How to Choose the Right Materials for Your Printed Circuit Board

Introduction

Fabricating a product with optimized commercial success is paramount, dependent on the choice of base materials employed to manufacture it. The decided-upon manufacturing substances have a substantial impact on the overall performance, longevity, and sustainability of products. For a product to survive in the market in the long run, it is mandatory to fulfill promises of excellence in order to gain the trust of customers. 

The same rules are upheld with the PCB boards and PCB board materials used in manufacturing. PCB board is a core and basic electronic component used in different dimensions of electronics. PCB use ranges from experts to school-going students for different ideas of prototyping, innovations, and even for school-related projects. In advance of using any PCB board, it is imperative to check all the PCB board manufacturing materials involved in the production of the product so the final products do not collapse or malfunction. To be safe from any dupes or doubles, concurrently essential is the manufacturer and supplier company of PCB boards from where a board is bought. To make your job as a PCB circuit board material selector easier, here is a PCB board material guide.

Types of PCB boards

 In order to be able to select the best PCB material as per the needs and requirements of the project, it is a pre-requisite to know what the different types of PCB boards and layers present in different PCB boards are. PCB has many types- Rigid, flexible, and rigid-flex. In turn, it has further types. Let’s first brush over the definitions of these types-

1. Rigid PCBs are those boards whose inner core is unbendable and very stiff. PCB board materials with high rigidity are opted for making this. The most common type of circuit board core materials used for this are types of flame retardant, and the most commonly used is FR-4 due to its high fire resistance in comparison to its other counterparts, such as FR1, FR2, FR3, etc.
2. Flexible PCBs are those boards that are made for flexibility and folding properties in order to fit the boards in places where rigid materials cannot be installed. However, flexibility in these boards comes at a cost, i.e., the core strength of the PCB board is lost. The most common types of PCB material used for this are polyimides (PI).
3. Rigid-Flex PCB: These are actually the hybrid models of rigid and flexible-type PCB boards. These boards possess layers of both rigid and flexible PCB boards. This is actually a very smart way to make sure the PCB board has strength as well as flexibility. The PCB materials used are the same as those used in typical rigid and flexible types.

PCB Board Layers

Regardless of the type of PCB boards, PCB boards feature different layers. These are:

1. Silkscreen layers: Each PCB board has a top silkscreen layer and a bottom silkscreen layer. These are the final layers applied to PCB boards. Annexing these layers to an almost ready and new PCB board is crucial to include all types of written data such as ratings, company logos, symbols, trademarks, or any other information graphics that need to be printed on PCB as the whole thing is done on silkscreen layer coatings.

2. Solder mask layers: These layers are implanted over the top and bottom copper layers with the aim of maintaining their safety against dust, corrosion, oxidation, corrosion scabbing of copper traces, and isolating the conducting paths. It is absolutely indispensable to use these additional layers over the copper layers.

3. Copper layers: Copper layers are the conducting layers present in PCB. PCB is a non-conductive board responsible for having required pathways for the conduction of current from one point to another. Copper is the duty-bound element that makes it possible. Copper is preferred over other conducting metals as a PCB board manufacturing material owing to its high conductivity, availability, solderability, cost, and easy-processing features.

4. Core layers: Core layers form the foundation structure of a PCB circuit board on which all the other layers rely. Core layers and pre-preg layers are considered the same in the sense that both use the same PCB materials on a molecular level. However, differences arise between the two during the manufacturing process. Core layers are made of core material reinforced with glass fabric, resulting in glass cloth, then glass cloth is bonded with epoxy resin and then cured as a whole unit, unlike pre-preg layers where epoxy resin is cured at a later stage. Core materials used vary in different PCB types; rigid PCBs make use of FR materials for stiffness, and flexible PCBs use PI materials for flexibility. Rigid-flex PCB types consist of a combination of PI and FR materials to allow different properties for different regions on a single PCB board. Once this core layer stack is cured, copper foils are applied on either side and if required, pre-preg layers are used between the core layer stack and copper foil layers.

5. Pre-preg layers: These layers act as inner insulation of the PCB board and do not allow short circuits between core PCB layers. Pre-preg layers are multi-functional in nature as they additionally bind the core materials or two different cores with each other when exposed to heat and pressure during the lamination process, behaving like an adhesive or glue, thus resulting in a rigid solid structure. Pre-preg materials are usually B-stage materials and consist of fiberglass, which can be tight or loose as per the needs and requirements, impregnated with uncured epoxy resin. Resin is cured during the lamination process of PCB assembly. 

Properties of different PCB Materials

It is noteworthy to mention that all PCB core materials are not compatible with all types of pre-preg materials due to varying di-electric constants between the two. Not only dielectric constant but there are a few more parameters that need to be considered while choosing PCB circuit board materials for core and pre-preg layers and are discussed as follows:

1. Electrical Properties:

1. Di-electric constant (D_K): The value of the dielectric constant is very important for upholding signal strength and for impedance matching. Pre-preg and core vary in di-electric constant values due to the delayed curing process of epoxy in pre-preg. Different di-electric constants make it difficult to calculate losses in PCB boards. The desired value of dielectric constant in PCB is between 2.5 and 4.5.
2. Di-electric loss (D_F):  Di-electric loss of the core has an impact on signal loss in PCB while dealing with radio and microwave frequency signals.
3. Resistivity: It is of critical importance to make sure the resistivity of both core and pre-preg materials is high because PCB can sometimes be subjected to large currents while testing different modules or devices; it should be able to have proper insulation in order not to cause any accidents or mishaps.

2. Thermal Properties:

1. Thermal conductivity: It defines the heat dissipation power of the PCB materials used in core and pre-preg. Its value should be high, typically ranging from 0.3-1.5 W/m.k.
2. Coefficient of thermal expansion: It measures the extent to which a substrate can expand once it is heated up. If the value rises to a large value, it can cause intercommunication problems.

3. Mechanical Properties:

1. Tensile strength: Tensile strength determines the force a PCB can endure. It should be as high as possible in both core and pre-preg layers. The value of tensile strength is responsible for making a PCB stiff and strong.
2. Elongation breakpoint: It is relevant in the case of flexible core PCB and shapes its flexibility power. Ideally, its value should lie between 2% and 5%.

4. Cost and processing feasibility: 

Striking a balance between performance parameters and the cost of PCB board materials is very necessary so that the public can afford it. Raising the quality of PCB comes at a cost, but it shouldn’t be sky-rocketingly high, which students can’t even afford. Finding a trade-off between quality and cost becomes the challenge post-manufacturing of PCB.

5. Environmental factors: 

Before starting the process of production of a PCB board, it is duty-bound to check the purpose it will be serving. Different conditions demand different parameters, such as high-temperature conditions, bending surfaces, cool temperatures, high mechanical stresses, pressure, etc. By aligning all the environmental conditions with PCB material properties, a good quality PCB can be produced.

6. Safety parameters: 

Safety parameters refer to the fire resistivity and chemical resistance of a PCB board. Usually, rigid core flame retardant materials are used to be safe from fire accidents. Similarly, a PCB board should be able to withstand chemically unsafe conditions. That is why flame ratings and chemical resistance values should be high for a PCB circuit board material.

Conclusion

Generally speaking, PCB material is very critical to producing a printed circuit board that is well-functional and robust. So, PCB board material should be chosen according to the layers, type, electrical properties, thermal properties, mechanical properties, cost and processing feasibility, environmental factors, safety parameters., etc.