compared to metals. Its low thickness ensures that FR4-based PCBs will be compatible with most microelectronics. As a bonus, it is considered a low-cost substrate.
FR4 epoxy is suitable for most final finishes, all the methods of soldering, and any silk screening. It sustains fluctuating environmental conditions well and has good insulation properties. FR4 is a basic choice for most manufacturers unless specific requirements to a component do not dictate choosing another material.
Polytetrafluoroethylene, or simply Teflon, is another polymer material serving as a PCB substrate. It has moderate flexibility and a superior weight to strength ratio. It also provides exceptional temperature fluctuation (-190~260℃) and chemical resistance.
PTFE laminates are rather expensive substrates, but they have a unique capability. Teflon naturally can transmit signals at high frequencies. While this does not sound very impressive, it is basically the only substrate that can be effectively used in PCBs for electronics that support 5G signal.
As a matter of fact, PTFE substrates have a specific niche of use, so there may be cases when Teflon is your sole option.
This imide polymer can basically be classified as a flexible one. But since the next material is way more popular in this category, let’s discuss PI separately.
You may have heard about the Kapton film that became indeed widespread. It is made of Polyimide. This material has a unique combination of properties, including high performance, great weight to strength ratio, good durability, and reliability. What’s even more interesting is that it offers great thermal stability, thermal resistance, high electrical conductivity, and excellent chemical resistance.
Such a blend of features makes Kapron film another versatile all-in-one choice. It fits the purposes of manufacturers that cannot predict all the conditions in the working environment.
As evident from its name, flexible substrates do not provide extra rigidity. Instead, they can be bent to a certain extent.
Due to this feature, PCBs may be inserted in microelectronics where free space is strictly limited. Or just in the devices that are expected to handle high stresses and temperatures in their working environments. It includes electronics for medical use, LED lighting, automobile components, and many more.
Let me take a wild guess: you think such substrates are highly advanced and difficult in production. But the truth? This is about ordinary polyethylene terephthalate (PET) – strong, lightweight plastic. It can easily be used to fabricate bottles for low-cost beverages. But it is also a core material in the production of flexible PCBs.
Besides their flexibility and heat resistance, PET-based PCBs have increased reliability, improved aesthetics, and are easy to install and repair.
Well, all that’s left are different metals that were widely used in the production of conventional PCBs. It includes copper, iron, steel alloys, aluminum, and others. It is not like they are completely obsolete, but they really do not have advantages over other materials but high rigidity, great mechanical durability, and good shelf life.
The drawback – their performance is relatively low, and the scope of application is strictly limited.
Well, we have already given you some clues while describing the types of substrate materials. Buts let’s run through the exact factors to consider while choosing the best PCB material for your design:
These characteristics are totally different from each other. But in the context of PCB design, they basically regulate the same aspect – the size of a piece of electronics in which a PCB can be inserted.
Microelectronics require circuit boards to fit into tight spaces. It includes sufficient dimensional stability, i.e., the capability of the substrate not to shrink or expand when a temperature lowers or rises respectively. Also, you should pay attention to the material stability under vibrations, shocks, and other impacting factors. Even the smallest change in the surface dimensions may cause a collapse of microelectronics.
On the other hand, a certain degree of flexibility naturally ensures that a PCB bends to fit in the operational space better.
This one is a no-brainer. You need to define what are the environmental conditions your PCBs will operate in. High or low temperatures, biological pollutants, metallic contaminants, chemicals, high pressure, excessive humidity, vibrations, mechanical stresses, everything at once. Guide your choice of material based on the resistance it can provide.
We also recommend looking for combinations between the substrate and surface finish, as propper post-processing may cover some weak points of the desired core layer.
These mostly concern thermal and electrical conductivity and mechanical properties. Perhaps unsurprisingly, if the chosen material failed to transfer electrical signals well under specific conditions, your PCB would not function at all.
Consider measuring a dielectric constant (DK) of a substrate. It will help you determine the amount of electrical energy that the material can transfer. The substrate should have a low Dk value to serve as reliable insulation for the copper traces. Additionally, the Dk value should be maintained as constant as possible to ensure continuous operation.
As for mechanical properties, the cases when a PCB did not sustain external stresses are not unheard of. As a rule of thumb, the substrate should be durable and strong enough to not collapse at normal operating conditions in the intended working environment. Intentional or accidental breaking electronics with a PCB does not count.
These include such cases as with 5G supporting electronics, or the need for highly rigid, metallic substrates. Under some circumstances, your options may be limited to the only choice. So be sure to carefully consider the specificities of both your PCB design and the type of electronics it is intended for.
In essence, you have a piece of electronics comprising several layers. The right combination of substrate materials, additional layers, copper foils, laminating conditions, types and methods of soldering, methods of silkscreening, and final finishes – is what makes a competitive PCB.
You should guide your decision regarding circuit boards’ materials based on the type of electronics and impacting factors. Refer to this guide on any matter, so your PCB fabrication is evidence-based.
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