What is the Fiber Weave Effect in a PCB Substrate?By ZM Peterson • May 27, 2020
Anyone working beyond microwave frequencies will need to consider new aspects of signal integrity they may have never considered. One of the many signal integrity aspects at high GHz frequencies is the fiber weave effect. This is an important aspect of the fiberglass weave that is less known among many designers, but it produces some odd effects at high frequencies and ultra-fast edge rates.
The image above is an SEM image of the fiberglass weave that is used to form PCB substrates. The fiber weave structure causes a PCB substrate to act like an inhomogeneous anisotropic material, where behavior of electromagnetic waves in the material depends on the direction waves flow in the material and the location in the material. The inhomogeneity in the substrate is periodic. It is this periodic inhomogeneity in the PCB substrate and the fibrous nature of the substrate that causes the fiber weave effect.
The signal integrity problems that arise in PCB substrates can be solved in some creative ways, but it requires understanding how you signals interact with the substrate as they are routed. It also requires judicious material selection to prevent signal degradation at a receiver. Let’s look at some of the important information needed to understand the fiber weave effect and how you can avoid signal integrity problems in mmWave and higher frequency ranges.
The Fiber Weave Effect and Your Signals
PCB laminates are formed from a square-grid of glass strands that are woven together into a panel. The panel is then filled with a resin to harden the panel and allow bonding to other laminate layers. The dielectric constants of these two materials are very different. The glass used to form the weave structure has a dielectric constant of approximately 6 with very low loss. The epoxy filler a dielectric constant approximately 3 and varying levels of loss, depending on the exact resin used in the laminate. The dielectric constant of PCB substrate materials is normally quoted as a single measurement, although this value depends on the measurement method and is basically equivalent to a length-averaged value for the two materials.
Due to the structure of a PCB substrate, the dielectric constant of the substrate varies in space. This applies to low-Dk and high-Dk substrates. This variation is very small (on the order of 1 mm or less), but it becomes important in some limiting cases:
- When a digital signal’s rise time is very fast
- When a signal’s bandwidth reaches very high frequencies
- When an analog signal’s wavelength is similar to the cavity size in the weave
The structure of different glass weaves leaves some open cavities that are normally filled with resin. It is these cavities that primarily cause the fiber weave effect. As a signal traverses a trace and the electromagnetic field passes through the substrate material, the dielectric constant seen by the signal varies along the length of the interconnect. These cavities in different standard glass weaves can be seen in a microscope image.
Cavities formed in glass weave substrates with different tightness.
It is these cavities in the substrate that cause variations in the signal velocity as the signal traverses an interconnect. These variations in signal velocity are responsible for the fiber weave effect. Although the fiber weave effect primarily refers to skew, it also refers to losses, dispersion, and cavity resonances in the substrate. Let’s look at each of these three effects.
Skew During Propagation
The fiber weave effect primarily refers to skew that accumulates along an interconnect. The skew between two ends of a differential pair, or between nets routed in parallel. When a trace in this arrangement is routed over a cavity in the substrate, the signal moves slightly faster, coming out of sync with the other signals. Over long distances, this skew accumulates and causes signals to desynchronize.
PCB design software applications are not set up to consider these effects simply because they can’t be addressed during the design phase. You can opt for a substrate with a tighter weave, which will cause the signal to experience less skew as it travels along an interconnect. Substrates that use NE-glass will have lower dielectric constant contrast as NE-glasses have Dk values near 4.5, which means accumulated skew will be smaller.
Maximum skew will accumulate when traces are routed over cavities in the substrate and when the other net is not routed over a cavity. This problem can’t really be solved by the designer unless you simply choose a substrate with a tighter weave. However, you may be able to orient your boards in a panel to appropriately minimize skew. This may require hand removal of finished boards from a panel; be sure to consult with your manufacturer on their capabilities (see below).
Skew arises when traces are routed over different portions of the fiber weave.
Different regions of the substrate will incur different levels of losses as a signal traverses the dielectric. Different sections of dielectric will attenuate the signal by different amounts, causing one signal level to drop relative to a signal on a parallel net. This arises due to reflection at dielectric interfaces in the substrate, which increases return loss. This is particularly important when routing differential pairs in the manner shown above as it reduces the common-mode rejection ratio of the receiver.
The dielectric constant of a substrate determines the impedance seen by a signal traveling along an interconnect. Dispersion is already present in a substrate, but at high GHz frequencies, the variation in the substrate’s dielectric constant and losses with location cause different impedance profiles to arise along the length of the trace. Again, these effects are minimal as long as traces are relatively short and you are working at low GHz wavelengths, but signal distortion due to dispersion in your PCB substrate becomes very obvious at high frequencies on long traces.
Cavity resonances are normally discussed within the high speed and high frequency communities in terms of resonances in waveguides and between conductors generally. However, anytime a travelling wave crosses an interface between different dielectrics, such as the interface between the resin and fiberglass in a PCB substrate, there is a back reflection and superposition in the cavity. The high refractive index contrast between the glass weave and the epoxy leads to relatively strong reflection, which is sufficient to cause superposition between incident and reflected waves in these substrate cavities.
This causes two electromagnetic waves travelling in opposite directions to interfere with each other in the epoxy regions in the substrate. A glass weave pitch of 60 mils is typical in FR4 materials, which gives a lowest order resonance frequency (half a wavelength) of approximately 45 GHz. In other words, any signal with bandwidth extending beyond 45 GHz will excite some resonance in the substrate around the trace. This resonance increases crosstalk in another nearby trace.
Cavity resonances due to the fiber weave effect can be treated as waves in a square box, making them easy to analyze.
Preventing the Fiber Weave Effect
Aside from choosing the appropriate substrate, early communication with your fabricator about your requirements for routing and board orientation on panels is critical. This can help you prevent excessive losses and skew from the fiber weave effect. Note that all of these effects can also be suppressed using a tighter substrate weave, while routing at an angle will only address the problem of skew. A substrate with NE-glass will have a lower refractive index, which will result in weaker reflection at the epoxy-glass interface and weaker resonances in the substrate cavities. Be sure to talk to your fabricator early in order to ensure your board will operate as intended. For a thorough review on the fiber weave effect in PCB substrates, take a look at this article in Applied Sciences (MDPI).
The design team at NWES specializes in high frequency and high speed PCB design. We know how to address the fiber weave effect in mmWave boards, and we know how to model impedance in real dielectrics to ensure signal integrity. We’re here to help innovative electronics companies design modern PCB and create cutting-edge technology. We’re also a digital marketing firm, and we provide SEO-driven content marketing services for the products we design. We’re the best choice to market your product because we’ve built it and we’ve used it. Contact NWES for a consultation.