The ubiquitous FR-4 printed circuit (PC) board has served us well for decades, but laminates with better and more-stable RF parameters are needed for the multi-GHz products now being designed for the multi-GHz spectrum, such as 5G.
In the world of printed circuit boards (PCBs or PC boards), FR-4 has been the “go to” substrate for decades. (There was an industry attempt to call them printed wiring boards—PW boards or PWBs—but that just never caught on.) This fire-resistant material (hence the “FR” designation) is composed of a matt of fiberglass and epoxy, is usually green in color (Figure 1), and is available in many versions to suit the diverse needs of the design objectives.
As the literal and figurative foundation of so many electronic products, PC boards have industry standards and standards-setting organizations such as NEMA and IPC, a global trade association for the printed-board and electronics-assembly industries, their customers and suppliers. Interestingly, IPC was founded in 1957 as the Institute for Printed Circuits, then changed the name to Institute for Interconnecting and Packaging Electronic Circuits, and then changed it to the more-anonymous IPC. (This is similar to the action of the Institute for Electrical and Electronic Engineering, which legally changed its name to “IEEE” and IBM which is the corporate name and not a nickname for what was previously known as International Business Machines.)
I won’t go into the details of FR-4 and its defining standards, as there are good online sources (including at Wikipedia [Refs. 1-3]). Due to the wide use of this substrate, it is available in standard thickness of 0.031 in (0.78 mm ), 0.062 in (1.57 mm) and 0.093 in (2.36 mm). The copper cladding can be on side only side or both sides, and the standard thickness of laminated cladding of lamination is usually 1 ounce copper/square foot of board or 35 µm (called “1 ounce” in the US); two and three-ounce is also in use as well as the thinner half-ounce laminate. Some PC boards do not use pre-clad boards but instead use an additive copper process, where copper is deposited where needed rather than chemical- or laser-etched away where it is not needed; each approach has electrical, cost, trace density/thickness, and other tradeoffs.
In many designs, multiple PCB layers are stacked to support ever-more complex and dense interconnects, and there are boards with 20 or more layers. Two sided and multilayer boards have plated-through holes connecting the various layers to provide both electrical and thermal conductivity. These vias (formal name is “vertical interconnect access”) have their own variations: they can be through hole, buried, or blind. The latter connects internal layers only and are a troubleshooting and repair nightmare.
FR-4 material is rugged, has fairly good electrical and mechanical specifications, and those numbers are reasonably stable over time and temperature. It does, however, dull and wear drill bits and cutting shears quickly, because it is fairly hard and abrasive (and the glass splinters you get from handling the edges are brutal).
In the days of through-hole components, before surface-mount technology (SMT) ICs took over the PC board world, a typical board could have hundreds of holes, so this was a significant consideration; fabricated PC boards were often priced primarily by their size and number of holes. The near-universal use of SMT devices has greatly reduced the number of holes needed to a just few as needed for mounting screws, larger components, some connectors, and other unique attachments.
Many engineers improvise small, custom enclosures of cladded FR-4, soldering the seams from one end to the other for full shielding of an entire circuit or a sub-circuit; the late Jim Williams and Bob Pease both show this technique in many of their prototypes. It’s uses go beyond electronics: I once cut two U-shaped pieces of FR-4 and sewed them as stiffeners into the handles of a family heirloom leather briefcase, Figure 2; I don’t want to do that again, ever!
FR-4 was not the first PCB material. Boards made of pressed phenolic-cotton paper, designated as FR-2, were the bridge between discrete-wired, hand-soldered circuits and FR-4.(Readers of a certain age may recall Magnavox ads which boasted that their TVs were carefully hand soldered, rather than being made by anonymous soldering on PC boards—talk about making lemonade when your story is a lemon. These phenolic boards were almost always one-sided, and designers would use top-side jumpers or even zero-ohm resistors (better for machine insertion) to overcome topological barriers to using a single-sided board.
Raw phenolic board is cheaper than FR-4, and can be punched instead of drilled, thus reducing manufacturing cost, time, and drill wear. It has not disappeared, either: I have disassembled many non-working appliances (whether attempting to repair them or just out of curiosity) and often seen single-sided phenolic boards used for much or all of the circuitry, especially for larger power components and switches where the attributes of FR-4 are not needed, (Figure 3).
The days of FR-4 as the dominant circuit-board laminate may, however, be coming to an end. That’s because the basic performance attributes and consistency of FR-4 are not suitable with the stringent needs of multi-GHz circuits. Among the linked electrical and mechanical parameters of greatest interest are dielectric constant (er), loss factor (tδ), dielectric breakdown voltage, leakage current, tensile strength, the shear strength, moisture absorption, the glass transition temperature (Tg), and the Z-axis thickness, along with their temperature coefficients.
Companies such as Rogers Corp. now offer different gigaHertz-friendly laminates, each with specific combinations of electrical and mechanical characteristics such as their RT/duroid and Magtex families. They also provide application notes and articles which discuss the subtleties of these boards and measurement, including the performance of PCB through-hole playing on 5G-related performance, [Ref. 7]. Measuring these parameters is also complex: an epsilometer jointly announced by Copper Mountain Technologies and Compass Technology Group uses computational electromagnetic modeling in addition to measurements to determine of the complex dielectric permittivity of sheets from 0.3 to 3-mm thick from 3 to 6 GHz, [Refs. 8, 9].
One thing is certain: even if you know and like FR-4, serious multi-GHz and 5G design needs to consider use of these advanced PC-board laminates, along with extremely sophisticated models and simulation covering all aspects of the physical design and assessment. There’s a lot of learning to do.
Have you ever had difficulties with FR-4 when used within reasonable situations? For example, did you need to go to thicker substrate of copper? Have you even used it for non-electronic or unconventional purposes or fixes?