In PCB manufacturing, one of the most important things is the choice of materials. Because the PCB industry has evolved, the materials we use to make them have to follow suit. So, today, various materials are readily available to manufacturers and designers.
But, not all of them have the same characteristics. Depending on the PCB design and intended functionality of the board, the designer has to make their choice of materials very carefully.
Every PCB has four layers. Typically, every board has a substrate part, laminate, a solder mask, and, of course, silkscreen.
But, not all layers are made equal. The types of PCB materials vary significantly for each layer. Depending on what you’re building and what you need it for, you can pick the materials of all four layers and tweak your PCB to make it fit your original design and purpose perfectly.
PCB Material: Lesson One — Materials of All PCB Layers
As mentioned, every PCB has four layers, and they can be made of different materials. Depending on the materials we chose, the board will have specific manufacturability. Also, it will function in a different way. That also means that its life cycle and durability also depend on the materials.
Every structure needs something that will serve as a base. We can’t build something over nothing, after all. In the case of PCBs, substrates play the base role.
Substrates are non-conductive and are made of dielectric materials. Those are usually glass or paper weave and epoxy resin, both of which are non-conductive. That means they can hold the components and other materials that conduct electricity without interfering with their work. Because they are non-conductive, we also call them “insulation materials” in the industry.
As you can see, we used the plural there. That’s because there are quite a few materials we can use for the substrate layer. We usually pick a substrate based on its ability to withstand heat and its electrical permeability (the dielectric constant).
Standard PCB substrate materials are:
- FR-1, FR-2, FR-3, FR-4 (the most common one), FR-5, FR-6
- Polytetrafluoroethylene (PTFE)
- CEM-1, CEM-2, CEM-3, CEM-4, CEM-5
- G-10 and G-11
- Aluminum or IMS (insulated metal substrate)
Depending on the substrate characteristics, we’ll either have a:
- Rigid PCB
- Flexible PCB
- Rigid-flex PCB
Aside from those three, we can also have an FR-4 substrate PCB. FR-4 is the most common PCB material, and it is a readily available substrate on the market. It’s everyone’s go-to because it’s not only versatile but also very affordable.
FR-4 is ideal for low-frequency multi-layer PCB design. It has an excellent strength-to-weight ratio, and that’s what makes it so cost-effective and popular.
It’s important to note that FR-4 is fire resistant. However, although versatile and a perfect fit for PCB prototyping, it’s not an ideal choice for high-density, high-speed boards.
Aluminum-nitrate and Alumina are the most common rigid substrates. Aside from those two, there are quite a few other ceramic-based substrates that will provide the same or similar thermal resistance and conductivity levels.
So, a rigid substrate PCB material is an ideal choice for high-frequency devices where the temperature plays a huge role. They can withstand temperatures of up to 350ºC without expanding or losing functionality.
What’s more, rigid substrate PCBs are strong, durable, and easy to maintain and repair if needed.
PTFE and advanced polyimide make flexible substrates that can bend a little without disrupting the PCB’s functionality.
The main feature of flexible substrates is that they are resistant to corrosion, radiation, and hazardous chemicals. What’s more, being shockproof as well as waterproof is something that comes in handy too.
Due to their characteristics, such as flexibility and waterproof capabilities, flex substrates are ideal for PCBs of devices that are wearable and will be exposed to hazardous environments.
Some would say that rigid-flex substrates are the best of both worlds. That might not necessarily be the case, but they are an ideal choice for lightweight PCBs. Making a lightweight PCB is of the utmost importance in some industries that value minimization over anything else. Furthermore, it also cuts on costs.
Rigid-flex substrates are easy to manufacture and allow for precise building. That’s an excellent characteristic that works in favor of PCBs that go into amplifiers, sensors, antennas, etc.
Laminate materials are responsible for PCBs’ stability and overall functionality. The laminate layer holds all other layers of PCBs together and imprints vital pathways into the substrate.
