FR-4 is technically woven fiberglass-reinforced sheets, which were epoxy laminated. For you to not be mistaken, FR-4 is neither core nor a prepreg as it has not been copper foil plated and laminated with other layers yet. So this material is the most basic, rigid layer intended for further processing – substrate. Do not confuse it with FR-4-based cores.
FR-4 is a standard rating by National Electrical Manufacturers Association) (NEMA). FR – is simply flame retardant here, which indicates its compliance with the UL94V-0 standard about the inflammability of plastic materials. And 4 – the exact type in the category of epoxy reinforced materials. It means that the laminate is woven glass fiber-based.
The popularity of FR-4 is well-founded on the following significant advantages:
The detailed properties may not be as impressive as ones of high-cost PCBs. But still, FR-4 offers a high cost to performance ratio making it popular among manufacturers of low and medium-cost electronics worldwide.
Speaking about other materials in the epoxy reinforced category, there are several types:
Despite all the above-mentioned strengths, FR-4 has substantial limitations, essential of which are the following:
Primarily, FR-4 serves as the base of any PCB. It is a foundation material for production, and it is laminated/plated with thin copper foil layers under high temperatures and pressure. Depending on the PCBs purpose, one or both sides of FR-4 sheet material can be laminated. This copper will be etched further to expose pads and routes – electrical connections. So most of FR-4 will be left uncovered.
FR-4 is the most rigid layer that defines most of the mechanical characteristics of PCBs, like its ability to bend and withstand external stresses, tensile strength, durability, etc. Prepregs can comprise one or multiple sheets of FR-4 mixed with insulation layers, copper foil, etc.
FR-4 is nearly the most popular material, but definitely not the only choice. Let’s compare it to two alternative laminates:
|Extremely versatile, but is a good choice for most of the low and medium-cost electronics only
|Nearly the only choice for high radio frequency applications and is a great choice for most expensive electronics
|Moderately versatile, and is a good choice for some specific types of low and medium-cost electronics
|Complexity of production
|Difficult in production and can rarely be fixed. Is expensive to rework
|Difficult in production, and can rarely be fixed. It is expensive to rework
|Mostly simply in production and can rarely be fixed. It is expensive to rework
|Has superior chemical resistance to acids, solvents, and petroleum products. Moderate corrosion resistance, depending on the finish. Is wear-resistant but not hydrophobic
|Has superior chemical resistance to acids, solvents, and petroleum products. High corrosion resistance
|Chemically inert, and cannot be damaged by most solvents and organic and inorganic acids. High corrosion resistance. Is wear-resistant and hydrophobic
|Impedance dielectric constant equals 4.5. The dissipation factor is not that low. Conductivity is moderate
|The dielectric constant can vary in the range 2.5 to 11 making impedance stability adjustable. The dissipation factor is low. Conductivity is high
|The dissipation factor is low making it more energy-efficient. Conductivity is moderate, and the dielectric constant equals 2.1
|Exceptionally durable and strong but not lightweight. Withstand external stresses
|Offers a great strength-to-weight ratio. It is a high-density material and is not intended to be bent or withstand high external stresses
|Durable and strong, but still can be bent and flexed even at low temperatures. Withstand external stresses
|Moderate thermal management variation. Slightly expanded, but suits most electronics that operate in a wide array of environments overall.
|Low thermal management variation. Does not expand, but does not suit electronics that operate in high-temperature environments overall.
|Moderate thermal management variation. Slightly expands but suits some electronics that operate in high-temperature environments overall
|Scope of application
|Most of the widespread low and medium-cost electronics. Does not suit any radio frequency applications
|High-cost and technologically advanced electronics. High radio frequency applications
|Low and medium-cost electronics that are intended for multiple operating environments. Some simple radio frequency applications
|Comparatively low-cost material
|One of the most expensive materials
|Comparatively affordable materials
Besides the above-mentioned, you may be offered the following types of laminates/substrates, which are not in the FR category, to use for your PCB productions: RF-35 Taconic material, which is a ceramic one and performs well for microwaves only. And Polymide substrate, which is a middle ground between Rogers and Teflon in the matter of their properties, applications, and costs.
Perhaps you are stuck with choosing a more suitable material from the epoxy reinforced category. Or not sure which brand offers a better FR-4. Either way, you may find these tips and advice to be useful:
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