Long ago, when I was working on new optoelectronic devices, the prototypes we built were pretty messy. Rather than splurge on top-quality materials, we used standardized, low-cost materials and measurement instruments just to get our devices to work. The devices did work, and all we sacrificed was aesthetics. But when it came time to show the feasibility of our devices in real applications, we knew we had to work with the right high-purity materials.
There are tradeoffs in choosing between cheaper, popular material and expensive, effective material. With the high popularity of FR4 among PCB designers and manufacturers, and more devices running at high speed or high frequency, it pays to know more about how FR4 affects signal integrity and propagation delay in your PCB.
FR4 and High-Speed/High-Frequency Design
Those familiar with high-speed design know that trace geometry, trace location, and board substrate all affect signal speed, impedance matching, and propagation delay. FR4 is not always the best option if you are designing a high-speed or high-frequency device. Most designers and engineers recommend using some other material as a substrate for high-frequency/high-speed devices.
Thanks to propagation delay, both analog waves, and digital pulses have critical link lengths that determine their transition to transmission line behavior. Once the links are behaving as transmission lines, impedance matching becomes critical in preventing ringing and resonance between sources and loads on your board. This transition depends on a comparison between the signal rise time and the propagation delay.
Adjacent traces on a single layer, as well as adjacent layers in multilayer boards, form a capacitor. The trace spacing and the dielectric constant of FR4 determine the equivalent capacitance. Impedance matching is critical in the transmission line regime, and this parasitic capacitance should be considered when designing the board, especially when working with high-speed/high-frequency signals.
Printed circuit board green electronic background
This all occurs due to the presence of a dielectric near conductive traces. The dielectric constant of the conductor determines the speed of a signal that travels along the conductor. If we imagine a trace to be suspended in a vacuum, the signal speed in the trace only depends on the conductor’s dielectric constant. In the presence of nearby dielectrics (like an FR4 substrate), the dielectric constant of the conductor takes on a different value.
This modified dielectric constant is called the effective dielectric constant. This is normally calculated by ignoring the effects of dispersion and absorption in the substrate. This is fine at low frequencies and switching speeds, but the same calculations yield the incorrect propagation delay at high frequencies and switching speeds. The board thickness also changes the effective dielectric constant of the board, which then alters the parasitic capacitance and required impedance matching.
FR4 boards could be used in high-speed devices when the layers are coated with high-speed laminates. These laminates involve less loss than FR4 and largely determine the effective dielectric constant in the traces. A combination of FR4 and a high-speed laminate may be preferable to an alternative material, depending on the costs involved.
Absorption and Dispersion
The propagation delay in a PCB trace depends on the dielectric constant of the substrate, the dimensions of the trace, and whether we are considering a stripline or microstrip trace. At very low frequencies and low switching speeds, the propagation delay is relatively insensitive to frequency/switching speed changes. But propagation delay becomes more sensitive at higher speeds and frequencies.
This occurs due to dispersion in the FR4 substrate. PCBs for RF, radar, and gigabit data applications need to be designed with an eye towards dispersion and its effects on propagation delay. Since these applications typically require a low noise floor, differential traces are used to suppress crosstalk. The propagation delay then affects the length matching tolerance between parallel traces.
Traces on FR4 tend to have higher losses than other PCB materials that are specialized for RF applications above 1 GHz. FR4 has negative dispersion and increasing loss tangent at increasingly higher frequencies. Compared to other materials that are specialized for high frequencies, dispersion actually increases the signal speed at higher frequencies, thus decreasing the propagation delay at higher frequencies.
Electromagnetic absorption in FR4 rapidly increases up to about 100 KHz and then increases steadily up to about 100 GHz. This causes traces on FR4 to have larger attenuation at high frequencies for a given board thickness. This is the primary reason that high-speed laminates are used on FR4 boards.
Thermal expansion design in a railroad track
Calculating the right value for propagation delay requires the right model for the material parameters of the substrate. The linear baseline models for dispersion and absorption are clearly incorrect above 4 GHz frequencies or switching speeds. The wideband Debye model is clearly the best model to use to describe these critical material parameters in FR4 over a broad frequency range.
If you look at the frequency spectrum in a digital pulse, you’ll find that digital signals have most of their intensity concentrated between the switching frequency and the knee frequency. The knee frequency is approximately one-third of the inverse of the signal rise time.
Since a digital pulse is really just a superposition of analog waves, dispersion affects each of these analog frequencies slightly differently. A good approximation is to consider dispersion only at the switching frequency. This approximation is acceptable for low to moderate dispersion.
Given the design tradeoffs that must be considered when selecting a substrate material, you’ll need PCB design software that gives you flexibility in choosing the right substrate. The CAD tools, intuitive layer stackup tools, and simulation features in Altium Designer 18.1 make it easy to design your next high-speed or high-frequency device on FR4. Talk to an Altium expert today to learn more.
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