One might not have to look far to find PTFE in everyday life; a quick check of the stovetop or kitchen cabinet might do the trick. PTFE, short for polytetrafluoroethylene and more commonly known as Teflon, is a remarkable material due to its exceptionally low coefficient of friction that made “nonstick” pans a household name. However, mechanical properties are not the end of PTFE’s appeal to industries: the excellent dielectric properties and high melting point render it highly suitable as an insulator. PTFE PCBs impart the best performance for microwave circuit applications but often use FR4 to reduce cost and improve structural integrity.
Manufacturing Considerations for High-Frequency Laminates
| Dimensional Stability | Multilayer Fabrication | Electrical Performance | |
|---|---|---|---|
| PTFE | Mediocre | Challenging | Excellent |
| Filled PTFE | Good | Acceptable | Great |
| Thermoset Hydrocarbon | Great | Robust | Good |
PTFE PCBs: A Manufacturer’s Perspective
PTFE offers exceptional electrical characteristics over more common FR4 laminates – so why isn’t it the default substrate material? Primarily, it is a cost issue born out of challenges working with the material during manufacturing: it is much softer than FR4, which complicates drilling (especially in the case of stackups consisting only of PTFE). Usually, manufacturers combine FR4 laminates with PTFE on the appropriate signal layers (i.e., high-speed/RF/microwave); PTFE’s low bond strength (which also affects the copper adhesion) complicates this process.
There are two approaches to solving PTFE bond strength issues:
- Sodium etching – A sodium solvent – historically sodium naphthalene, but more recently (di)ethylene glycol dimethyl ether due to safety concerns during processing of the former – reacts with the PTFE, allowing it to form hydrogen bonds, which are the backbone of board adhesion. Glycol ethers use a higher temperature during application to drastically improve bond strength. Sodium-etched PTFE degrades with exposure to UV radiation, heat, and abrasion; storage requires temperature- and moisture-controlled environments.
- Plasma etching – An electrically charged gas defluorinates the surface to support hydrogen bonding.
Sodium etching provides superior bond strength to plasma etching, making it the preferred option for most applications. Some manufacturers may favor plasma etching as the plasma etch chamber is equipment common to flex/rigid-flex printed circuits and doesn’t require treatment with specialty chemicals. Regardless of the method, etching improves the PTFE substrate’s bond strength over unetched PTFE.
As mentioned, all-PTFE stackups are rare due to bond strength issues; high-temperature bonding is a possibility, but this can also age the materials and affect shelf or service life. Instead, manufacturers prefer a hybrid stackup that combines the strengths of PTFE with FR4 on the appropriate layers. Manufacturers can process this hybrid stackup more efficiently, saving on PTFE laminate costs while offering the desired electrical characteristics and additional mechanical benefits. Furthermore, marrying the PTFE and FR4 layers can reduce system size by combining what would otherwise be discrete printed circuit assemblies joined by a cable/wire harness into a singular, unified product. Not only does this save on enclosure spaces in designs where the form factor is a concern, but eliminating (or at least reducing) the number of off-board connections improves reliability and assembly costs.
Comparing PTFE and High-Frequency Laminate Options
Variability of High-Frequency Laminates
| Temperature-Dependent Dielectric Constant (%/25℃) | Moisture Absorption
(%) |
Impedance Tolerance
(%) |
Thickness Tolerance (%) | |
|---|---|---|---|---|
| FR4* | 1 | 0.5 | 10 | 15 |
| PTFE | .16 | 0.02 | 1 | 7 |
| Filled PTFE | .017 | 0.1 | 2 | 7 |
| Thermoset Hydrocarbon | .067 | 0.06 | 2 | 7 |
*: Included as control
There are other options available to manufacturers beyond pure-PTFE and hybrid PTFE-FR4 stackups. As a laminate, ceramic-filled PTFE has material properties between PTFE and FR4; while its dispersion factor is twice that of pure PTFE (i.e., electromagnetic waves dissipate twice as much energy over identical transmission paths), its losses are still low overall (in comparison, FR4 has a dissipation factor approximately ten times that of ceramic-filled PTFE). Thermoset hydrocarbon substrates have twice as much electrical loss as ceramic-filled PTFEs but even better manufacturability, making them even more similar to FR4s in production.
Thermal properties may also influence the choice of PTFE substrate. Ceramic-filled PTFE PCBs shine in this context even if microwave speeds are not in play: ceramics grant much improved thermal dissipation for high-power circuits over traditional FR4 substrates. Additionally, ceramic-filled PTFE exhibits the best-controlled impedance for applications with large temperature swings due to a low temperature-dependent dielectric constant that ensures constancy across extreme temperatures. Comparatively, PTFE and hybrid PTFE-FR4 experience drastic differences in dielectric constant and perform best with reasonable swings around room temperature.
Heightened requirements for controlled impedance can also propel the use of PTFE laminates in a design. Regarding moisture, stackups containing PTFE exhibit much better hygroscopicity than FR4 alone. The same low surface energy that inhibits bond strength between laminates also prevents water absorption and may be employed similarly to boards using ceramic-filled PTFE purely for temperature reasons. The moisture absorption distorts the dielectric constant relative to the manufacturer’s datasheet, throwing the entire design off-kilter. More generally, PTFE and other high-frequency laminates present much lower impedance tolerances: whereas FR4 impedance values can vary by up to 10%, the specialty laminates confer a much more reasonable 1~2% to the design – a necessity for complex and high-performance layouts.
Your Contract Manufacturer for Specialty Laminates
PTFE PCBs give designers and manufacturers a broader range of material properties, primarily for high-speed microwave circuitry, but some environmental factors can also spur the choice. In general, while PTFE PCBs are more difficult for the manufacturer to process (and therefore more costly), high-end performance may dictate their use, and manufacturers can tamp down on costs by using hybrid-FR4 stackups when possible. Here at VSE, we’re a team of engineers committed to building electronics for our customers, and that includes a full review of stackup design materials to ensure maximum cost-performance benefit. We’ve been realizing life-saving and life-changing devices with our manufacturing partners for over forty years.
