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Goodbye, FR-4, we’re going to miss you

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—that 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.

assembles PCB circuit board
Figure 1. The green, FR-4 PC board has become so common we often don’t even consciously realize the major role it plays and the technical standards which define it. (Image source: Yuppee Magazine)

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!

Figure 2. Alternative uses for PCBs include stiffening this briefcase. Bare boards also work well as trivets for your kitchen table.

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).

Figure 3. PC boards using a matrix of paper and phenolic (designated as FR-2) is a low-cost alternative to FR-4 for some applications; it is often used in single-sided designs with larger components and less-critical design constraints. (Image source: The Engineering Projects)

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?

References

  1. Wikipedia, “Printed circuit board
  2. Wikipedia, “Via (electronics)
  3. Wikipedia, “FR-4
  4. SEEED Studio, “Printed Circuit Board(PCB) Material Types and Comparison
  5. Microwaves101, “FR-4
  6. All About Circuits, “Which Via Should I Choose? A Guide to Vias in PCB Design
  7. Microwave Journal, Evaluating PCB Plated Through Holes For 5G Applications
  8. John W. Schultz, Compass Technology Group, “A New Dielectric Analyzer for Rapid Measurement of Microwave Substrates up to 6 GHz
  9. Microwave Journal, Epsilometer Measures Dielectric Properties to 6 GHz
  10. Microwave Journal, Laminate Materials Simultaneously Increase μ and ε, Reducing Antenna Size
  11. Microwave Journal, Characterizing Circuit Materials at mmWave Frequencies

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8 comments on “Goodbye, FR-4, we’re going to miss you

  1. Martin Rowe
    September 11, 2019

    Sorry Bill, but I must disagree. FR-4 will live on for many years. Why? because it’s cheap compared to other PCB substrates. In the signal integrity world of high-speed digital signals, the dealth of FR-4 has been predicted for a long time. But, new signal processing in for form of error correction, pre-emphasis, and updated modulation (PAM4) keep extending the life of FR-4. Then, there are all those lower-speeds (you know megabits/sec) applications where FR-4 works just fine.

  2. eldercosta
    September 12, 2019

    FR-4 is often green-ish (very light green). However, the green on Figure 1 is not from FR-4, it is from the solder mask. It could be blue, red, black, grey, white (possibly any color one may think of).

  3. antedeluvian
    September 12, 2019

    Apropos of your handle stiffener, you may find my blog “Strange Brews: Designs That Shouldn’t Involve a PCB, but Do!” interesting.
    https://www.eetimes.com/author.asp?section_id=36&doc_id=1320132#

  4. Martin Rowe
    September 12, 2019

    I worked in a PCB and flexible circuit factory while in high school and for a couple of summers in college. A scar on my left arm is from a wave solder machine.

  5. GSKrasle
    September 16, 2019

    I once worked on a product that was to have a pretty small and thin PCB, but the prototypes were made many many times larger than the target size, with lots of alternate circuits, debug hardware and oh-SOOO-many jumpers, all the circuitry on the top-side. Unfortunately, they were fabricated with the same thin material (yes, FR4). No matter how many “feet” were added, it would flex too much when jumpers or connectors were changed, and resistors, capacitors, and especially inductors would break.
    I finally found a cheap easy and effective solution: I used wood-screws to mount them to slabs of foam insulation panels intended for walls. So I ended-up with 7.5cm thick plystyrene “PCBs” because the FR4 was too floppy.

  6. David Ashton
    September 18, 2019

    I used to know Phenolic board by the acronym SRBP – Synthetic Resin Bonded Paper. A rose by any other name would smell as sweet. Except it does NOT smell sweet when heated to extract components. If it gets heated to the point of charring it smells truly awful (not to be done inside or without ventilation). So I am very careful when extracting components from this stuff and only do it when I really need.

    • Martin Rowe
      September 18, 2019

      Is that board you’re smelling or the burned component? I love the smell of molten silicon in the morning. 🙂

  7. tomcircuit
    October 14, 2019

    Maybe there are still applications that use Phenolic PCB substrate, but I wouldn’t be surprised if those were actually CEM1. CEM1 (single sided) and CEM3 (double sided) are paper-glass-epoxy composites, flame retardant, and a bit cheaper than FR4. Once nice thing about CEM is that the boards can be scored and snapped apart very cleanly. It seems that these materials really never die, they just fade into ubiquity!

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