Introduction
PCB Circuit Boards incorporate a number of materials of various types, shapes, and sizes. These materials come together so as to help the circuit boar perform its functions to the highest accurate degree. It’s only logical that the materials you use will affect the performance and durability of your PCB.
Here we’ll be discussing PCB ‘substrate’ materials, their types, and how to choose the best one for you.
What is a PCB Substrate?
The PCB substrate in a PCB is the base material, the physical component that holds all the traces and components. A good quality PCB depends on the type of substrate material you use. A good PCB would have a solid durable structure and the ability to hold its various components.
The PCB industry continues to evolve and with it we saw the evolution of substrates used in printed circuit boards. The most common type is FR-4, a fiberglass-epoxy laminate, low-cost, provides good insulation between copper layers, and helps with signal integrity. You will also find substrates made from polymers and ceramics. 
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Types of Substrates:
There are various types and variations of substrates but we’ll be discussing the 5 most common types. These include:
- FR-2: FR stands for “flame resistant”. This type of substrate is made up of synthetic resin bonded paper, composed of paper saturated with a plasticized phenol-formaldehyde resin. This type is usually used in inexpensive electronics and consumer products. Looking at its properties, it is not as good as FR-4 but is significantly cheaper. It is better suited for simpler applications.
- FR-4: This is the most commonly used substrate type. FR-4 is a glass-reinforced epoxy laminate. It is composed of woven fiberglass material imposed with a flame-resistant epoxy resin binder. This type of substrate is much stronger as durable as compared to FR-2. While FR-2 is used in single-layer PCBs, FR-4 is more widely used as it is ideal for single and multi-layer PCBs. This is why they are used in higher-end electronics and devices.
- RF: RF or microwave substrates are made of thermoplastics, thermoset resins, or PTFE (polytetrafluoroethylene), a “fluoropolymer” with a low coefficient of friction and exceptional dielectric properties. Mechanically it is not the best but it shows excellent electrical performance.
- Flex: Circuit boards are both flexible and rigid, flex circuits are designed to be more flexible. The substrate used in this case is flex substrate, made of polymers like polyimide and PET (Polyethylene Terephthalate). Flexible circuits are used in places where rigid circuits will not perform well.
- Metal: Metalcore or IMS (Insulated Metallic Substrate) PCBs are an alternative to standard circuit boards. They are used for PCBs that would be exposed to larger mechanical loads, or dimensional instability, or in PCBs where large amounts of heat must be conducted away from sensitive components. A ceramic core or metalcore substrate will help here.
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Choosing the right substrate for your PCB:
As mentioned before, the material you use affects the quality and performance of your PCB. PCBs are now created to be more thin, flexible yet durable. So what kind of substrate do they require? Polyimide and Mylar substrates offer greater flexibility, elasticity, and environmental resistance to meet these new evolving requirements. 
Materials commonly used in substrates include those mentioned above and FR-1, PTDFE, G-10, etc. Usually, if your PCB is more focused on lowering costs and versatility of use then you can use lightweight polyester. Lightweight polyesters will give your PCB flexibility but it will diminish the quality of performance, and so it is recommended only if you wish for lower costs.
For traditional non-flex PCBs what sort of substrate would be more suitable? It depends on the board’s requirements and the board’s application. For example, you may need to consider the frequency level requirements, will it be able to accommodate high power/temperature, what density or complexity will the board have?
The amount of heat a PCB can resist before losing its ability to perform well is its Tg (glass transition temperature). In this case, the heat resistance needs to be higher than the subjected heat. High-performance PCBs usually have a Tg of 170°C while more standard PCBs center around 130°C. Standard PCBs usually use FR-4 as we’ve mentioned before, its Tg point is around 135°C. PTFE is also a good choice for higher frequency, high power boards having a Tg ranging from 160-280°C.
Polyimide used for more flexible circuit boards offers a Tg of around 250°C, making it extremely resistant to higher temperatures. Polyimide will however be more costly than FR-4.
At the end of the day, you want to make sure you study all the details, do your research, and see which material would be better suited to the job. Choosing poor materials will lead to a poorly designed PCB regardless of how talented a manufacturer you hire. 
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