Imagine this: you're an expert in PCB manufacturing, and discussing this topic won't bother you at all. Well, it is possible, but only if you have extensive knowledge of PCB components and materials.
Chances are, the posts you've read on the Internet are full of conflicting information. Do you need something complete and clear? Just stick around here!
The PCB is essentially a layered structure consisting of a laminate substrate, copper, solder mask, silkscreen, and additional finishes. The following are the steps involved in the production of a PCB.
Prepreg
The main substrate of a PCB is a layer of glass fabric on which other materials can be deposited. Prepreg is a substrate coated with resin or fiberglass that holds the epoxy in place. The types of substrates are FR4, polyimide, Teflon, etc. The final stage of substrate preparation is drying in a prepreg machine. Once the liquid epoxy has covered the glass, it is cooled to a heat-setting point where it hardens again. The result is a rigid and thin material with a special weight-to-strength ratio.
Laminate
Copper Clad Laminates are made by combining a sheet of prepreg with a thin layer of copper foil. The layers are bonded together under heat and pressure, basically called lamination in the original sense of the word. A manufacturer may require a more advanced or robust PCB, and in this case, they can laminate several prepreg layers with copper foil or even put additional layers of dielectric and adhesive in between. For example, such PCBs are used in electronics for 5G communication infrastructure.
Copper Foil
Copper is known for its excellent electrical conductivity and is the material used for PCBs. Two types of copper foil are fully compatible with any PCB - electrodeposited (ED) and rolled annealed (RA) copper foil. The copper layer is etched from the laminate to leave tiny traces that form the circuit pattern. Specially designed trace patterns will connect the board's components and help it interact with other electronic devices.
Solder Mask
Solder mask is a coating made of polymer resin and applied to the surface of the PCB to prevent PCB degradation. There are different types of solder masks available, such as top and bottom solder mask, epoxy liquid solder mask, liquid photoimaging (LPI) ink solder mask, and dry film photoimaging solder mask. The main function of the solder mask is to ensure that the copper will not deteriorate due to moisture and oxidation. Additionally, the solder mask may have additional properties such as better resistance, enhanced conductivity, etc.
Nomenclature
Silkscreen, white legend or nomenclature is the outermost layer of a PCB consisting of text, letters or numbers printed on the surface of the component. This layer contains information about the PCB version, identification number and warning symbols. It also indicates where the text points to, the reference designator, and the polarity of the part. The information indicated in the form of a white legend is absolutely necessary to manually assemble or test a PCB. The material for this layer is a non-conductive epoxy or acrylic ink, which is heat-treated in an oven.
When it comes to PCB manufacturing, there are various materials that can be used, but we will focus on the core layers or substrates that are commonly used in modern electronics.
FR4 epoxy resin is a fiberglass or epoxy laminate that serves as the substrate for most current PCBs. It is flame retardant, lightweight, moderately rigid, waterproof, and has good mechanical strength. Its low thickness ensures that it is compatible with most microelectronics, and it is considered a low-cost substrate. FR4 epoxy is suitable for most final finishes, all welding methods, and any screen printing. It withstands fluctuating environmental conditions well and has good insulating properties, making it the basic choice for most manufacturers unless specific requirements demand another material.
Polytetrafluoroethylene, or Teflon, is another polymer material used as a PCB substrate. It has moderate flexibility and a superior weight-to-strength ratio. It also has excellent temperature fluctuation (-190~260℃) and chemical resistance. PTFE laminates are fairly expensive substrates, but they have unique features. Teflon naturally transmits high frequency signals, making it the only substrate that can be effectively used for PCBs in electronics that support 5G signals.
Polyimide (PI) is a flexible polymer that is used in Kapton film, a popular substrate for its high performance, high weight-to-strength ratio, good durability, and reliability. It also has excellent thermal stability, heat resistance, high electrical conductivity, and excellent chemical resistance. This makes it a versatile all-in-one option that serves the purpose of manufacturers who cannot predict all conditions in a work environment.
Flexible substrates, on the other hand, provide no additional rigidity but can bend to a certain degree. This feature makes them ideal for microelectronics where free space is strictly limited or for equipment that is expected to handle high pressures and temperatures in its operating environment, such as medical electronics, LED lighting, and automotive parts. Flexible substrates are made from polyethylene terephthalate (PET), a strong, lightweight plastic that is also used to create bottles for low-cost beverages. PET-based PCBs offer higher reliability, better aesthetics, and are easier to install and repair.
Finally, there are traditional PCBs made from various metals such as copper, iron, alloy steel, and aluminum. While they are not obsolete, they have no advantages over other materials besides high rigidity, mechanical durability, and long shelf life. Their performance is relatively low, and their range of applications is severely limited.
Dimensional Tolerances, Stability, and Flexibility:
When it comes to PCB design, dimensional tolerances, stability, and flexibility are crucial factors that affect the dimensions of the electronic components that can be plugged into the PCB. Microelectronics, in particular, requires circuit boards that fit into tight spaces and have sufficient dimensional stability. The substrate material should also be stable under vibration, shock, and other external factors. Even the slightest change in surface dimensions can cause microelectronic devices to fail. Additionally, a certain degree of flexibility is required to allow the PCB to bend to fit the operating space.
Resistance:
The environmental conditions under which the PCB will operate should be taken into consideration when choosing the substrate material. The material should be able to withstand high or low temperatures, biological contamination, metal contamination, chemicals, high pressure, excessive humidity, vibration, and mechanical stress. It is important to choose a material that can provide the necessary resistance to operate under these conditions. It is also recommended to look for a combination of substrate and surface finish, as proper post-processing can cover weak spots in the desired core layer.
Specific Attributes:
The thermal and electrical conductivity, as well as mechanical properties, are important specific attributes to consider. The chosen material should transmit electrical signals well under certain conditions to ensure the PCB functions properly. Measuring the dielectric constant (DK) of the substrate can help determine how much electrical energy the material can transfer. The substrate should have a low Dk value to act as reliable insulation for copper traces. Additionally, the Dk value should be kept as constant as possible to ensure continuous operation. As for mechanical properties, the substrate should be strong and durable enough to withstand normal operating conditions in the intended work environment.
Specific Requirements:
Specific requirements may arise in cases where 5G supports electronics or requires highly rigid metal substrates. In some cases, the options for substrate materials may be limited to only one option. Therefore, it is important to carefully consider the specifics of your PCB design and the type of electronics it will be used for.
In conclusion, when choosing the best substrate material for your PCB design, it is important to consider the dimensional tolerances, stability, flexibility, resistance, specific attributes, and requirements. Careful consideration of each factor can help ensure the proper functioning of your electronics.