FR4 PCBs: Substrates and Stackups

Designing a PCB starts with the circuit, but the days when the substrate material is an afterthought are long gone. Performance rests on the design’s mechanical, electrical, and thermal considerations, and substrates lacking one or more of these areas are likely to exhibit poor quality and reliability over the device’s service life. FR4 is an informal standard for PCB manufacturing – it works well with existing procedures and processes (many of which evolved around it), performs admirably in most settings, and is highly cost-effective. Before deciding whether an FR4 PCB is right for a design, weighing the various parameters a board must meet is necessary.

Why Are FR4 PCBs So Common? The DFM Perspective

Certainly, FR4 is the “default” substrate choice for most circuit board applications due to its wide range of suitability. FR4 is a broad class of materials that fall under flame retardant (hence, FR) woven fiberglass impregnated with an epoxy resin binder that is flame resistant (i.e., self-extinguishing). As the designation of FR4 materials is vast, it’s essential to limit the scope to high-quality substrates that are robust enough to withstand multiple lamination cycles (a necessary characteristic for microvias found in HDI boards) with a high glass transition temperature (Tg) and compatible with lead-free solders for RoHS compliance.

Most manufacturing processes favor FR4 boards in terms of yield and general cost-effectiveness. Comparatively, pure or partial PTFE substrates (used in high-frequency boards where the lossy nature of FR4 becomes apparent) often require specific accommodations to increase yield and product quality. FR4 materials also exhibit excellent reliability, with most defects related to thermal cycling and coefficient of thermal expansion (CTE) differences between the surrounding epoxy-glass matrix and copper plating of via barrels. The CTE (specifically, the z-axis CTE – planar CTE values for FR4 substrates and copper are fairly consistent) for a high-performance PCB should be less than <50 ppm/℃ (i.e., a .005% increase in thickness per degree Celsius). A high Tg (>185℃) will also ensure straightforward processing, as the CTE increases dramatically at temperatures above the Tg.

FR4 Material Characteristics

So far, the focus has been on the characteristics of FR4 materials that lend themselves well to manufacturing. When selecting substrate material , the designer’s first thought is not necessarily how well it behaves for the manufacturer but the electrical properties necessary for the controlled impedance of the transmission lines – the dielectric constant (Dk) and loss tangent (tan(??)/Df). While designers tend to treat these values as static, it’s vital to understand they (like all material properties) have temperature dependence. The TcDk, or temperature-dependent dielectric constant, indicates how much the dielectric constant varies with a change in temperature for boards experiencing extreme temperature cycling during operation.?

Temperature variation concerns can also extend to overall thermal routing that prevents undue temperature buildup that saps performance and accelerates material aging. Aside from special substrate considerations like metal core PCBs (which require additional fabrication adjustments), FR4 presents the highest thermal conductivity substrate. Material hygroscopy is a similar, sometimes overlooked environmental aspect or the tendency to absorb atmospheric moisture. FR4 tends to be the most hygroscopic of available board material classes, which can cause impedance variations and potential damage to sensitive components.

Incorporating FR4 Into Hybrid Stackups

A novel approach to building a printed circuit stackup is individually evaluating the layer needs. In some respects, this is already the case: designers and manufacturers can select from multiple different material thicknesses based on the needs of the via structures and controlled impedance transmission lines . Generally, the material’s characteristics show slight variance – the materials necessary for an HDI, high-power, or RF design will remain constant throughout the stackup. Hybrid multilayer PCBs advance this thought by mixing and matching materials accordingly, so the high-frequency laminates (as an example) are only necessary for the corresponding layers of the stackup.

In hybrid stackups, FR4 is a cost-effective substrate relative to more exotic materials, providing greater reliability during manufacturing and service life operations. With some reservations, hybrid-FR4 PCBs can mix and match laminates more effectively using the FR4 material to “average out” the material properties during manufacturing and operation. Since most manufacturing techniques and technology develop around FR4, manufacturability concerns do not rise considerably in this method. FR4-PTFE hybrids do need to alter the sequence of specific chemical treatments to reduce defects, but this modified format is easily implementable from standard progression.

From a reliability standpoint, FR4 confers multiple advantages. Suppose the electrical characteristics of PTFE (as an example) are not uniform throughout a printed circuit. FR4 improves manufacturing reliability due to a closer CTE match to the deposited and etched copper features, reducing the stress-strain buildup around the copper-substrate junctions that can be responsible for numerous defects.

Your Contract Manufacturer Knows FR4s And More

FR4 PCBs are the bedrock of manufacturing, but design considerations should always be the primary decider of material appropriateness. FR4 is a jack-of-all-trades, doing most things well but not exceptionally so; DFM will want to focus on simulation and the complexity of the stackup to determine whether a different laminate material (or hybrid) will achieve optimal performance. Here at VSE, we’re a team of engineers committed to building electronics for our customers, and that includes a detailed review of the design to ensure the best possible stackup configuration. With our manufacturing partners, we’ve been realizing life-saving and life-changing devices for over forty years.

If?you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards , look no further than VSE. Contact us today to learn more about partnering with us for your next project.