What does the lamination process look like? Well, we stack internal layers together and then place them under extreme pressure and heat. A photosensitive dry resist laminates the board, and then we slowly release the pressure and let the board cool off.
Laminate materials have several vital characteristics that allow them to make a PCB functional in different ways. Laminate materials can provide different levels of:
- Tensile strength
- Shear strength
- Dielectric constant
- Loss factor
- Glass transition temperature
- Fire resistance
- Change of thickness due to temperature
Usual types of PCB laminate materials are:
We’ve already spoken about FR-4 because it’s the most popular PCB material. It performs well and is highly versatile. When it comes to the characteristics we mentioned in the previous paragraph, FR-4 has stellar marks for each one.
An improved version of the most common material, high-performance FR-4 is a great choice for multi-layer PCBs. Although equally impressive as standard FR-4, it also has some additional perks, such as increased Tg. That means it can withstand higher temperatures without changing shape.
Also, unlike the standard FR-4, high-performance FR-4 has low dielectric properties, which means it is a better fit for high-frequency circuits.
High Tg Epoxy Laminate
As the name suggests, this particular laminate material is ideal for multi-layer PCBs because it has higher heat resistance capabilities and can withstand high levels of moisture and hazardous chemicals.
Aside from being a great fit for multi-layer PCBs, BT Epoxy is also the first choice for designers and manufacturers aiming for lead-free PCBs. BT Epoxy is somewhat superior to other laminate materials because it has outstanding electrical, thermal, and mechanical properties — BT Epoxy scores high on all fronts.
This particular laminate material can go both ways and is a great choice for flexible and rigid-flex, high-density boards. The main advantage of polyimide is extreme environmental stability. That makes it a great choice for advanced PCBs that go into military, automotive, and aerospace devices.
Copper Clad (CCL)
Ideal for high-voltage circuits, CCL affects the transmission speed of the entire board. What’s more, it also prevents excess energy loss and the impedance of the signal in the circuit.
The last laminate material option is the Teflon, which is another material ideal for high-frequency applications. It’s an unconventional material, but it does have specific advantages that make it worthwhile.
Solder masks are the last layer of PCBs. They are the first (or last) line of defense. Their main purpose is to protect the entire board from oxidation. Furthermore, they prevent solder bridges from forming.
Overall, the solder mask layer is vital for the longevity and stability of the entire board. These thin layers of polymer coat entire boards and go around the vias and pads in order to provide optimal protection.
Now, the material used, as mentioned, is a polymer. Generally speaking, all solder masks are made from two different materials:
- Epoxy liquid
- Dry film
Epoxy is the most common choice when it comes to PCB solder masks. This thermosetting polymer is printed onto the board. On the other hand, dry film materials come in thin sheets of film and are vacuumed onto the board.
A Few Parting Words — What to Consider While Choosing the Material
Picking the right materials for a specific PCB design can make or break an entire project. Each layer of PCBs can be of a specific material, and we make a choice based on our boards’ intended purpose. Some boards require highly specific materials of substrate, laminate, and solder mask layers, while others only need one layer of specific material.
The best way to pick a PCB material is to pay attention to its:
Thermal properties — Because some boards need to be able to function properly under high temperatures, we need to make a smart choice when it comes to PCB material selection. We should go for ones that have high levels of:
- Thermal conductivity
- Decomposition temperature
- Glass transition temperature
- Thermal expansion
Electrical properties — PBCs are, of course, electrical devices. That is why it’s vital that every PCB material we use has excellent electrical properties such as:
- Electrical strength
- Volume resistivity
- Surface resistivity
- Low loss tangent
Don’t Know Which PCB Material to Pick? Ask the Knowledgeable Staff of MKTPCB!
Because there are a lot of materials available out there, it’s somewhat understandable not to know all their advantages by heart. Luckily, you don’t have to!
Here at MKTPCB, we can help you pick the best materials for your project. With over ten years of experience, we not only know all their properties by heart, but we can also advise on which materials fit together and how to make the right choice for your PCB project. So, don’t hesitate to contact us!